Page 260 - Computer Science Class 11 Without Functions
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greet
Non-negative 0 1 2 3 4
indices
h e l l o
Negative –5 –4 –3 –2 –1
indices
Fig 11.1: String greet indexing
11.1.2 Strings are Immutable!
Strings in Python are immutable, i.e., a string cannot be modified. An attempt to modify a string will yield an
error. For instance, an attempt to replace the character at index 1 ('a') with 'o' in the string 'Kamal' results in
an error:
>>> name = 'Kamal'
>>> name[1] = 'o'
Traceback (most recent call last):
File "<pyshell#55>", line 1, in <module>
name[1] = 'o'
TypeError: 'str' object does not support item assignment
Strings in Python are immutable, i.e., a string cannot be modified. An attempt to modify a string will yield an error.
However, it is perfectly fine to create a new string and assign it to an existing variable. For example,
>>> name = 'Kamal'
>>> name
'Kamal'
>>> name = 'Komal'
>>> name
'Komal'
11.1.3 Invalid Indices
An index outside the valid range of indices is said to be out of range. An attempt to access a component of a string
using an index outside of the valid range of indices will result in an error. For instance, the set of valid indices for data
stored in greet ('hello') are 0, 1, 2, 3, and 4; and -5, -4, -3, -2 and -1. Thus, accessing index 7,
which is outside these ranges, will yield an error as shown below:
>>> greet[7]
Traceback (most recent call last):
File "<pyshell#7>", line 1, in <module>
greet[7]
IndexError: string index out of range
Set of valid indexes for accessing string characters include [0, len(string)-1] and [-len(string), -1]. An attempt to
access a component of a string using an index outside of the valid range of indices will result in an error.
11.1.4 Length of a String
To find the length of a string, Python provides the built-in function len() which returns the length of a string. For
example,
258 Touchpad Computer Science-XI

