C Function Parameters

Parameters are the variables a function receives; arguments are the values you give when you call it. In int add(int a, int b), a and b are parameters; in add(2, 3), 2 and 3 are the arguments.

Several parameters

Parameters are separated by commas, each with its own type. The arguments are matched by position.

#include <stdio.h>

void introduce(const char *name, int age) {
    printf("%s is %d years old.\n", name, age);
}

int main(void) {
    introduce("Asha", 12);
    introduce("Ravi", 14);
    return 0;
}

Asha is 12 years old.
Ravi is 14 years old.

const char *name is how a function receives a string it will only read; const promises it will not change it.

Pass by value

C always passes a copy of each argument. Changing the parameter inside the function does not change the caller's variable:

#include <stdio.h>

void tryToChange(int n) {
    n = 100;
    printf("inside: n = %d\n", n);
}

int main(void) {
    int n = 5;
    tryToChange(n);
    printf("outside: n = %d\n", n);
    return 0;
}

inside: n = 100
outside: n = 5

To let a function change a variable, pass its address and use a pointer parameter, as swap does in the Pointers topic. A function can also simply return a new value, which is often clearer.

Passing arrays

When an array is passed, the function receives a pointer to its first element, not a copy. That has two consequences: the function can change the array's elements, and it cannot know the array's length, so the length is passed too.

#include <stdio.h>

int total(const int values[], int count) {
    int sum = 0;
    for (int i = 0; i < count; i++) {
        sum += values[i];
    }
    return sum;
}

void doubleAll(int values[], int count) {
    for (int i = 0; i < count; i++) {
        values[i] *= 2;
    }
}

int main(void) {
    int marks[] = {10, 20, 30, 40};
    int n = sizeof(marks) / sizeof(marks[0]);
    printf("total: %d\n", total(marks, n));
    doubleAll(marks, n);
    printf("after doubling: %d %d %d %d\n", marks[0], marks[1], marks[2], marks[3]);
    return 0;
}

total: 100
after doubling: 20 40 60 80

const int values[] documents that total only reads the array; the compiler stops any attempt to change it.

Returning more than one result

A function returns at most one value. For two results, return one and pass a pointer for the other, or pass pointers for both:

#include <stdio.h>

void minMax(const int a[], int n, int *min, int *max) {
    *min = *max = a[0];
    for (int i = 1; i < n; i++) {
        if (a[i] < *min) *min = a[i];
        if (a[i] > *max) *max = a[i];
    }
}

int main(void) {
    int a[] = {7, 2, 9, 4};
    int lo, hi;
    minMax(a, 4, &lo, &hi);
    printf("min %d, max %d\n", lo, hi);
    return 0;
}

min 2, max 9

Practice exercises

1. Write void printArray(const int a[], int n) and use it to print two different arrays.
2. Write a function that divides two integers and returns both the quotient and the remainder through pointers.
3. Write void reverse(int a[], int n) that reverses an array in place.