C Language: Program to check given character string for palindrome

A palindrome is a word, phrase, number or other sequence of units that can be read the same way in either direction. Eg: Katak, 121 etc

Following C program shows the simple way check, if the given character string is a palindrome or not.

Following code is available under GNU General Public License v3 or above.

/*
 * Copyright (c) 2009 Raghavendra Nayak
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

/*
 * C program to reverse given string, and to check whether it is a palindrome.
 */

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

/**
 * @brief function to return the given string length.
 * @param[in] string the input string.
 * @return the string length.
 */
unsigned int get_string_length(const char *string)
{
    int i;
    int string_length = 0;

    for (i = 0; string[i] != 0 && string[i] != '\n'; ++i)
        ++string_length;
    return string_length;
}

/**
 * @brief function to reverse the given string.
 * @param[in] string the input string.
 * @param[out] rev_string the reversed string would be placed in this variable.
 * @return none.
 */
void reverse(const char *in_string, char *rev_string)
{
    int i, j;
    unsigned int length;

    length = get_string_length(in_string);

    for (i = length-1, j = 0; i >= 0; --i, ++j)
        rev_string[j] = in_string[i];

    /* mark the end of string */
    rev_string[j] = 0;
}

int main()
{
    const int max_size = 100;
    int string_length = 0;

    char string[max_size];
    char reverse_string[max_size];

    printf("\nPlease enter input string: ");
    /* Note: we are not using gets, because gets is deprecated and can be
       dangerous. */
    fgets(string, max_size, stdin);

    /* remove any new line from input string,
       if fgets has put '\n' at the end of string */
    string_length = get_string_length(string);
    string[string_length] = 0;

    /* get the reversed string */
    reverse(string, reverse_string);

    /* now compare */
    if (!strcmp(string, reverse_string)) {
        printf("\nEntered string = %s, reversed string = %s."
               "\n%s = %s, hence %s is a palindrome.\n",
               string, reverse_string, string, reverse_string, string);
    } else {
        printf("\nEntered string = %s, reversed string = %s."
               "\n%s != %s, hence %s is not a palindrome.\n",
               string, reverse_string, string, reverse_string, string);
    }
    return 0;
}

Links:

• Wikipedia article on palindrome
• Browse this source code at Google Code