ppal.c

Jonas Altrock ew20b126@technikum-wien.at

To overview.

The whole source file ppal.c.

/* compile with `clang -std=c99 -Wall -o ppal ppal.c` */

Include C-libraries for input/output and allocation of memory.

#include <stdio.h>
#include <stdlib.h>

Task

Read a string and find out whether it is a palindrome. Use pointer arithmetics.

Solution

Pre-processor macro that defines the length of our input buffer. The longest we will read in is 80 characters, plus 1 for null-termination.

#define MAX_CHARS_WITH_NUL 81

main() takes no arguments for this program.

int main() {

calloc initializes the memory to all 0s, so we always have a null-terminated string.

    char *buffer = calloc(MAX_CHARS_WITH_NUL, sizeof(char));
    char c;
    int length = 0;

Read in one character at a time, until newline. This is written a bit complicated, because we use the assignment to c as an expression here by wrapping it in parentheses.

Usually it is not good style to have a side-effect like this (side-effect meaning the program state changes) in a context where we are asking a question and want a true or false answer. It can cause bugs due to being sensitive to the order in which the parts of the condition are evaluated.

    while ((c = getchar()) != '\n' && length < MAX_CHARS_WITH_NUL) {

Ignore everything that is not a-z.

        if ((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')) {

If it is an uppercase character, transform it to lowercase.

            if ((c >= 'A' && c <= 'Z')) {

Lowercase a-z are exactly 32 positions above the uppercase variants, see man ascii.

                c += 32;
            }

            buffer[length] = c;
            length += 1;
        }
    }

Now to check whether the input string is a palindrome. By default we assume it is a palindrome, until we find a contradiction.

    int is_palindrome = 1;

Empty string and single char are palindromes. For everything longer than 1 char we use two pointers, from the start and end of the string, and move them towards each other until they meet.

    if (length > 1) {
        char *left, *right;
        left = buffer;
        right = buffer + (length - 1);

Iterate until pointers meet or go past each other.

        while (left < right) {

If the characters at the mirrored positions are unequal, we know we do not have a palindrome.

            if (*left != *right) {
                is_palindrome = 0;
            }

            left += 1;
            right -= 1;
        }
    }

Print the result.

    if (is_palindrome) {
        printf("palindrome\n");
    } else {
        printf("not a palindrome\n");
    }

Don’t forget to free the allocated memory.

    free(buffer);
    return 0;
}