Question

I am new to python and I am trying to implement rfind function using find.

def rfind_imp(s,t):
    ss = s
    fpos = 0
    while(True):             
         fpos = s.find(t,fpos)
         ss=ss[fpos:]

         if(ss):    
           fpos = fpos +1    
         else:
           return fpos

print rfind_imp("I saw a donkey,I saw a saw "," ")
print "I saw a donkey,I saw a saw ".rfind(" ")

But the issue I am facing is I am getting 16th charcter as rfind value. Can some one help me find where I am doing a mistake. Also any suggestions for better implementation of rfind using find should be helpful.

Answer 1

I think your algorithm should be implemented like this:

def rfind_imp(s, t):
    fpos = -1
    while True:
         pos = s.find(t, fpos + 1)
         if pos == -1:
            return fpos
         fpos = pos

Another algorithm that’s probably faster for strings with many ocurrences of t:
(Edit: made it work for strings with more than one character)

def rfind2(s, t):
    pos = s[::-1].find(t[::-1])
    if pos == -1:
        return pos
    return len(s) - pos - len(t)