为什么我的代码不适用于所有回文？

Question

I've been writing some code to take a string and check it to see if it is a palindrome or not. It seems to work on most things except for "axya" or any other non palindrome that starts and ends with the same letters. What am I doing wrong?

Code is written in python.

str = str.replace(' ','')
str = list(str)
var_a = 0
var_b = 0
var_c = 0

while var_a < len(str) - 1:
  var_b = str[var_a]
  if len(str) - var_a == len(str) - 1:
    var_c = str[len(str) -2]
  else:
    if len(str) - var_a < len(str) - 1:
      var_a = var_a + 1
      var_c = str[len(str) - var_a]
    else:
      var_c = str[len(str) - 1]\
  if var_b == var_c:
    return 'true'
  else:
    return 'false'
  var_a += 1

Answer 1

Note this part:

while var_a < len(str) - 1:
  # ...
  if var_b == var_c:
    return 'true'
  else:
    return 'false'

The function returns no matter var_b equals or not equals to var_c , so the loop only iterates once, that's why it only tests the start and end characters. You should keep looping to test the rest characters if they are the same.

Answer 2

While there certainly could be a way to do this with loops, there is no need for this:

>>> a = "racecar"
>>> a == a[::-1]
True
>>> b = "axya"
>>> b == b[::-1]
False
>>> 

The [] is an excellent operator for strings and lists. string[::-1] will return the string reversed. Check if that's the same as the string forwards, and you've got a palindrome!

To actually answer your question though :P

You're returning 'false' before checking all your letters.

  if var_b == var_c:
    return 'true'
  else:
    return 'false'

should be more like:

# if these letters are different, the word is not a palindrome
if var_b != var_c:
  return 'false'
else:
  pass # keep looping

your program should not return true until the loop has terminated from var_a reaching the end, without finding any differences along the way.