Page 289 - Computer Science Class 11 Without Functions
P. 289
The strings 'red', 'green', 'blue', 'pink', 'orange', 'white', and 'black' are stored at index
0, 1, 2, 3, 4, 5, and 6 respectively. The indexes 0, 1, 2, 3, 4, 5, 6, correspond to the negative indexes -7, -6,
-5, -4, -3, -2, and -1, respectively. To access an element in a list, the list object is followed by an opening square bracket,
the index to be accessed, and a closing square bracket. For example,
>>> colors[3]
'pink'
>>> colors[-2]
'white'
Unlike strings, lists are mutable. So, we can replace an item in a list with another item. For example,
An empty list can be defined using square brackets [ ] or by using the list() constructor with no arguments. Here
are both methods:
Method 1: Using square brackets []
emptyList = []
Method 2: Using the list() constructor with no arguments:
emptyList = list()
12.1.1 List Derived from a String
Given a string as an argument, the list() function returns a list. For example,
>>> vowels = 'aeiou'
>>> list(vowels)
['a', 'e', 'i', 'o', 'u']
12.1.2 Length of a List: len()
We have already used the len() function to find the length of a string. We can also apply the len() function to find
the number of elements in a list. For example,
>>> len(colors)
7
12.1.3 Slicing
In the last chapter, we used slicing for accessing a subsequence of elements in a string. In this chapter, we will use
slicing to access the subsequences of lists and tuples. Recall the following syntax for specifying a slice:
<start>:<finish> [:<step>]
Note that the step is optional. Below we give some examples of slicing,
>>> colors[3:6]
['yellow', 'orange', 'white']
The slice [3:6] yields a list comprising the elements colors[3], colors[4], and colors[5], but not
colors[6].
The slice [:3] yields the list comprising the list elements up to index 3 (excluding the element at index 3, For
example,
>>> colors][:3]
['red', 'green', 'blue']
Note that colors[-1] and colors[6] refer to the same position in list colors. So, the slice [1, -1] yields the
list comprising the list elements colors[1], colors[2], …, colors[5] excluding colors[6].
>>> colors][1:–1]
['green', 'blue', 'yellow', 'orange', 'white']
The slice [::2] yields a list, comprising the alternate elements of the list, because the step size is 2:
>>> colors[::2]
['red', 'blue', 'orange', 'black']
Lists and Tuples 287

