String Indexing in Python

Strings are used for processing text data in Python. While processing strings, we often need to access a certain part of the string. In this article, we will see how we can extract parts of a string using indexing in Python. 

What is String Indexing ?

If we have an ordered sequence or container object such as a string, a list, or a tuple, we can access the elements of the objects using their relative position in the sequence. The relative position of the elements in the ordered sequence is termed as index. With Indexing, we can access any element from an ordered sequence using the indices. 

In python, string indexing is zero based. It means that we start the counting from 0 and the first character of the string is assigned the index 0, the second character is assigned the index 1, the third character is assigned the index 2 and so on. 

We can understand this using the following example.

Suppose that we have a string “PythonForBeginners”

Here, the index of the letter “P” is 0. The index of the letter “y” is 1. The index of letter ”t” is 2, The index of letter “h” is 3 and so on. The index of the last letter “s” is 17.

In python, we can use positive as well as negative numbers for string indexing. Let us discuss them one by one.

String Indexing using Positive Numbers

As we have seen above, Strings are indexed using positive numbers from 0 to string length -1. We can access a character at any position between 0 to (length of the string) -1 using positive indices as follows.

myString = "PythonForbeginners"
index = 0
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))
index = 1
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))
index = 2
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))
index = 3
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))
index = 17
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))

Output:

Character at index 0 in the string 'PythonForbeginners' is P.
Character at index 1 in the string 'PythonForbeginners' is y.
Character at index 2 in the string 'PythonForbeginners' is t.
Character at index 3 in the string 'PythonForbeginners' is h.
Character at index 17 in the string 'PythonForbeginners' is s.

Always remember that an index greater than or equal to the string length will cause an IndexError exception as follows.

myString = "PythonForbeginners"
index = 20
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))

Output:

Traceback (most recent call last):
  File "/home/aditya1117/PycharmProjects/pythonProject/string12.py", line 3, in <module>
    character = myString[index]
IndexError: string index out of range

You can either avoid the IndexError exception by checking the value of index before accessing any character in the string. Alternatively, you can use python try except block to handle the exceptions if they occur.

Here, I will suggest you to use the try except blocks. Checking the index every time we access a character may be redundant and costly if we are accessing using indices less than the length of the string. While using try except blocks, the program will not check the value of index each time we access a character from the string. If IndexError occurs, it will be handled by the code in the except block. 

Indexing using Negative Numbers

We can also use negative indices for accessing characters from a string. In python, the last character of the string is assigned an index -1. The second last character is assigned an index -2. Similarly, the first character of the string is assigned an index -(length of the string).

We can understand this using the following example.

Suppose that we have a string “PythonForBeginners”

Here, the index of the letter “s” is -1. The index of the letter “r” is -2. The index of letter ”n” is -3 , The index of letter “n” is -4 and so on. The index of the first letter “P” is -18.

You can verify this using the following program.

myString = "PythonForbeginners"
index = -1
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))
index = -2
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))
index = -3
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))
index = -4
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))
index = -18
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))

Output:

Character at index -1 in the string 'PythonForbeginners' is s.
Character at index -2 in the string 'PythonForbeginners' is r.
Character at index -3 in the string 'PythonForbeginners' is e.
Character at index -4 in the string 'PythonForbeginners' is n.
Character at index -18 in the string 'PythonForbeginners' is P.

While using negative numbers as indices, Make sure that you do not pass an index less than  -(length of the string). Otherwise, your program will run into an IndexError as follows.

myString = "PythonForbeginners"
index = -20
character = myString[index]
print("Character at index {} in the string '{}' is {}.".format(index, myString, character))

Output:

Traceback (most recent call last):
  File "/home/aditya1117/PycharmProjects/pythonProject/string12.py", line 3, in <module>
    character = myString[index]
IndexError: string index out of range

Conclusion

In this article, we have studied string indexing in python. We have seen how we can use negative as well as positive numbers to access characters from a string. To study more about strings in python, you can read this article on string concatenation.