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1 .. Objectives |
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2 .. ---------- |
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3 |
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4 .. At the end of this tutorial, you should know -- |
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5 |
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6 .. 1. How to define strings |
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7 .. #. Different ways of defining a string |
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8 .. #. How to concatenate strings |
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9 .. #. How to print a string repeatedly |
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10 .. #. Accessing individual elements of the string |
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11 .. #. Immutability of strings |
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12 |
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13 .. Prerequisites |
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14 .. ------------- |
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15 |
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16 .. 1. getting started with ipython |
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17 |
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18 .. Author : Madhu |
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19 Internal Reviewer : Punch |
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20 External Reviewer : |
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21 Language Reviewer : Bhanukiran |
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22 Checklist OK? : <15-11-2010, Anand, OK> [2010-10-05] |
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23 |
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24 Script |
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25 ------ |
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26 |
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27 {{{ Show the slide containing the title }}} |
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28 |
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29 Hello friends. Welcome to this spoken tutorial on Getting started with |
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30 strings. |
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31 |
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32 {{{ Show the slide containing the outline }}} |
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33 |
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34 In this tutorial, we will look at what we really mean by strings, how |
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35 Python supports the use of strings and some of the operations that can |
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36 be performed on strings. |
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37 |
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38 {{{ Shift to terminal and start ipython }}} |
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39 |
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40 To begin with let us start ipython, by typing:: |
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41 |
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42 ipython |
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43 |
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44 on the terminal |
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45 |
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46 So, what are strings? In Python anything within either single quotes |
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47 or double quotes or triple single quotes or triple double quotes are |
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48 strings. |
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49 |
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50 {{{ Type in ipython the following and read them as you type }}}:: |
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51 |
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52 'This is a string' |
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53 "This is a string too' |
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54 '''This is a string as well''' |
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55 """This is also a string""" |
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56 'p' |
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57 "" |
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58 |
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59 Note that it really doesn't matter how many characters are present in |
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60 the string. The last example is a null string or an empty string. |
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61 |
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62 Having more than one control character to define strings is handy when |
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63 one of the control characters itself is part of the string. For |
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64 example:: |
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65 |
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66 "Python's string manipulation functions are very useful" |
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67 |
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68 By having multiple control characters, we avoid the need for |
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69 escaping characters -- in this case the apostrophe. |
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70 |
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71 The triple quoted strings let us define multi-line strings without |
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72 using any escaping. Everything within the triple quotes is a single |
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73 string no matter how many lines it extends:: |
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74 |
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75 """Having more than one control character to define |
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76 strings come as very handy when one of the control |
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77 characters itself is part of the string.""" |
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78 |
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79 We can assign this string to any variable:: |
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80 |
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81 a = 'Hello, World!' |
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82 |
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83 Now 'a' is a string variable. String is a collection of characters. In |
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84 addition string is an immutable collection. So all the operations that |
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85 are applicable to any other immutable collection in Python works on |
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86 string as well. So we can add two strings:: |
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87 |
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88 a = 'Hello' |
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89 b = 'World' |
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90 c = a + ', ' + b + '!' |
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91 |
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92 We can add string variables as well as the strings themselves all in |
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93 the same statement. The addition operation performs the concatenation |
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94 of two strings. |
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95 |
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96 Similarly we can multiply a string with an integer:: |
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97 |
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98 a = 'Hello' |
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99 a * 5 |
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100 |
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101 gives another string in which the original string 'Hello' is repeated |
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102 5 times. |
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103 |
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104 Following is an exercise that you must do. |
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105 |
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106 %% %% Obtain the string ``%% -------------------- %%`` (20 hyphens) |
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107 without typing out all the twenty hyphens. |
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108 |
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109 Please, pause the video here. Do the exercise and then continue. |
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110 |
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111 :: |
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112 |
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113 s = "%% " + "-"*20 + " %%" |
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114 |
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115 Let's now look at accessing individual elements of strings. Since, |
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116 strings are collections we can access individual items in the string |
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117 using the subscripts:: |
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118 |
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119 a[0] |
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120 |
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121 gives us the first character in the string. The indexing starts from 0 |
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122 for the first character and goes up to n-1 for the last character. We |
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123 can access the strings from the end using negative indices:: |
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124 |
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125 a[-1] |
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126 |
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127 gives us the last element of the string and |
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128 :: |
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129 |
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130 a[-2] |
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131 |
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132 gives us second element from the end of the string |
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133 |
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134 Following is an exercise that you must do. |
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135 |
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136 %% %% Given a string, ``s = "Hello World"``, what is the output of:: |
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137 |
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138 s[-5] |
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139 s[-10] |
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140 s[-15] |
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141 |
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142 Please, pause the video here. Do the exercise and then continue. |
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143 |
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144 :: |
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145 |
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146 s[-5] |
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147 |
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148 gives us 'W' |
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149 :: |
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150 |
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151 s[-10] |
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152 |
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153 gives us 'e' and |
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154 :: |
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155 |
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156 s[-15] |
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157 |
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158 gives us an ``IndexError``, as should be expected, since the string |
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159 given to us is only 11 characters long. |
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160 |
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161 Let us attempt to change one of the characters in a string:: |
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162 |
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163 a = 'hello' |
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164 a[0] = 'H' |
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165 |
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166 As said earlier, strings are immutable. We cannot manipulate a |
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167 string. Although there are some methods which let us manipulate |
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168 strings, we will look at them in the advanced session on strings. In |
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169 addition to the methods that let us manipulate the strings we have |
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170 methods like split which lets us break the string on the specified |
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171 separator, the join method which lets us combine the list of strings |
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172 into a single string based on the specified separator. |
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173 |
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174 {{{ Show summary slide }}} |
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175 |
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176 This brings us to the end of another session. In this tutorial session |
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177 we learnt |
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178 |
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179 * How to define strings |
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180 * Different ways of defining a string |
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181 * String concatenation and repetition |
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182 * Accessing individual elements of the string |
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183 * Immutability of strings |
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184 |
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185 {{{ Show the "sponsored by FOSSEE" slide }}} |
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186 |
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187 This tutorial was created as a part of FOSSEE project, NME ICT, MHRD India |
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188 |
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189 Hope you have enjoyed and found it useful. |
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190 Thank you! |
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191 |