42 * Structure of lists |
43 * Structure of lists |
43 * Access list elements |
44 * Access list elements |
44 * Append elements to lists |
45 * Append elements to lists |
45 * Delete elements from lists |
46 * Delete elements from lists |
46 |
47 |
47 List is a compound data type, it can contain data of other data |
48 List is a compound data type, it can contain data of mutually |
48 types. List is also a sequence data type, all the elements are in |
49 different datatypes. List is also a sequence data type, all the |
49 order and the order has a meaning. |
50 elements are arranged in a given order. |
50 |
51 |
51 .. #[[Anoop: "all the elements are in order and **there** order has a |
52 .. #[[Anoop: "all the elements are in order and **there** order has a |
52 meaning." - I guess something is wrong here, I am not able to |
53 meaning." - I guess something is wrong here, I am not able to |
53 follow this.]] |
54 follow this.]] |
54 |
55 |
67 Lets now see how to define a non-empty list. We do it as,:: |
68 Lets now see how to define a non-empty list. We do it as,:: |
68 |
69 |
69 nonempty = ['spam', 'eggs', 100, 1.234] |
70 nonempty = ['spam', 'eggs', 100, 1.234] |
70 |
71 |
71 Thus the simplest way of creating a list is typing out a sequence |
72 Thus the simplest way of creating a list is typing out a sequence |
72 of comma-separated values (items) between square brackets. |
73 of comma-separated values (or items) between two square brackets. |
73 All the list items need not be of the same data type. |
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74 |
74 |
75 As we can see lists can contain different kinds of data. In the |
75 As we can see lists can contain different kinds of data. In the |
76 previous example 'spam' and 'eggs' are strings and 100 and 1.234 are |
76 previous example 'spam' and 'eggs' are strings whereas 100 and 1.234 are |
77 integer and float. Thus we can put elements of heterogenous types in |
77 integer and float respectively. Thus we can put elements of different types in |
78 lists including list itself. |
78 lists including lists itself. This property makes lists heterogeneous |
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79 data structures. |
79 |
80 |
80 .. #[[Anoop: the sentence "Thus list themselves can be one of the |
81 .. #[[Anoop: the sentence "Thus list themselves can be one of the |
81 element types possible in lists" is not clear, rephrase it.]] |
82 element types possible in lists" is not clear, rephrase it.]] |
82 |
83 |
83 Example :: |
84 Example :: |
84 |
85 |
85 listinlist=[[4,2,3,4],'and', 1, 2, 3, 4] |
86 listinlist=[[4,2,3,4],'and', 1, 2, 3, 4] |
86 |
87 |
87 We access list elements using the index. The index begins from 0. So |
88 We access an element of a list using its corresponding index. Index of |
88 for list nonempty, nonempty[0] gives the first element, nonempty[1] |
89 the first element of a list is 0. So for the list nonempty, nonempty[0] |
89 the second element and so on and nonempty[3] the last element. :: |
90 gives the first element, nonempty[1] the second element and so on and |
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91 nonempty[3] the last element. :: |
90 |
92 |
91 nonempty[0] |
93 nonempty[0] |
92 nonempty[1] |
94 nonempty[1] |
93 nonempty[3] |
95 nonempty[3] |
94 |
96 |
110 nonempty[-1] |
112 nonempty[-1] |
111 nonempty[-2] |
113 nonempty[-2] |
112 nonempty[-4] |
114 nonempty[-4] |
113 |
115 |
114 -1 gives the last element which is the 4th element , -2 second to last |
116 -1 gives the last element which is the 4th element , -2 second to last |
115 and -4 gives the fourth from last element which is first element. |
117 and -4 gives the fourth from the last which, in this case, is the first element. |
116 |
118 |
117 We can append elements to the end of a list using append command. :: |
119 We can append elements to the end of a list using the method append. :: |
118 |
120 |
119 nonempty.append('onemore') |
121 nonempty.append('onemore') |
120 nonempty |
122 nonempty |
121 nonempty.append(6) |
123 nonempty.append(6) |
122 nonempty |
124 nonempty |
132 Please, pause the video here. Do the exercise and then continue. |
134 Please, pause the video here. Do the exercise and then continue. |
133 |
135 |
134 The solution is on your screen |
136 The solution is on your screen |
135 |
137 |
136 |
138 |
137 As we can see non empty appends 'onemore' and 6 at the end. |
139 As we can see nonempty is appended with 'onemore' and 6 at the end. |
138 |
140 |
139 Using len function we can check the number of elements in the list |
141 Using len function we can check the number of elements in the list |
140 nonempty. In this case it 6 :: |
142 nonempty. In this case it is 6 :: |
141 |
143 |
142 len(nonempty) |
144 len(nonempty) |
143 |
145 |
144 |
146 |
145 |
147 |
151 |
153 |
152 |
154 |
153 deletes the element at index 1, i.e the second element of the |
155 deletes the element at index 1, i.e the second element of the |
154 list, 'eggs'. The other way is removing element by content. Lets say |
156 list, 'eggs'. The other way is removing element by content. Lets say |
155 one wishes to delete 100 from nonempty list the syntax of the command |
157 one wishes to delete 100 from nonempty list the syntax of the command |
156 should be |
158 would be |
157 |
159 |
158 .. #[[Anoop: let x = [1,2,1,3] |
160 .. #[[Anoop: let x = [1,2,1,3] |
159 now x.remove(x[2]) |
161 now x.remove(x[2]) |
160 still x is [2,1,3] so that is not the way to remove |
162 still x is [2,1,3] so that is not the way to remove |
161 element by index, it removed first occurrence of 1(by |
163 element by index, it removed first occurrence of 1(by |
173 nonempty |
175 nonempty |
174 nonempty.remove('spam') |
176 nonempty.remove('spam') |
175 nonempty |
177 nonempty |
176 |
178 |
177 If we check now we will see that the first occurence 'spam' is removed |
179 If we check now we will see that the first occurence 'spam' is removed |
178 thus remove removes the first occurence of the element in the sequence |
180 and therefore `remove` removes the first occurence of the element in the sequence |
179 and leaves others untouched. |
181 and leaves others untouched. |
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182 |
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183 One should remember this that while del removes by index number, |
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184 `remove` removes on the basis of content being passed on. For instance |
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185 if :: |
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186 |
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187 k = [1,2,1,3] |
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188 del([k[2]) |
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189 |
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190 gives us [1,2,3]. :: |
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191 |
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192 k.remove(x[2]) |
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193 |
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194 will give us [2,1,3]. Since it deletes the first occurence of what is |
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195 returned by x[2] which is 1. |
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196 |
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197 |
180 |
198 |
181 |
199 |
182 |
200 |
183 |
201 |
184 |
202 |