String Length, Compare and Concatenate Functions in C

Algorithm

1. Read two words with scanf, each stored in a char array.
2. my_length counts characters until the terminating '\0'.
3. my_compare walks both strings while the characters match and returns the difference at the first mismatch, or 0 if they are equal.
4. my_concat moves to the end of the destination and copies the source after it, including its terminator.
5. Print the lengths, the comparison result and the concatenated string.

C program

#include <stdio.h>

int my_length(const char *s) {
    int n = 0;

    while (s[n] != '\0') {
        n++;
    }
    return n;
}

int my_compare(const char *a, const char *b) {
    int i = 0;

    while (a[i] != '\0' && a[i] == b[i]) {
        i++;
    }
    return (unsigned char)a[i] - (unsigned char)b[i];
}

void my_concat(char *dest, const char *src) {
    int i = 0;
    int j = 0;

    while (dest[i] != '\0') {
        i++;
    }
    while (src[j] != '\0') {
        dest[i] = src[j];
        i++;
        j++;
    }
    dest[i] = '\0';
}

int main(void) {
    char first[50], second[50], joined[100];
    int cmp;

    printf("Enter first word:\n");
    scanf("%49s", first);
    printf("Enter second word:\n");
    scanf("%49s", second);

    printf("Length of \"%s\" = %d\n", first, my_length(first));
    printf("Length of \"%s\" = %d\n", second, my_length(second));

    cmp = my_compare(first, second);
    if (cmp < 0) {
        printf("\"%s\" is less than \"%s\"\n", first, second);
    } else if (cmp > 0) {
        printf("\"%s\" is greater than \"%s\"\n", first, second);
    } else {
        printf("\"%s\" and \"%s\" are equal\n", first, second);
    }

    joined[0] = '\0';
    my_concat(joined, first);
    my_concat(joined, second);
    printf("Concatenated: %s\n", joined);
    return 0;
}

Sample input and output

Hello
World

Enter first word:
Enter second word:
Length of "Hello" = 5
Length of "World" = 5
"Hello" is less than "World"
Concatenated: HelloWorld

apple
apple

Enter first word:
Enter second word:
Length of "apple" = 5
Length of "apple" = 5
"apple" and "apple" are equal
Concatenated: appleapple

How the program works

The functions take const char * for input strings and char * only for the destination, which is the one that changes. Because arrays are passed as pointers, my_concat changes the caller's array without returning it.

my_compare returns the difference of the first mismatching characters, which is the same convention as the library strcmp. Casting to unsigned char keeps the result correct for characters outside the ASCII range.

Viva questions

Common mistakes

• Forgetting the terminator in my_concat, so printf prints garbage after the text.
• Using a destination array that is too small for the joined result.
• Comparing strings with == on the pointers instead of the characters.