String Functions strlen, strcpy, strcat and strcmp in C

Algorithm

1. Read two words with scanf (no spaces) into first and second.
2. Use strlen to print the length of first.
3. Copy first into copy with strcpy, then append second to copy with strcat.
4. Compare first and second with strcmp and report which is smaller, or that they are equal.
5. Print the joined string and its length.

C program

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

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

    printf("Enter two words:\n");
    if (scanf("%49s %49s", first, second) != 2) {
        printf("Error: enter two words\n");
        return 1;
    }

    printf("Length of first: %d\n", (int)strlen(first));

    strcpy(copy, first);
    strcat(copy, second);
    printf("Joined: %s\n", copy);
    printf("Length of joined: %d\n", (int)strlen(copy));

    cmp = strcmp(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);
    }
    return 0;
}

Sample input and output

Good Morning

Enter two words:
Length of first: 4
Joined: GoodMorning
Length of joined: 11
Good is less than Morning

abc abc

Enter two words:
Length of first: 3
Joined: abcabc
Length of joined: 6
abc and abc are equal

How the program works

strcpy must run before strcat, because strcat appends to whatever the destination already holds. copy is sized for both words plus the terminator, which is why it is larger than first and second.

strcmp returns a negative, zero or positive value according to the first differing character, so the comparison is made by its sign rather than by the exact number.

Viva questions

Common mistakes

• Calling strcat on an empty uninitialised buffer, which can read garbage before appending.
• Comparing strings with == instead of strcmp, which compares the addresses.