palindrome.c

/*
 * Copyright (C) 2020 Raghavendra Nayak
 *
 * Permission to use, copy, modify, and/or distribute this software for any purpose
 * with or without fee is hereby granted.

 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND
 * FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS
 * OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER
 * TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF
 * THIS 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;
}