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Lesson 12 - Functions in the C language

Today's C lesson is about a very important topic - functions. We already know that we write code into the main() function. This was all well and good for our small educational programs which usually could only do a single thing. However, consider writing a program which is thousands of lines long. I'm sure you agree it'd be very hard to work with such a "noodle" of code if it was all in a single file and a single function. Even more so, if we wanted to perform the same command sequences at multiple places in our program. In this case, we'd have to copy it all over again or to jump from one program place to another. Both options are very confusing.

Functional decomposition

We sometimes refer to splitting a program into multiple functions as a functional decomposition. Don't let the term intimidate you, we'll just think about what our application needs to do and then create a function in our source code for each of those tasks. In real-life applications, we usually create auxiliary functions as well. For example, a function for printing the application menu, or splitting a complex function to various simpler functions to keep the program readable.

Functions are sometimes called subroutines or subprograms. If a function doesn't return a value (more on this further along), it may be referred to as a procedure in some programming languages. Functions for larger applications, where there is a lot of them, are gathered into multiple modules/libraries. You know these very well already, e.g. from writing #include <stdio.h> which loads the library (module) for working with standard input/output (with the console). Similarly, mathematical functions are gathered in the math.h system library. We'll also learn to create said modules/libraries later on.

Creating functions

A function is a logical block of code which we write once and then call it multiple times without needing to write it all over again. We'll declare functions in the global scope, somewhere above the main() function. They'll look similar to main(). Let's add a function to our source code which will write "Hi, welcome!".

We'll show the entire source code just to be illustrative:

#include <stdio.h>
#include <stdlib.h>

void greet(void)
    printf("Hi, welcome!\n");

int main(int argc, char** argv)
    return (EXIT_SUCCESS);

The first void in the function definition specifies that the function doesn't return any value. The second void is there for a similar reason. It indicates that the function doesn't have any input parameters. Now, we have to call the function to execute it. Of course, we'd only be able to do so after we declare it, otherwise, the compiler wouldn't recognize the function (that's why we declared our function before main()). Add the following line to main():

greet(); // calling the function

The result:

Console application
Hi, welcome!

Functions with parameters

A function can have any number of input parameters (they're sometimes called arguments) which we write into the parentheses in its definition. We influence a function's behavior by parameters. Consider a situation we want to greet our users by their names. So let's extend our function of a name parameter and specify it later with a concrete value when calling the function:

void greet(char name[])
    printf("Hi %s, welcome here!\n", name);

Now, we'll modify the calling of the function in main() like this:

greet("Carl"); // calling the function

If we wanted to greet multiple people, we wouldn't have to write printf("Hi ... for each of them. Instead, we'll simply call our function:


The result:

Console application
Hi Carl, welcome!
Hi David, welcome!
Hi Mary, welcome!

The function's return value

A function can also return a value. Let's leave our greeting example and create a function for computing the area of a rectangle. Furthermore, we'll make it so we're able to use the result in another calculations. Therefore, we won't write the result but return it as the return value. Every function can return 1 value using the return command which will also terminate the function, so any other code after it won't be executed. We specify the data type of the return value before the function definition. Add the following function to your program:

int rectangle_area(int width, int height)
    int result = width * height;
    return result;

In real-world applications, our function would probably compute something more complex, so it'd actually be worthwhile to implement. However, as an example, a simple rectangle will serve just fine. We name functions using lowercase letters, whole words and using under_scores instead of spaces. Although the C language is full of abbreviations, you should avoid them at all costs. For example, birth_date() is much clearer than bird() which makes it hard to tell what it even does at the first sight.

If we wanted to print the area of a rectangle now, we'd simply call our function straight in the printf() function. First, the rectangle's area will be computed. Then, this value will be returned and passed as an input parameter to printf() which will print it. Let's try it out by entering 10 and 20 cm as width and height:

printf("The area of the rectangle is: %d cm^2", rectangle_area(10, 20));

Console application
The area of the rectangle is: 200 cm^2

If you find it confusing, feel free to use an auxiliary variable:

int area = rectangle_area(10, 20);
printf("The area of the rectangle is: %d cm^2", area);

However, we didn't decide to return the result as the function's return value to simply print it. Let's take advantage of this and compute the sum of the areas of two rectangles:

int total_area = rectangle_area(10, 20) + rectangle_area(20, 40);
printf("The sum of the areas of the rectangles is: %d cm^2", total_area);

The result:

Console application
The sum of the areas of the rectangles is: 1000 cm^2

Regarding previous exercises we did earlier on in the course: you may try to modify some of them and split them up into functions. According to good software design practices, a source code should always be split up into functions (and ideally into libraries/modules, more on this later on) to keep it clear. We omitted this at the beginning to keep things simple, but now, please, keep this in mind :)

The main advantage of using functions is clarity and keeping code shorter (we can write something once and call it a hundred times from multiple places in our program). If we decide to modify the function, we would only have to do it at one place and this change will affect all of the function calls immediately which decreases the possibility of making an error, significantly. In the greeting example, we could simply change the greeting text once in the function and it'd affect all three of the calls. If we didn't have the code in a function, we'd have to modify 3 sentences and it's very likely we'd make a typo somewhere.


To sum it all up, let's take a little peek into an advanced topic - recursion. A recursive function is a function which calls itself in its body. Such a function needs some information to determine when it should end. Otherwise, it'd call itself, then it'd call itself again and this would end with the program terminating due to insufficient memory. Recursion is used very often in various algorithms.

In functional programming languages, recursion is used instead of loops. Take for example, a for loop which sums up the numbers from 1 to 10. We could achieve the same result with recursion as well. The function would call itself over and over with a number which increases by one 1 or it would terminate itself (depending on what the current number is).

int loop(int current_index, int final_index, int sum)
    if (current_index == final_index)
        return sum;
    return loop(current_index + 1, final_index, sum + current_index);

We'd call the function like this:

printf("%d", loop(0, 10, 0)); // beginning of the recursion

We could do the same using a for loop:

int sum = 0;
int a;
for (a = 0; a < 10; a++)
    sum += a;
printf("%d", a);

As you can see, reading a code using recursion is not as easy as reading a code using loops. But that's not all. Using recursion creates additional memory requirements since parameters and return values have to be passed over and over again. Generally speaking, most programs which use recursion can be rewritten to not do so. Let's create a sample program that computes a factorial. We'll make versions with and without recursion.

int factorial(int x)
    if (x == 1)
        return 1
    return x * factorial(x - 1);

We'd call the function like this:

printf("%d", factorial(10));

Here's the alternative using loops:

int result = 1;
int x = 10;
int i;
for (i = 2; i <= x; i++)
    result *= i;
printf("%d", result);

You may run into recursion in existing source codes or at job interviews. However, it's probably better to avoid recursion, at least for now. Recursion is also able to waste the entire call stack quickly and terminate the program. Furthermore, it's difficult to understand. If you're confused by it, you're going to get more acquainted with it in the algorithms course where there's enough space to explain it in further detail.

In the next lesson, Structures in the C language, we'll introduce one of the basic aspects of the C language - structures.



Article has been written for you by David Capka
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