Python String index() Method

Finding the string(substring) in a string is an application that has many uses in day-to-day life. Python offers this using a function index(), which returns the position of the first occurrence of a substring in a string. 

Syntax: 

ch.index(ch1, begp, endp)

Parameters: 

• ch1 : The string to be searched for.
• begp (default : 0) : This function specifies the position from where search has to be started. 
• endp (default : string_len) : This function specifies the position from where search has to end.

Return Value: 

Returns the first position of the substring found.

Exception: 

Raises ValueError if argument string is not found. 

Example 1

Python

# Python code to demonstrate the working of
# index()

# initializing target string
ch = "geeksforgeeks"

# initializing argument string
ch1 = "geeks"

# using index() to find position of "geeks"
# starting from 2nd index
# prints 8
pos = ch.index(ch1,2)

print ("The first position of geeks after 2nd index : ",end="")
print (pos)

Output:

The first position of geeks after 2nd index : 8

Note: The index() method is similar to find(). The only difference is find() returns -1 if the searched string is not found and index() throws an exception in this case.

Example 2: Exception

ValueError: This error is raised in the case when the argument string is not found in the target string.

Python

# Python code to demonstrate the exception of
# index()

# initializing target string
ch = "geeksforgeeks"

# initializing argument string
ch1 = "gfg"

# using index() to find position of "gfg"
# raises error
pos = ch.index(ch1)

print ("The first position of gfg is : ",end="")
print (pos)

Output:

Traceback (most recent call last):
  File "/home/aa5904420c1c3aa072ede56ead7e26ab.py", line 12, in 
    pos = ch.index(ch1)
ValueError: substring not found

Example 3

Practical Application: This function is used to extract the suffix or prefix length after or before the target word. The example below displays the total bit length of instruction coming from AC voltage given information in a string.

Python

# Python code to demonstrate the application of
# index()

# initializing target strings
voltages = ["001101 AC", "0011100 DC", "0011100 AC", "001 DC"]

# initializing argument string
type = "AC"

# initializing bit-length calculator
sum_bits = 0

for i in voltages :

    ch = i

    if (ch[len(ch)-2]!='D'):
       # extracts the length of bits in string
       bit_len = ch.index(type)-1

       # adds to total
       sum_bits = sum_bits + bit_len

print ("The total bit length of AC is : ",end="")
print (sum_bits)

Output: 

The total bit length of AC is : 13