Strings and Functions

A string is an array of char values, so a function gets it as a parameter just like any other array:

#include <stdio.h>

void introduce(const char name[]) {
    printf("My name is %s.\n", name);
}

int main(void) {
    char friend_name[] = "Tom";
    introduce("Anna");
    introduce(friend_name);
    return 0;
}

This program prints:

My name is Anna.
My name is Tom.

You can pass a text in quotes or a char array. Two things are different from the arrays of numbers in the previous exercise:

• There is no count. A string ends with the null character '\0', so the function can find its end by itself: with a loop that stops at '\0', or with strlen.
• Before the type, there's the word const, which you know from constants. It is a promise that the function only reads the string and doesn't change it. If the function tried, for example with name[0] = 'X';, the program wouldn't compile: error: read-only variable is not assignable (you can't assign to a variable that is only for reading). Write const whenever a function only reads a string. Then anyone who reads the function knows at a glance that the text stays as it is.

A function can go through a string in a loop and return a result. This one counts how many times a letter appears in a text:

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

int count_letter(const char text[], char letter) {
    int count = 0;
    int length = strlen(text);
    for (int i = 0; i < length; i++) {
        if (text[i] == letter) {
            count++;
        }
    }
    return count;
}

int main(void) {
    printf("%d\n", count_letter("banana", 'a'));
    printf("%d\n", count_letter("Mississippi", 's'));
    return 0;
}

This program prints:

3
4

A function without const may change the string. Just like with other arrays, the change stays in the array of the caller:

#include <ctype.h>
#include <stdio.h>

void make_upper(char text[]) {
    for (int i = 0; text[i] != '\0'; i++) {
        text[i] = toupper(text[i]);
    }
}

int main(void) {
    char message[] = "hello";
    make_upper(message);
    printf("%s\n", message);
    return 0;
}

This program prints:

HELLO

One thing a function can't do is return an array, so it can't return a new string the way it returns a number. When a function should produce a text, it writes it into a char array that the caller passes to it, just as make_upper changes message.