C recursive function palindrome

Question

I'm trying to do a recursive function with C to check if a word is a palindrome or not (read in both ways).

It's the first time I use this kind of function, but I have a problem, I don't know why it doesn't work, if you could help me, there's my code, thanks :

#include <stdio.h> 
#include <string.h>

int palindrome(char c[100],int i, int j)
{
    if (j == i)
    {
        return 1;
    }

    // Si le premier et le dernier caractère
    // sont les mêmes alors, on peut commencer les tests
    if(c[i] == c[j])
    {
        // On fais les tests pour chaque caractère de la chaine
        return palindrome(c, i++, j--);
    } else {
        return 0;
    }

    return 0;
}

int main(void)
{
    char chaine[100] = "radar";
    int pal;

    pal = palindrome(chaine, 0, strlen(chaine)); // Returns : 0 -> False / 1 -> True
    printf("%d", pal);
    return 0;
}

Answer 1

The problem is your passing j value as strlen(chaine) instead of strlen(chaine) - 1.

#include <stdio.h>
#include <string.h>

int palindrome(char c[100],int i, int j) {
    if (j == i)
        return 1;
    else if(c[i] == c[j])
        return palindrome(c, ++i, --j);
    else
        return 0;
}

int main() {
    char chaine[100] = "radar";
    int pal;
    pal = palindrome(chaine, 0, strlen(chaine) -1 ); // Returns : 0 -> False / 1 -> True
    printf("%d", pal);
    return 0;
}

Answer 2

If you can achieve what you are looking for without using recursion do the following. Read the word into a list on and then reverse the word into another list and if they match you have a palindrome.

Answer 3

Modified code :
1) If first index is 0 , then last index should be strlen(chaine) - 1.
2) You were passing same values of i and j in the recursive call , you need to use pre-increment and post-increment operators.

#include <stdio.h> 
#include <string.h>

int palindrome(char c[100],int i, int j)
{

    if (j == i)
    {
        return 1;
    }

    if(c[i] == c[j]) // Si le premier et le dernier caractère sont les mêmes alors, on peut commencer les tests
    {

        // On fais les tests pour chaque caractère de la chaine
        // increment and decrement first 
        // You were passing same values every time (0 and 4)
        return palindrome(c, ++i, --j);

    }
    else
    {
        return 0;
    }

    return 0;

}

int main()
{

    char chaine[100] = "radar";
    int pal;
    // Last index should be strlen(chaine) - 1
    pal = palindrome(chaine, 0, strlen(chaine)-1); // Returns : 0 -> False / 1 -> True
    printf("%d", pal);

    return 0;

}

Answer 4

You have several problems with your recursion. First, i++ and j-- pass the values of i and j with the post increment/decrement applied as a side effect after the next call to palindrome has already been made. Instead you need:

        return palindrome (c, i + 1, j - 1);

Next, what happens if chaine has an even number of characters? i never equals j . You need to update your test to break recursion to:

    if (j <= i)
        return 1;

Lastly, you are attempting to read the nul-terminating character when you call palindrome in main() using strlen(chaine) , instead you need:

    pal = palindrome (chaine, 0, strlen (chaine) - 1);

Putting that together, you could do something like:

#include <stdio.h> 
#include <string.h>

int palindrome (const char *c, int i, int j)
{
    if (j <= i)
        return 1;

    if (c[i] == c[j])
        return palindrome (c, i + 1, j - 1);

    return 0;
}

int main (int argc, char **argv) {

    char *chaine = argc > 1 ? argv[1] : "radar";
    int pal;

    pal = palindrome (chaine, 0, strlen (chaine) - 1);

    printf ("chaine : '%s' %s a palindrome\n", chaine, pal ? "is" : "is not");

    return 0;
}

Example Use/Output

$ ./bin/recursive_palindrome
chaine : 'radar' is a palindrome

$ ./bin/recursive_palindrome amanaplanacanalpanama
chaine : 'amanaplanacanalpanama' is a palindrome

$ ./bin/recursive_palindrome catfish
chaine : 'catfish' is not a palindrome

(for the curious, it is "a man a plan a canal panama" )