Python | Get the starting index for all occurrences of given substring

Given a string and a substring, the task to find out the starting index for all the occurrences of a given substring in a string. Let’s discuss a few methods to solve the given task.

Method #1: Using Naive Method

# Python3 code to demonstrate 
# to find all occurrences of substring in 
# a string 
  
# Initialising string 
ini_string = 'xbzefdgstbzefzexezef'
  
# Initialising sub-string 
sub_string = 'zef'
  
# Printing initial string and sub-string 
print ("initial_strings : ", ini_string, "\nsubstring : ", sub_string) 
  
res = [] 
flag = 0
k = 0
  
# Finding all occurrences of substring 
# in a string using Naive method 
for i in range(0, len(ini_string)): 
    k = i 
    flag = 0
    for j in range(0, len(sub_string)): 
        if ini_string[k] != sub_string[j]: 
            flag = 1
        if flag: 
            break
        k = k + 1
    if flag == 0: 
        res.append(i) 
  
  
# printing result( 
print ("resultant positions", str(res)) 

Output:

initial_strings :  xbzefdgstbzefzexezef 
substring :  zef
resultant positions [2, 10, 17]

&nsbp;
Method #2: Using list comprehension

# Python3 code to demonstrate 
# to find all occurrences of substring in 
# a string 
  
# Initialising string 
ini_string = 'xbzefdgstbzefzexezef'
  
# Initialising sub-string 
sub_string = 'zef'
  
# Printing initial string and sub-string 
print ("initial_strings : ", ini_string, "\nsubstring : ", sub_string) 
  
res = [] 
# Finding all occurrences of substring 
# in a string using list comprehension 
res = [i for i in range(len(ini_string))  
       if ini_string.startswith(sub_string, i)] 
  
# printing result( 
print ("resultant positions", str(res)) 

Output:

initial_strings :  xbzefdgstbzefzexezef 
substring :  zef
resultant positions [2, 10, 17]

Method #3: Using regex

# Python3 code to demonstrate 
# to find all occurrences of substring in 
# a string 
import re 
  
# Initialising string 
ini_string = 'xbzefdgstbzefzexezef'
  
# Initialising sub-string 
sub_string = 'zef'
  
# Printing initial string and sub-string 
print ("initial_strings : ", ini_string, "\nsubstring : ", sub_string) 
  
res = [] 
# Finding all occurrences of substring 
# in a string using re.finditer 
res = [m.start() for m in re.finditer(sub_string, ini_string)] 
  
# printing result( 
print ("resultant positions", str(res)) 

Output:

initial_strings :  xbzefdgstbzefzexezef 
substring :  zef
resultant positions [2, 10, 17]

My Personal Notes

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