Python palindrome program not working

Question

I\'ve written a simple program in python which checks if the sentence is palindrome. But I can\'t figure out why isn\'t it working. The results is always False. Does anyone know

Answer 1

Since the mistake is already explained and the obvious s == s[::-1] is already taken, I'll just throw a possibly minimal version of the original into the mix:

def isPalindrome(s):
    s = s.strip().lower()
    return not s or s[0] == s[-1] and isPalindrome(s[1:-1])

Note that you don't need replace(" ", ""). Space on the outside is removed with strip() now, and spaces on the inside will be removed by the strip() later, in a call deeper into the recursion (if we don't stop earlier because a s[0] == s[-1] fails).

Answer 2

You aren't returning the result of the function.

replace :

if word[0] == word[-1]:
    isPalindrome(word[1:-1])

with

if word[0] == word[-1]:
    return isPalindrome(word[1:-1])

Answer 3

You're making this way more complicated than it has to be:

def palindrome(sentence):
    sentence = sentence.strip().lower().replace(" ", "")
    return sentence == sentence[::-1]

sentence[::-1] uses string slicing to reverse the characters in the string.

A slightly more verbose solution that shows how the logic of the return statement above works:

def palindrome(sentence):
    sentence = sentence.strip().lower().replace(" ", "")
    if sentence == sentence[::-1]:
        return True
    else:
        return False

Answer 4

You need to replace "input" with "raw_input". Also, you are calling isPalindrome recursively and there is a mistake here as well. It should be:

if word[0] == word[-1]:
    return isPalindrome(word[1:-1])
else:
    return False

Check the corrected code below:

def isPalindrome(word):
    # Removes all spaces, and lowercase the word.
    word = word.strip().lower()
    word = word.replace(" ", "")

    # If the length of the word is less than 1, means its a palindrome
    if (len(word) <= 1):
        return True

# Compares the first and the last character of the word.
# If it is the same, calls the function again with the same word, without its first and last characters.
# If its not the same, its not palindrome
    if word[0] == word[-1]:
        return isPalindrome(word[1:-1])
    else:
        return False


sentence = raw_input("Enter a sentence: \n")

if (isPalindrome(sentence)):
    print("The sentence %s is palindrome." % sentence)
else:
    print("The sentence %s is NOT palindrome" % sentence)

Answer 5

I presume this is an assignment and the recursion is necessary, obviously return word == word[::-1] is simpler but is not really relevant. You can write your recursive function a bit more succinctly:

def isPalindrome(word):
    if not word:
        return True
    return word[0] == word[-1] and isPalindrome(word[1:-1])

word[0] == word[-1] will either be True or False so you will either reach an empty string where not word will True so the recursion ends and the function returns True or word[0] == word[-1] will be False so the function will return False as and isPalindrome(word[1:-1]) will never be evaluated.

I would also maybe do the lowering outside of the function:

def isPalindrome(word):
    if not word:
        return True
    return word[0] == word[-1] and isPalindrome(word[1:-1])


sentence = input("Enter a sentence: \n")
sentence = sentence.strip().lower()
sentence = sentence.replace(" ", "")
if isPalindrome(sentence):
    print("The sentence %s is palindrome." % sentence)
else:
    print("The sentence %s is NOT palindrome" % sentence)

Answer 6

You algorithm is fine, the only problem is that you don't return the true result through the recursion, you have to return the isPalindrome result when calling it recursively:

else:
    if word[0] == word[-1]:
        return isPalindrome(word[1:-1]) #this changed
    else:
        return False