Question

#i couldnt find the difference in the code 
    >>> def match_ends(words):
     # +++your code here+++
     count=0
     for string in words:
      if len(string)>=2 and string[0]==string[-1]:
       count=count+1
     return count

>>> match_ends(['', 'x', 'xy', 'xyx', 'xx'])
2
>>> 
>>> def match_ends(words):
    # +++your code here+++
     count=0
     for string in words:
      if string[0]==string[-1] and len(string)>=2:
       count=count+1
     return count

>>> match_ends(['', 'x', 'xy', 'xyx', 'xx'])

   Traceback (most recent call last):
   File "<pyshell#26>", line 1, in <module>
   match_ends(['', 'x', 'xy', 'xyx', 'xx'])
   File "<pyshell#25>", line 5, in match_ends
   if string[0]==string[-1] and len(string)>=2:
   IndexError: string index out of range

I couldn’t find the difference in the code except the if condition if len(string)>=2 and string[0]==string[-1]: in the first function and if string[0]==string[-1] and len(string)>=2: in the second function

Answer 1

In the first, you first check if there are enough characters to test against, in the second you don’t:

if len(string)>=2 and string[0]==string[-1]:

and

if string[0]==string[-1] and len(string)>=2:

and pass in an empty string:

match_ends(['', 'x', 'xy', 'xyx', 'xx'])

The empty string has length 0, and no character at index 0:

>>> len('')
0
>>> ''[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

The if boolean expression is being evaluated left-to-right, and the string[0]==string[-1] expression is evaluated before the len(string)>=2 test, and then fails for that empty string.

In the other version, the len(string)>=2 part is evaluated first, found to be False for the empty string (0 is not greater or equal to 2) and Python then doesn’t need to look at the other half of the and expression at all, since there is no chance the and expression will become True whatever the second half evaluates to.

See Boolean expressions in the python documentation:

The expression x and y first evaluates x; if x is false, its value is returned; otherwise, y is evaluated and the resulting value is returned.