functions.md

Functions

To make our program more easy to debug, read and to prevent duplicating code. We should create our own functions.

Here is a function that returns the maximum of two integers.

int find_max(int a, int b)
{
    if (a > b) return a;
    else return b;
}

• The return value is int

• The function name is find_max

• The parameters are integers a and b, these are variables, scoped to the function only.

• The body of the function between {} is where the functions logic is written.

From the programs main function, we call the function `find_max' by passing in arguments to the functions parameters.

The functions return value is used in an expression and result stored to variable result.

int main(void)
{
    int result = find_max(10, 12) + 10;
    printf("result: %d\n", result);
    return 0;
}

Output:

result: 22

Full program code:

#include <stdio.h>

int find_max(int a, int b)
{
    if (a > b) return a;
    else return b;
}

int main(void)
{
    int result = find_max(10, 12) + 10;
    printf("result: %d\n", result);
    return 0;
}

Variable scope

Variables defined inside {} are only available inside the {}. For example, variables a and b are not available to main function. Variable result is not available to find_max function.

Variables a and b have function level or local scope to find_max function.

Variable result has function level or local scope to main function.

Variables cannot be used before there have been declared.

Function definition

Functions are typically split into two parts.

1. Function declaration
2. Function definition - function body

This makes the program more readable and prevents the main function from being buried depth in the code file.

Function declaration

int find_max(int a, int b); // Declaration

Function definition

int find_max(int a, int b)
{
    if (a > b) return a;
    else return b;
}

Full program code:

#include <stdio.h>

int find_max(int a, int b);

int main(void)
{
    int result = find_max(10, 12) + 10;
    printf("result: %d\n", result);
    return 0;
}

int find_max(int a, int b)
{
    if (a > b) return a;
    else return b;
}

The main function is now more prominent in the code, and the find_max function now appears below the main function.

Void functions

A void function is a function that returns no value. Used to accept inputs from user or print text.

void function declaration:

void print_two(float a, float b);

Full program:

#include <stdio.h>

void print_two(float a, float b);

int main(void)
{

    float x = 2.5;
    float y = 4.2;
    print_two(x, y);
    return 0;
}

void print_two(float a, float b)
{
    printf("%f\n%f\n", a, b);
    // No return needed.
}

Passing arrays to functions

We have seen that to create an array and assign values to it, we can use: -

int a[] = {2,4,9,1,3,5};

In C programming this array a is not a variable that can be used in an expression, but a memory address in the computers memory.

Printing a as shown below will output a hexadecimal number which is the memory address. The %p refers to the placeholder for a pointer.

printf("a: %p\n", a);

Output

a: 0x7ffe9d685b70

To print the contents of an array, we need to iterate through every element and print the value.

for (int i = 0; i < 6; i++)
    printf("a[%d]=%d\n", i, a[i]);

Ouput:

a[0]=2
a[1]=4
a[2]=9
a[3]=1
a[4]=3
a[5]=5

When we pass an array to a function we are passing the memory address (pointer) of the array.

void add_one(int array[], int length)
{
    printf("array: %p\n", array); 
    for(int i = 0; i < length; i++) array[i] += 1;
}

The function above, takes in an array memory address and iterates through every index, incrementing the index value.

Note this may not be the desired outcome, as we may not always want to update the arrays contents.

Putting this all together we have the program below:

#include <stdio.h>

void add_one(int array[], int length);

int main(void)
{

    int a[] = {2,4,9,1,3,5};
    printf("a: %p\n", a);

    add_one(a, 6);

    for (int i = 0; i < 6; i++)
        printf("a[%d]=%d\n", i, a[i]);

    return 0;
}

void add_one(int array[], int length)
{
    printf("Array passed to function: %p\n", array); 
    for(int i = 0; i < length; i++) array[i] += 1;
}

Output:

a: 0x7ffd1bccf360
Array passed to function: 0x7ffd1bccf360
a[0]=3
a[1]=5
a[2]=10
a[3]=2
a[4]=4
a[5]=6