String Functions: string.h

You can't use =, + or == on strings in C. Instead, use the functions in <string.h>:

strlen: the length

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

int main() {
    char word[20] = "banana";
    printf("%s has %d letters\n", word, (int) strlen(word));
    return 0;
}

Output

banana has 6 letters

strcpy and strcat: copy and join

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

int main() {
    char full[50];
    strcpy(full, "Hello");
    strcat(full, ", ");
    strcat(full, "World");
    printf("%s\n", full);
    return 0;
}

Output

Hello, World

Common mistake: Making the destination too small. strcpy and strcat don't check the size – if the text doesn't fit, they write past the end of the array. Always make the array big enough.

strcmp: are two strings equal?

strcmp returns 0 when the two strings are the same. (A negative or positive number means the first one comes before or after the other in dictionary order.)

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

int main() {
    char password[20];
    scanf("%s", password);
    if (strcmp(password, "open123") == 0) {
        printf("Access granted\n");
    } else {
        printf("Wrong password\n");
    }
    return 0;
}

Input (what the user types)

open123

Output

Access granted

Common mistake: Writing if (password == "open123"). That compares memory addresses, not the text, and is almost always false. Use strcmp.

Looping over a string

Go through a string letter by letter until you reach '\0':

#include <stdio.h>

int main() {
    char text[30] = "hello world";
    int vowels = 0;
    for (int i = 0; text[i] != '\0'; i++) {
        char c = text[i];
        if (c == 'a' || c == 'e' || c == 'i' || c == 'o' || c == 'u') {
            vowels++;
        }
    }
    printf("Vowels: %d\n", vowels);
    return 0;
}

Output

Vowels: 3