Udacity while loop question

[liquidmetalrob]

Hi!
I'm doing the Intro to Computer Science course on Udacity and I'm trying to:

#Define a procedure, find_last, that takes as input
# two strings, a search string and a target string,
# and returns the last position in the search string
# where the target string appears, or -1 if there
# are no occurrences.
#
# Example: find_last('aaaa', 'a') returns 3

# Make sure your procedure has a return statement.

This is my attempt, but it won't work. Can anyone explain what I did wrong?

def find_last(text,s):
   x = -1
   b = text[x]
   while b != s:
       x = x-1
   return x

[Larz60+]

the following line is not valid code

b = text[x]

what is this supposed to do?

[liquidmetalrob]

b = text[x]

This is supposed to assign to b the letter of the string at position x, which is assigned to -1 at the top.
I've tested it and it works though.

text = 'abcabcabcabc'
x = -1
b = text[x]
print b

The result that is printed is 'c'.

[buran]

the following line is not valid code

b = text[x]

what is this supposed to do?

Actually it is :-)

>>> text = 'sometext'
>>> x=-1
>>> b=text[x]
>>> b
't'
>>>

[ichabod801]

The point is that when you change x, b is not changed. So you are always testing the last letter of the string (b). You want to test directly on text[x].

However, that is only going to work for single character search strings. If you have a two character search string, like find_last('aababcabcd', 'ab'), it will never match. You might want to look at the startswith method of strings.

[sparkz_alot]

And stop using single letter variables.

[nilamo]

Luckily, that's actually already built into python.

>>> find_last = lambda haystack, needle: haystack.rfind(needle)
>>> find_last("ababababspam", "ab")
6
>>> find_last("ababababspam", "cat")
-1
>>> help(str.rfind)
Help on method_descriptor:

rfind(...)
   S.rfind(sub[, start[, end]]) -> int

   Return the highest index in S where substring sub is found,
   such that sub is contained within S[start:end].  Optional
   arguments start and end are interpreted as in slice notation.

   Return -1 on failure.