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Functions as Modules

In C, we use functions as the basic unit of modular design. Modular design is the practice of designing a system in modules. A module is any entity with an interface and an implementation. The interface provides an abstraction (a simplified view of an entity which omits unimportant details) of the module.

An interface is a contract/intersection between the system and the environment. Credits: Robert Elder

Functions act as modules in C. The definition of the function determines the interface. Take for example the following:

#include <stdio.h>

void count_to(int number){
    printf("Let's count from 1 to %d\n",number);
    for (int i=1;i<=number;i++){

int main(){
    return 0;

↗ Open 📋 Copy c/functions/count_to.c

By just typing count_to(number), we can use the function. This is the interface. The implementation is the actual body of the function. Appreciate how we do not need to know what the body of the function looks like in order to use it. We only need to know that we must pass an integer into the function.

We can use a simple diagram to represent this module: