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14 <tr><th colspan="3" align="center">Lists and Tuples</th></tr> |
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24 <div class="article" title="Lists and Tuples"> |
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25 <div class="titlepage"> |
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26 <div><div><h2 class="title"> |
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27 <a name="id2609714"></a>Lists and Tuples</h2></div></div> |
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28 <hr> |
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29 </div> |
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30 <div class="toc"> |
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31 <p><b>Table of Contents</b></p> |
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32 <dl> |
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33 <dt><span class="section"><a href="ar01.html#common-list-operations">1. Common List Operations</a></span></dt> |
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34 <dd><dl> |
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35 <dt><span class="section"><a href="ar01.html#indexing">1.1. Indexing</a></span></dt> |
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36 <dt><span class="section"><a href="ar01.html#concatenating">1.2. Concatenating</a></span></dt> |
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37 <dt><span class="section"><a href="ar01.html#slicing">1.3. Slicing</a></span></dt> |
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38 <dt><span class="section"><a href="ar01.html#multiplication">1.4. Multiplication</a></span></dt> |
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39 <dt><span class="section"><a href="ar01.html#membership">1.5. Membership</a></span></dt> |
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40 <dt><span class="section"><a href="ar01.html#length-maximum-and-minimum">1.6. Length, Maximum and Minimum</a></span></dt> |
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41 <dt><span class="section"><a href="ar01.html#changing-elements">1.7. Changing Elements</a></span></dt> |
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42 <dt><span class="section"><a href="ar01.html#deleting-elements">1.8. Deleting Elements</a></span></dt> |
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43 <dt><span class="section"><a href="ar01.html#assign-to-slices">1.9. Assign to Slices</a></span></dt> |
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44 </dl></dd> |
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45 <dt><span class="section"><a href="ar01.html#none-empty-lists-and-initialization">2. None, Empty Lists, and Initialization</a></span></dt> |
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46 <dt><span class="section"><a href="ar01.html#nested-lists">3. Nested Lists</a></span></dt> |
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47 <dt><span class="section"><a href="ar01.html#list-methods">4. List Methods</a></span></dt> |
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48 <dd><dl> |
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49 <dt><span class="section"><a href="ar01.html#append">4.1. append</a></span></dt> |
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50 <dt><span class="section"><a href="ar01.html#count">4.2. count</a></span></dt> |
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51 <dt><span class="section"><a href="ar01.html#extend">4.3. extend</a></span></dt> |
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52 <dt><span class="section"><a href="ar01.html#index">4.4. index</a></span></dt> |
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53 <dt><span class="section"><a href="ar01.html#insert">4.5. insert</a></span></dt> |
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54 <dt><span class="section"><a href="ar01.html#pop">4.6. pop</a></span></dt> |
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55 <dt><span class="section"><a href="ar01.html#remove">4.7. remove</a></span></dt> |
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56 <dt><span class="section"><a href="ar01.html#reverse">4.8. reverse</a></span></dt> |
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57 <dt><span class="section"><a href="ar01.html#sort">4.9. sort</a></span></dt> |
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58 </dl></dd> |
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59 <dt><span class="section"><a href="ar01.html#tuples">5. Tuples</a></span></dt> |
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60 <dd><dl><dt><span class="section"><a href="ar01.html#common-tuple-operations">5.1. Common Tuple Operations</a></span></dt></dl></dd> |
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61 <dt><span class="section"><a href="ar01.html#additional-syntax">6. Additional Syntax</a></span></dt> |
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62 <dd><dl> |
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63 <dt><span class="section"><a href="ar01.html#range">6.1. range()</a></span></dt> |
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64 <dt><span class="section"><a href="ar01.html#for">6.2. for</a></span></dt> |
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65 </dl></dd> |
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66 <dt><span class="section"><a href="ar01.html#conclusion">7. Conclusion</a></span></dt> |
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67 </dl> |
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68 </div>Lists |
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69 <p><a name="x_1"></a>Python provides an intuitive way to represent a group items, called |
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70 <span class="emphasis"><em>Lists</em></span>. The items of a |
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71 <span class="emphasis"><em>List</em></span> are called its elements. Unlike C/C++, elements can be of any type. A |
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72 <span class="emphasis"><em>List</em></span> is represented as a list of comma-sepated elements with square brackets around them: |
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73 </p> |
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74 <pre class="programlisting">>>> a = [10, 'Python programming', 20.3523, 23, 3534534L] |
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75 >>> a |
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76 [10, 'Python programming', 20.3523, 23, 3534534L] |
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77 |
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78 |
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79 </pre> |
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80 <div class="section" title="1. Common List Operations"> |
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81 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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82 <a name="common-list-operations"></a>1. Common List Operations</h2></div></div></div> |
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83 <p><a name="x_2"></a>The following are some of the most commonly used operations on |
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84 <span class="emphasis"><em>Lists</em></span>. |
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85 </p> |
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86 <div class="section" title="1.1. Indexing"> |
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87 <div class="titlepage"><div><div><h3 class="title"> |
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88 <a name="indexing"></a>1.1. Indexing</h3></div></div></div> |
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89 <p><a name="x_3"></a>Individual elements of a |
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90 <span class="emphasis"><em>List</em></span> can be accessed using an index to the element. The indices start at 0. One can also access the elements of the |
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91 <span class="emphasis"><em>List</em></span> in reverse using negative indices.: |
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92 </p> |
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93 <pre class="programlisting">>>> a[1] |
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94 'Python programming' |
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95 >>> a[-1] |
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96 3534534L |
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97 |
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98 </pre> |
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99 <p><a name="x_4"></a>It is important to note here that the last element of the |
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100 <span class="emphasis"><em>List</em></span> has an index of -1. |
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101 </p> |
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102 </div> |
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103 <div class="section" title="1.2. Concatenating"> |
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104 <div class="titlepage"><div><div><h3 class="title"> |
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105 <a name="concatenating"></a>1.2. Concatenating</h3></div></div></div> |
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106 <p><a name="x_5"></a>Two or more |
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107 <span class="emphasis"><em>Lists</em></span> can be concatenated using the + operator: |
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108 </p> |
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109 <pre class="programlisting">>>> a + ['foo', 12, 23.3432, 54] |
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110 [10, 'Python programming', 20.3523, 'foo', 12, 23.3432, 54] |
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111 >>> [54, 75, 23] + ['write', 67, 'read'] |
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112 [54, 75, 23, 'write', 67, 'read'] |
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113 |
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114 |
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115 </pre> |
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116 </div> |
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117 <div class="section" title="1.3. Slicing"> |
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118 <div class="titlepage"><div><div><h3 class="title"> |
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119 <a name="slicing"></a>1.3. Slicing</h3></div></div></div> |
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120 <p><a name="x_6"></a>A |
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121 <span class="emphasis"><em>List</em></span> can be sliced off to contain a subset of elements of the |
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122 <span class="emphasis"><em>List</em></span>. Slicing can be done by using two indices separated by a colon, where the first index is inclusive and the second index is exclusive. The resulting slice is also a |
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123 <span class="emphasis"><em>List</em></span>.: |
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124 </p> |
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125 <pre class="programlisting">>>> num = [1, 2, 3, 4, 5, 6, 7, 8, 9] |
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126 >>> num[3:6] |
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127 [4, 5, 6] |
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128 >>> num[0:1] |
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129 [1] |
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130 >>> num[7:10] |
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131 [7, 8, 9] |
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132 |
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133 </pre> |
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134 <p><a name="x_7"></a>The last example showed how to access last 3 elements of the |
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135 <span class="emphasis"><em>List</em></span>. There is a small catch here. The second index 10 actually refers to the 11th element of the |
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136 <span class="emphasis"><em>List</em></span> which is still valid, even though it doesn't exist because the second index is exclusive and tells the Python interpreter to get the last element of the |
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137 <span class="emphasis"><em>List</em></span>. But this can also be done in a much easier way using negative indices: |
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138 </p> |
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139 <pre class="programlisting">>>> num[-3:-1] |
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140 [7, 8, 9] |
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141 |
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142 </pre> |
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143 <p><a name="x_8"></a>Excluding the first index implies that the slice must start at the beginning of the |
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144 <span class="emphasis"><em>List</em></span>, while excluding the second index includes all the elements till the end of the |
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145 <span class="emphasis"><em>List</em></span>. A third parameter to a slice, which is implicitly taken as 1 is the step of the slice. It is specified as a value which follows a colon after the second index: |
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146 </p> |
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147 <pre class="programlisting">>>> num[:4] |
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148 [1, 2, 3, 4] |
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149 >>> num[7:] |
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150 [8, 9] |
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151 >>> num[-3:] |
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152 [7, 8, 9] |
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153 >>> num[:] |
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154 [1, 2, 3, 4, 5, 6, 7, 8, 9] |
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155 >>> num[4:9:3] |
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156 [5, 8] |
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157 >>> num[3::2] |
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158 [4, 6, 8] |
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159 >>> num[::4] |
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160 [1, 5, 9] |
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161 |
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162 |
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163 </pre> |
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164 </div> |
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165 <div class="section" title="1.4. Multiplication"> |
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166 <div class="titlepage"><div><div><h3 class="title"> |
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167 <a name="multiplication"></a>1.4. Multiplication</h3></div></div></div> |
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168 <p><a name="x_9"></a>A |
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169 <span class="emphasis"><em>List</em></span> can be multiplied with an integer to repeat itself: |
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170 </p> |
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171 <pre class="programlisting">>>> [20] * 5 |
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172 [20, 20, 20, 20, 20] |
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173 >>> [42, 'Python', 54] * 3 |
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174 [42, 'Python', 54, 42, 'Python', 54, 42, 'Python', 54] |
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175 |
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176 |
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177 </pre> |
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178 </div> |
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179 <div class="section" title="1.5. Membership"> |
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180 <div class="titlepage"><div><div><h3 class="title"> |
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181 <a name="membership"></a>1.5. Membership</h3></div></div></div> |
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182 <p><a name="x_a"></a> |
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183 <span class="strong"><strong>in</strong></span> operator is used to find whether an element is part of the |
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184 <span class="emphasis"><em>List</em></span>. It returns |
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185 <span class="strong"><strong>True</strong></span> if the element is present in the |
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186 <span class="emphasis"><em>List</em></span> or |
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187 <span class="strong"><strong>False</strong></span> if it is not present. Since this operator returns a Boolean value it is called a Boolean operator: |
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188 </p> |
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189 <pre class="programlisting">>>> names = ['Guido', 'Alex', 'Tim'] |
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190 >>> 'Tim' in names |
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191 True |
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192 >>> 'Adam' in names |
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193 False |
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194 |
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195 |
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196 </pre> |
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197 </div> |
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198 <div class="section" title="1.6. Length, Maximum and Minimum"> |
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199 <div class="titlepage"><div><div><h3 class="title"> |
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200 <a name="length-maximum-and-minimum"></a>1.6. Length, Maximum and Minimum</h3></div></div></div> |
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201 <p><a name="x_b"></a>Length of a |
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202 <span class="emphasis"><em>List</em></span> can be found out using the len function. The max function returns the element with the largest value and the min function returns the element with the smallest value: |
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203 </p> |
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204 <pre class="programlisting">>>> num = [4, 1, 32, 12, 67, 34, 65] |
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205 >>> len(num) |
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206 7 |
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207 >>> max(num) |
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208 67 |
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209 >>> min(num) |
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210 1 |
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211 |
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212 |
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213 </pre> |
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214 </div> |
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215 <div class="section" title="1.7. Changing Elements"> |
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216 <div class="titlepage"><div><div><h3 class="title"> |
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217 <a name="changing-elements"></a>1.7. Changing Elements</h3></div></div></div> |
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218 <p><a name="x_c"></a>Unlike Strings |
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219 <span class="emphasis"><em>Lists</em></span> are mutable, i.e. elements of a |
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220 <span class="emphasis"><em>List</em></span> can be manipulated: |
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221 </p> |
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222 <pre class="programlisting">>>> a = [1, 3, 5, 7] |
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223 >>> a[2] = 9 |
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224 >>> a |
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225 [1, 3, 9, 7] |
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226 |
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227 |
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228 </pre> |
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229 </div> |
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230 <div class="section" title="1.8. Deleting Elements"> |
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231 <div class="titlepage"><div><div><h3 class="title"> |
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232 <a name="deleting-elements"></a>1.8. Deleting Elements</h3></div></div></div> |
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233 <p><a name="x_d"></a>An element or a slice of a |
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234 <span class="emphasis"><em>List</em></span> can be deleted by using the |
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235 <span class="strong"><strong>del</strong></span> statement: |
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236 </p> |
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237 <pre class="programlisting">>>> a = [1, 3, 5, 7, 9, 11] |
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238 >>> del a[-2:] |
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239 >>> a |
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240 [1, 3, 5, 7] |
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241 >>> del a[1] |
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242 >>> a |
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243 [1, 5, 7] |
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244 |
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245 |
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246 </pre> |
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247 </div> |
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248 <div class="section" title="1.9. Assign to Slices"> |
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249 <div class="titlepage"><div><div><h3 class="title"> |
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250 <a name="assign-to-slices"></a>1.9. Assign to Slices</h3></div></div></div> |
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251 <p><a name="x_e"></a>In the same way, values can be assigned to individual elements of the |
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252 <span class="emphasis"><em>List</em></span>, a |
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253 <span class="emphasis"><em>List</em></span> of elements can be assigned to a slice: |
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254 </p> |
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255 <pre class="programlisting">>>> a = [2, 3, 4, 5] |
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256 >>> a[:2] = [0, 1] |
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257 [0, 1, 4, 5] |
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258 >>> a[2:2] = [2, 3] |
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259 >>> a |
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260 [0, 1, 2, 3, 4, 5] |
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261 >>> a[2:4] = [] |
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262 >>> a |
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263 [0, 1, 4, 5] |
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264 |
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265 </pre> |
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266 <p><a name="x_f"></a>The last two examples should be particularly noted carefully. The last but one example insert elements or a list of elements into a |
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267 <span class="emphasis"><em>List</em></span> and the last example deletes a list of elements from the |
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268 <span class="emphasis"><em>List</em></span>. |
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269 </p> |
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270 </div> |
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271 </div> |
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272 <div class="section" title="2. None, Empty Lists, and Initialization"> |
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273 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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274 <a name="none-empty-lists-and-initialization"></a>2. None, Empty Lists, and Initialization</h2></div></div></div> |
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275 <p><a name="x_10"></a>An |
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276 <span class="emphasis"><em>Empty List</em></span> is a |
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277 <span class="emphasis"><em>List</em></span> with no elements and is simply represented as []. A |
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278 <span class="emphasis"><em>None List</em></span> is one with all elements in it being |
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279 <span class="strong"><strong>None</strong></span>. It serves the purpose having a container list of some fixed number of elements with no value: |
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280 </p> |
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281 <pre class="programlisting">>>> a = [] |
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282 >>> a |
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283 [] |
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284 >>> n = [None] * 10 |
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285 >>> n |
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286 [None, None, None, None, None, None, None, None, None, None] |
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287 |
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288 |
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289 </pre> |
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290 </div> |
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291 <div class="section" title="3. Nested Lists"> |
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292 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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293 <a name="nested-lists"></a>3. Nested Lists</h2></div></div></div> |
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294 <p><a name="x_11"></a>As mentioned earlier, a List can contain elements of any data type. This also implies a |
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295 <span class="emphasis"><em>List</em></span> can have a |
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296 <span class="emphasis"><em>Lists</em></span> themselves as its elements. These are called as |
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297 <span class="emphasis"><em>Nested Lists</em></span>. There is no limit on the depth of the |
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298 <span class="emphasis"><em>Nested Lists</em></span>: |
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299 </p> |
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300 <pre class="programlisting">>>> a = [1, [1, 2, 3], 3, [1, [1, 2, 3]], 7] |
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301 |
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302 |
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303 </pre> |
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304 </div> |
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305 <div class="section" title="4. List Methods"> |
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306 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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307 <a name="list-methods"></a>4. List Methods</h2></div></div></div> |
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308 <p><a name="x_12"></a>A method is a function that is coupled to an object. More about objects and its methods are discussed in Advanced Python module. In general, a method is called like:</p> |
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309 <pre class="programlisting">object.method(arguments) |
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310 |
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311 </pre> |
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312 <p><a name="x_13"></a>For now, it is enough to know that a list of elements is an object and so |
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313 <span class="emphasis"><em>List</em></span> methods can be called upon them. Also some of the methods change the |
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314 <span class="emphasis"><em>List</em></span> in-place, meaning it modifies the existing list instead of creating a new one, while other methods don't. It must be noted as we run through the |
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315 <span class="emphasis"><em>List</em></span> methods. |
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316 </p> |
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317 <p><a name="x_14"></a>Some of the most commonly used |
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318 <span class="emphasis"><em>List</em></span> methods are as follows: |
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319 </p> |
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320 <div class="section" title="4.1. append"> |
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321 <div class="titlepage"><div><div><h3 class="title"> |
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322 <a name="append"></a>4.1. append</h3></div></div></div> |
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323 <p><a name="x_15"></a>The |
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324 <span class="emphasis"><em>append</em></span> method is used to append an object at the end of the list: |
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325 </p> |
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326 <pre class="programlisting">>>> prime = [2, 3, 5] |
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327 >>> prime.append(7) |
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328 >>> prime |
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329 [2, 3, 5, 7] |
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330 |
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331 </pre> |
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332 <p><a name="x_16"></a>It is important to note that append changes the |
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333 <span class="emphasis"><em>List</em></span> in-place. |
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334 </p> |
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335 </div> |
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336 <div class="section" title="4.2. count"> |
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337 <div class="titlepage"><div><div><h3 class="title"> |
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338 <a name="count"></a>4.2. count</h3></div></div></div> |
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339 <p><a name="x_17"></a>The |
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340 <span class="emphasis"><em>count</em></span> method returns the number of occurences of a particular element in a list: |
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341 </p> |
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342 <pre class="programlisting">>>> [1, 4, 4, 9, 9, 9].count(9) |
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343 3 |
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344 >>> tlst = ['Python', 'is', 'a', 'beautiful', 'language'] |
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345 >>> tlst.count('Python') |
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346 1 |
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347 |
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348 |
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349 </pre> |
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350 </div> |
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351 <div class="section" title="4.3. extend"> |
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352 <div class="titlepage"><div><div><h3 class="title"> |
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353 <a name="extend"></a>4.3. extend</h3></div></div></div> |
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354 <p><a name="x_18"></a>The |
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355 <span class="emphasis"><em>extend</em></span> method extends the list on which it is called by the list supplied as argument to it: |
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356 </p> |
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357 <pre class="programlisting">>>> a = [1, 2, 3] |
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358 >>> b = [4, 5, 6] |
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359 >>> a.extend(b) |
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360 [1, 2, 3, 4, 5, 6] |
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361 |
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362 </pre> |
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363 <p><a name="x_19"></a>This is an in-place method. This method is equivalent to using the + operator, but using the + operator returns a new list.</p> |
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364 </div> |
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365 <div class="section" title="4.4. index"> |
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366 <div class="titlepage"><div><div><h3 class="title"> |
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367 <a name="index"></a>4.4. index</h3></div></div></div> |
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368 <p><a name="x_1a"></a>The |
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369 <span class="emphasis"><em>index</em></span> method returns the index position of the element in the list specified as argument: |
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370 </p> |
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371 <pre class="programlisting">>>> a = [1, 2, 3, ,4, 5] |
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372 >>> a.index(4) |
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373 3 |
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374 |
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375 |
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376 </pre> |
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377 </div> |
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378 <div class="section" title="4.5. insert"> |
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379 <div class="titlepage"><div><div><h3 class="title"> |
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380 <a name="insert"></a>4.5. insert</h3></div></div></div> |
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381 <p><a name="x_1b"></a>The |
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382 <span class="emphasis"><em>insert</em></span> method is used to insert an element specified as the second argument to the list at the position specified by the first argument: |
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383 </p> |
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384 <pre class="programlisting">>>> a = ['Python', 'is', 'cool'] |
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385 >>> a.insert(2, 'so') |
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386 >>> a |
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387 ['Python', 'is', 'so', 'cool'] |
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388 |
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389 </pre> |
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390 <p><a name="x_1c"></a>The |
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391 <span class="emphasis"><em>insert</em></span> method changes the |
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392 <span class="emphasis"><em>List</em></span> in-place. |
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393 </p> |
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394 </div> |
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395 <div class="section" title="4.6. pop"> |
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396 <div class="titlepage"><div><div><h3 class="title"> |
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397 <a name="pop"></a>4.6. pop</h3></div></div></div> |
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398 <p><a name="x_1d"></a>The |
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399 <span class="emphasis"><em>pop</em></span> method removes an element from the list. The index position of the element to be removed can be specified as an argument to the |
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400 <span class="emphasis"><em>pop</em></span> method, if not it removes the last element by default: |
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401 </p> |
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402 <pre class="programlisting">>>> a = [1, 2, 3, 4, 5] |
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403 >>> a.pop() |
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404 >>> a |
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405 5 |
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406 >>> a.pop(2) |
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407 >>> a |
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408 3 |
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409 |
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410 </pre> |
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411 <p><a name="x_1e"></a>The |
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412 <span class="emphasis"><em>pop</em></span> method changes the |
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413 <span class="emphasis"><em>List</em></span> in-place. |
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414 </p> |
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415 </div> |
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416 <div class="section" title="4.7. remove"> |
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417 <div class="titlepage"><div><div><h3 class="title"> |
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418 <a name="remove"></a>4.7. remove</h3></div></div></div> |
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419 <p><a name="x_1f"></a>The |
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420 <span class="emphasis"><em>remove</em></span> method removes the first occurence of an element supplied as a parameter: |
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421 </p> |
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422 <pre class="programlisting">>>> a = [1, 2, 3, 4, 2, 5, 2] |
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423 >>> a.remove(2) |
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424 >>> a |
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425 [1, 3, 4, 2, 5, 2] |
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426 |
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427 |
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428 </pre> |
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429 </div> |
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430 <div class="section" title="4.8. reverse"> |
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431 <div class="titlepage"><div><div><h3 class="title"> |
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432 <a name="reverse"></a>4.8. reverse</h3></div></div></div> |
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433 <p><a name="x_20"></a>The |
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434 <span class="emphasis"><em>reverse</em></span> method reverses elements in the list. It is important to note here that |
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435 <span class="emphasis"><em>reverse</em></span> method changes the list in-place and doesn't return any thing: |
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436 </p> |
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437 <pre class="programlisting">>>> a = ['guido', 'alex', 'tim'] |
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438 >>> a.reverse() |
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439 >>> a |
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440 ['tim', 'alex', 'guido'] |
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441 |
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442 |
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443 </pre> |
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444 </div> |
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445 <div class="section" title="4.9. sort"> |
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446 <div class="titlepage"><div><div><h3 class="title"> |
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447 <a name="sort"></a>4.9. sort</h3></div></div></div> |
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448 <p><a name="x_21"></a>The |
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449 <span class="emphasis"><em>sort</em></span> method is used to sort the elements of the list. The |
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450 <span class="emphasis"><em>sort</em></span> method also sorts in-place and does not return anything: |
|
451 </p> |
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452 <pre class="programlisting">>>> a = [5, 1, 3, 7, 4] |
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453 >>> a.sort() |
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454 >>> a |
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455 [1, 3, 4, 5, 7] |
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456 |
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457 </pre> |
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458 <p><a name="x_22"></a>In addition to the sort method on a |
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459 <span class="emphasis"><em>List</em></span> object we can also use the built-in |
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460 <span class="strong"><strong>sorted</strong></span> function. This function takes the |
|
461 <span class="emphasis"><em>List</em></span> as a parameter and returns a sorted copy of the list. However the original list is left intact: |
|
462 </p> |
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463 <pre class="programlisting">>>> a = [5, 1, 3, 7, 4] |
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464 >>> b = sorted(a) |
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465 >>> b |
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466 [1, 3, 4, 5, 7] |
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467 >>> a |
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468 [5, 1, 3, 7, 4] |
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469 |
|
470 |
|
471 </pre> |
|
472 </div> |
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473 </div> |
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474 <div class="section" title="5. Tuples"> |
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475 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
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476 <a name="tuples"></a>5. Tuples</h2></div></div></div> |
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477 <p><a name="x_23"></a> |
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478 <span class="emphasis"><em>Tuples</em></span> are sequences just like |
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479 <span class="emphasis"><em>Lists</em></span>, but they are immutable. In other words |
|
480 <span class="emphasis"><em>Tuples</em></span> provides a way to represent a group of items, where the group of items cannot be changed in any way. The syntax of a |
|
481 <span class="emphasis"><em>Tuple</em></span> is also very similar to |
|
482 <span class="emphasis"><em>List</em></span>. A |
|
483 <span class="emphasis"><em>Tuple</em></span> is represented with the list of items, called elements of the |
|
484 <span class="emphasis"><em>Tuple</em></span> separated by comma, with the entire list being enclosed in parenthesis. It is not compulsory to use parenthesis around a |
|
485 <span class="emphasis"><em>Tuple</em></span> but it may be necessary in some of the cases: |
|
486 </p> |
|
487 <pre class="programlisting">>>> a = 1, 2, 3 |
|
488 >>> a |
|
489 (1, 2, 3) |
|
490 >>> b = 1, |
|
491 >>> b |
|
492 (1,) |
|
493 |
|
494 </pre> |
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495 <p><a name="x_24"></a>It is interesting to note the second example. Just a value followed by a comma automatically makes that an element of a |
|
496 <span class="emphasis"><em>Tuple</em></span> with only one element. It is also important to note that, irrespective of input having a parenthesis, the output always has a parenthesis. |
|
497 </p> |
|
498 <p><a name="x_25"></a>The first example is also known as |
|
499 <span class="emphasis"><em>Tuple packing</em></span>, because values are being packed into a tuple. It is also possible to do |
|
500 <span class="emphasis"><em>Tuple unpacking</em></span> which is more interesting. It is better to understand that by example. Say we have a co-ordinate pair from which we need to separate x and y co-ordinates: |
|
501 </p> |
|
502 <pre class="programlisting">>>> a = (1, 2) |
|
503 >>> x, y = a |
|
504 >>> x |
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505 1 |
|
506 >>> y |
|
507 2 |
|
508 |
|
509 </pre> |
|
510 <p><a name="x_26"></a> <span class="emphasis"><em>Tuple unpacking</em></span> also has several other use-cases of which the most interesting one is to swap the values of two variables. Using programming languages like C would require anywhere around 10 lines of code and an extra temporary variable to do this (including all the #include stuff). Python does it in the most intuitive way in just one line. Say we want to swap the co-ordinates in the above example: |
|
511 </p> |
|
512 <pre class="programlisting">>>> x, y = y, x |
|
513 >>> x |
|
514 2 |
|
515 >>> y |
|
516 1 |
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517 |
|
518 </pre> |
|
519 <div class="section" title="5.1. Common Tuple Operations"> |
|
520 <div class="titlepage"><div><div><h3 class="title"> |
|
521 <a name="common-tuple-operations"></a>5.1. Common Tuple Operations</h3></div></div></div> |
|
522 <p><a name="x_27"></a>There is no need to introduce all the |
|
523 <span class="emphasis"><em>Tuple</em></span> operations again, since |
|
524 <span class="emphasis"><em>Tuples</em></span> support the following operations that |
|
525 <span class="emphasis"><em>List</em></span> supports in exactly the same way: |
|
526 </p> |
|
527 <div class="blockquote"><blockquote class="blockquote"><div class="itemizedlist"><ul class="itemizedlist" type="*"> |
|
528 <li class="listitem" style="list-style-type: *"><p><a name="x_28"></a>Indexing</p></li> |
|
529 <li class="listitem" style="list-style-type: *"><p><a name="x_29"></a>Concatenating</p></li> |
|
530 <li class="listitem" style="list-style-type: *"><p><a name="x_2a"></a>Slicing</p></li> |
|
531 <li class="listitem" style="list-style-type: *"><p><a name="x_2b"></a>Membership</p></li> |
|
532 <li class="listitem" style="list-style-type: *"><p><a name="x_2c"></a>Multiplication</p></li> |
|
533 <li class="listitem" style="list-style-type: *"><p><a name="x_2d"></a>Length, Maximum, Minimum</p></li> |
|
534 </ul></div></blockquote></div> |
|
535 <p><a name="x_2e"></a>The following examples illustrate the above operations:</p> |
|
536 <pre class="programlisting">>>> a = (1, 2, 3, 4, 5, 6) |
|
537 >>> a[5] |
|
538 6 |
|
539 >>> b = (7, 8, 9) |
|
540 >>> a + b |
|
541 (1, 2, 3, 4, 5, 6, 7, 8, 9) |
|
542 >>> a[3:5] |
|
543 (4, 5) |
|
544 >>> 5 in a |
|
545 True |
|
546 >>> c = (1,) |
|
547 >>> c * 5 |
|
548 (1, 1, 1, 1, 1) |
|
549 >>> len(a) |
|
550 6 |
|
551 >>> max(a) |
|
552 6 |
|
553 >>> min(a) |
|
554 1 |
|
555 |
|
556 </pre> |
|
557 <p><a name="x_2f"></a>However the following |
|
558 <span class="emphasis"><em>List</em></span> operations are not supported by |
|
559 <span class="emphasis"><em>Tuples</em></span> because |
|
560 <span class="emphasis"><em>Tuples</em></span> cannot be changed once they are created: |
|
561 </p> |
|
562 <div class="blockquote"><blockquote class="blockquote"><div class="itemizedlist"><ul class="itemizedlist" type="*"> |
|
563 <li class="listitem" style="list-style-type: *"><p><a name="x_30"></a>Changing elements</p></li> |
|
564 <li class="listitem" style="list-style-type: *"><p><a name="x_31"></a>Deleting elements</p></li> |
|
565 <li class="listitem" style="list-style-type: *"><p><a name="x_32"></a>Assigning to slices</p></li> |
|
566 </ul></div></blockquote></div> |
|
567 <p><a name="x_33"></a>Similarity to |
|
568 <span class="emphasis"><em>Lists</em></span> leads to the questions like, why not |
|
569 <span class="emphasis"><em>Lists</em></span> only? Why do we even want |
|
570 <span class="emphasis"><em>Tuples</em></span>? Can we do the same with |
|
571 <span class="emphasis"><em>Lists</em></span>? And the answer is |
|
572 <span class="strong"><strong>Yes</strong></span> we can do it, but |
|
573 <span class="emphasis"><em>Tuples</em></span> are helpful at times, like we can return Tuples from functions. They are also returned by some built-in functions and methods. And also there are some use cases like co-ordinate among other things. So |
|
574 <span class="emphasis"><em>Tuples</em></span> are helpful. |
|
575 </p> |
|
576 </div> |
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577 </div> |
|
578 <div class="section" title="6. Additional Syntax"> |
|
579 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
580 <a name="additional-syntax"></a>6. Additional Syntax</h2></div></div></div> |
|
581 <p><a name="x_34"></a>The following additional syntax are introduced to make it easier to operate on |
|
582 <span class="emphasis"><em>Lists</em></span>. |
|
583 </p> |
|
584 <div class="section" title="6.1. range()"> |
|
585 <div class="titlepage"><div><div><h3 class="title"> |
|
586 <a name="range"></a>6.1. range()</h3></div></div></div> |
|
587 <p><a name="x_35"></a>The |
|
588 <span class="emphasis"><em>range</em></span> function takes at least one argument and 2 additional optional arguments. If two or more arguments are specified, the range function returns a list of natural numbers starting from the first argument passed to it to the second argument. The third argument, if specified is used as a step. Suppose only one argument is specified, then |
|
589 <span class="emphasis"><em>range</em></span> function returns a list of natural numbers starting from 0 upto the argument specified: |
|
590 </p> |
|
591 <pre class="programlisting">>>> range(5, 10, 2) |
|
592 [5, 7, 9] |
|
593 >>> range(2, 15) |
|
594 [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14] |
|
595 >>> range(12) |
|
596 [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11] |
|
597 |
|
598 </pre> |
|
599 </div> |
|
600 <div class="section" title="6.2. for"> |
|
601 <div class="titlepage"><div><div><h3 class="title"> |
|
602 <a name="for"></a>6.2. for</h3></div></div></div> |
|
603 <p><a name="x_36"></a>The |
|
604 <span class="strong"><strong>for</strong></span> keyword is used as a part of the looping construct. Unlike for loops in other languages, Python's for is used to iterate through the elements of sequences like |
|
605 <span class="emphasis"><em>Lists</em></span>, |
|
606 <span class="emphasis"><em>Tuples</em></span>, |
|
607 <span class="emphasis"><em>Dictionaries</em></span>, etc. The syntax of the for loop consists of |
|
608 <span class="strong"><strong>for</strong></span>, followed by a variable to hold the individual or the current element of the list during iteration and |
|
609 <span class="strong"><strong>in</strong></span>, followed by the sequence and a semicolon(':') The next line which is part of the |
|
610 <span class="strong"><strong>for</strong></span> loop, i.e the statements that are part of the loop should start with a new intend: |
|
611 </p> |
|
612 <pre class="programlisting">>>> names = ['Guido', 'Alex', 'Tim'] |
|
613 >>> for name in names: |
|
614 ... print "Name =", name |
|
615 ... |
|
616 Name = Guido |
|
617 Name = Alex |
|
618 Name = Tim |
|
619 |
|
620 |
|
621 </pre> |
|
622 </div> |
|
623 </div> |
|
624 <div class="section" title="7. Conclusion"> |
|
625 <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
|
626 <a name="conclusion"></a>7. Conclusion</h2></div></div></div> |
|
627 <p><a name="x_37"></a>This section on |
|
628 <span class="emphasis"><em>Lists</em></span> and |
|
629 <span class="emphasis"><em>Tuples</em></span> introduces almost all the necessary machinary required to work on |
|
630 <span class="emphasis"><em>Lists</em></span> and |
|
631 <span class="emphasis"><em>Tuples</em></span>. Topics like how to use these data structures in bigger more useful programs will be introduced in the subsequent chapters. |
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632 </p> |
|
633 </div> |
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634 </div> |
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635 <div class="navfooter"> |
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636 <hr> |
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637 <table width="100%" summary="Navigation footer"> |
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642 <td width="40%" align="right"> </td> |
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643 </tr> |
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644 <tr> |
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645 <td width="40%" align="left" valign="top">Chapter 1. List and Tuples </td> |
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646 <td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td> |
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