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 to 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