How to count the number of sub-sequences in a string?

Question

This program is to count the number of sub-strings in a string.

Test case:
string ABCDCDC
pattern CDC

The answer should be 2 but I am getting 0 .

def count_substring(string, sub_string):
  for i in range(0,len(string)-len(sub_string)+1):
    count=0
    for j in range(0,len(sub_string)):
       if(string[i+j]!=sub_string[j]):
           break;
    if j==len(sub_string):
       count=count+1
    return count

Answer 1

At the end of the for j in range(0,len(sub_string)) loop j can be at most len(sub_string)-1 (if you don't break early). So if j==len(sub_string): is always False . You want if j==len(sub_string)-1: or simply else: (meaning you've reached the end of the for loop) instead.

Also count = 0 should be initialized at the start, not in the first for loop:

def count_substring(string, sub_string):
    count=0
    for i in range(0,len(string)-len(sub_string)+1):
        for j in range(0,len(sub_string)):
            if(string[i+j]!=sub_string[j]):
                break;
        else:
            count=count+1
    return count

Answer 2

Strings are sequences of 1-character substrings in Python, so you can use the built-in count() method — in other words, you don't have to re-invent the wheel and write your own function to do it.

test_string = 'ABCDCDC'
sub_string = 'CDC'

print(test_string.count(sub_string))  # -> 2