python - Number of occurrences of a substring in a string

Question

I need to count the nunber of times the substring 'bob' occurs in a string.

Example problem: Find the number of times 'bob' occurs in string s such that

"s = xyzbobxyzbobxyzbob"  #(here there are three occurrences)

Here is my code:

s = "xyzbobxyzbobxyzbob"

numBobs = 0

while(s.find('bob') >= 0)
   numBobs = numBobs + 1
   print numBobs

Since the find function in Python is supposed to return -1 if a substring is unfound the while loop ought to end after printing out the incremented number of bobs each time it finds the substring.

However the program turns out to be an infinite loop when I run it.

Answer 1

For this job, str.find isn't very efficient. Instead, str.count should be what you use:

>>> s = 'xyzbobxyzbobxyzbob'
>>> s.count('bob')
3
>>> s.count('xy')
3
>>> s.count('bobxyz')
2
>>>

---

Or, if you want to get overlapping occurrences, you can use Regex:

>>> from re import findall
>>> s = 'bobobob'
>>> len(findall('(?=bob)', s))
3
>>> s = "bobob"
>>> len(findall('(?=bob)', s))
2
>>>