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Udacity while loop question - liquidmetalrob - Jul-21-2017
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
RE: Udacity while loop question - Larz60+ - Jul-21-2017
the following line is not valid code
b = text[x]what is this supposed to do?
RE: Udacity while loop question - liquidmetalrob - Jul-21-2017
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 bThe result that is printed is 'c'.
RE: Udacity while loop question - buran - Jul-21-2017
(Jul-21-2017, 01:00 PM)Larz60+ Wrote: 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' >>>
RE: Udacity while loop question - ichabod801 - Jul-21-2017
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.
RE: Udacity while loop question - sparkz_alot - Jul-21-2017
And stop using single letter variables.
RE: Udacity while loop question - nilamo - Jul-21-2017
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.