Functions

A function in C is a piece of code we write once and reuse many times

#include <stdio.h>

void greet(char name[]) {
    printf("Hello, %s!\n", name);
}

int main(void) {
    greet("Ritchie");
    greet("Thompson");
    greet("Kernighan");
    return 0;
}

Output

Hello, Ritchie!
Hello, Thompson!
Hello, Kernighan!

Let's break down the function header

void greet(char name[])

• void: the function does NOT return anything (it just prints)
• greet: the name of the function
• (char name[]): the parameter list. char name[] means "a string" (a piece of text). We'll talk about strings in a dedicated lesson

The format specifier %s in printf is for strings, just like %d is for ints

A function can also return a value. Instead of void, we put the return type

#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int main(void) {
    int result = add(2, 3);
    printf("%d\n", result);
    return 0;
}

Output 5

The function add takes two ints and returns an int. The return keyword sends the value back, and the function exits immediately when return runs

We can also use the result directly in another expression

#include <stdio.h>

int add(int a, int b) {
    return a + b;
}

int main(void) {
    printf("%d\n", add(2, 3) * 10);   // 50
    return 0;
}

Important detail in C: a function must be declared before it is used. That's why add is written above main. If we put it below, the compiler reads main first, sees add being called, doesn't know what add is, and complains

#include <stdio.h>

int main(void) {
    int x = add(2, 3);   // ERROR: add is not yet known
    return 0;
}

int add(int a, int b) {   // declared too late
    return a + b;
}

There is a workaround: forward declarations (also called prototypes). We write the function header at the top, the body anywhere

#include <stdio.h>

int add(int a, int b);   // prototype

int main(void) {
    printf("%d\n", add(2, 3));
    return 0;
}

int add(int a, int b) {   // body, written later
    return a + b;
}

Notice the ; at the end of the prototype: it's a declaration, not a body. For now, the simplest is to write your helper functions above main and stop worrying

Mission: The Terminal's Arithmetic Module

In an old-time computing center, every terminal needed its own quick arithmetic module, so operators wouldn't have to do the same calculations by hand every time.

Write a function calculator that takes three parameters: int number1, int number2, and char operator (a single character like '+', '-', '*', '/'; note the single quotes for a single char in C).

The function should print the result of the operation, in the format number1 operator number2 = result. If the operator is not recognized, print Invalid operator.

1. Handle addition ('+'), subtraction ('-'), multiplication ('*'), and division ('/')
2. Use %c as the format specifier for a single char
3. Use == to compare the char (this works in C because chars are, under the hood, small integers)

In main, call calculator five times, with exactly these sets of arguments, in order:

• 14, 12, '+'
• 10, 3, '-'
• 5, 4, '*'
• 10, 2, '/'
• 1, 1, 'x'

Example

Your program should print

14 + 12 = 26
10 - 3 = 7
5 * 4 = 20
10 / 2 = 5
Invalid operator