Palindrome program in python

Question

P.S: I'm new to python (and programming in general), however, I've got this question with regards to checking if a word is a palindrome. Here's my code:

def checkPal(word):
    newWord = ""
    for j in range(len(word)-1, 0):
        newWord = newWord + word[j]
        print(newWord) #this doesn't print anything
    if(newWord==word):
        print("Palandrome!!")
    else:
        print("sorry")

So this doesn't throw any errors, however, it doesn't work too! I am aware there are other ways of doing it, like:

str(n) == str(n)[::-1]

This works like a charm. But I'd just like to know what's wrong with my code. I can't seem to find a solution. I've tried item assignments separately too:

Word = ""
toAdd = "LetsTryThis"
Word = Word + toAdd[0]
print(Word)

The output:

The output is as expected, but this doesn't seem to work in the for loop of my checkPal function. See comment.

Thanks for the help!

you can just reverse the string and compare with original string to check palindrome — iamdeowanshi, Mar 22 '17 at 16:17
and I am voting this because this doesn't satisfy stackoverflow standards — iamdeowanshi, Mar 22 '17 at 16:17
range(len(word) -1, 0) will give a range where the start is greater than the end. So the loop will no execute. You need to use a step of -1 as nbro suggested. — chatton, Mar 22 '17 at 16:19
wow. Works like a charm! I didn't see anything about passing 3 parameters in range().. I'll try reading a bit more about that. Thanks! — lancev.veigas3, Mar 22 '17 at 16:26
Good question here that would be worth a read - http://stackoverflow.com/questions/931092/reverse-a-string-in-python — Craicerjack, Mar 22 '17 at 16:30
for j in range(len(word)-1,-1,-1) worked well. however, I want to know why my code didn't work. according to what I've written the assignment should create a new string variable with the reverse of the earlier string. Later if the two strings are true, it would show true. What went wrong? — lancev.veigas3, Mar 22 '17 at 16:34

score 2 · Accepted Answer · 2017-03-24T10:00:10.253

If you want a range in reverse order (initial value greater than final value) you need to specify a negative step (-1 in your case)

range(5,0)       # doesn't work
[]

range(5,0,-1)    # works
[5, 4, 3, 2, 1]

But this is an inefficient way of reversing a string, use word[::-1] instead (will be way more efficient)

if word == word[::-1]:
    print('Palindrome')
else:
    print('Not a palindrome')

However if word is long, it will take a lot of effort to reverse it and then compare the whole word with the original one.

Instead you can just iterate once from 0 to len(word)/2 and compare the first element with the last one, the second with the penultimate, etc... Until one of them fail, otherwise it's a palindrome.

def is_palindrome(word):
    is_palindrome = True
    for i in range(len(word)/2):
        if word[i] != word[-i-1]:
            is_palindrome = False
            break

    # you can print if it's a palindrome:
    if is_palindrome:
        print('Palindrome')
    else:
        print('Not a palindrome')

    # or just return a boolean answer
    return is_palindrome

# test cases
is_palindrome('adda')    # even palindrome
is_palindrome('adfda')   # odd palindrome
is_palindrome('asda')    # even non-palindrome
is_palindrome('adfsa')   # odd non-palindrome

You also need to be careful about the endpoints, because the correct start and step for the reverse of a given range aren't just the original start and step switched around. `reversed(range(...))` is usually easier to work with. — user2357112, Mar 22 '17 at 16:23
"You are building a newWord step by step and then comparing it to the whole word (this will not work)" - that would actually work. I don't see why you think it wouldn't. Maybe you thought the comparison was happening at each step. — user2357112, Mar 22 '17 at 16:34
True, he was just reversing the word in the loop. Will correct my answer — , Mar 22 '17 at 16:35
Thanks for the help! Learnt quite a few things after raising the question. The alternate solution of comparing the first element with the last one, the second with the penultimate, etc was a great idea.. Thanks. Solved my problem. — lancev.veigas3, Mar 24 '17 at 03:13

Palindrome program in python

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