Functions are an elementary part of programming languages. With them, programs can be simplified and presented in a manageable and reusable way (modularisation). Functions also help us to keep our code organised and understandable.
Functions act as jumps in the program. In the event of a function call, the program jumps to the position where the function is defined and then returns after the function has been executed. Most programming languages have many built-in or native functions. To find out how the built-in p5.js functions work, and what parameters they expect, we need to look at the documentation. We have already used many inbuilt functions using calls such as stroke() or ellipse().
There are two parts of a function. First, there is the function declaration, where the name of the function along with its properties and behaviours are defined. Next, there is the function call, where the function is then actually executed. Functions may be called multiple times.
Functions can also accept input values (called parameters) that are sent to the function when called. These input parameters can change the behaviour of the function.
Functions can also return a value. This means when we return to the line where the function was called, it has now carried a number with it. In p5js, functions that have no return value are declared with “function” return type.
This is how the function is named, its return type and parameters defined with sudo code.
datatype functionName (parameter,.....)
{
<function contents>
<possible return value>
}
This is how we execute a function:
functionName(parameter,.....) ;
If you want to draw three smiley faces out of ellipses or arcs you could repeat all steps three times. Or better, we can create a function that draws a smiley face, and then call this function three times. This makes our code tidier, shorter, simpler to understand and much more modular. This example shows how to use functions for creating legible and simple blocks (modularization). The best part is, that we can easily continue to use our smiley function to draw hundreds of smileys with minimal code.
function setup()
{
createCanvas(500,300);
stroke(255);
strokeWeight(3);
noFill();
}
function draw()
{
background(0);
smiley(100, height/2);
smiley(250, height/2);
smiley(400, height/2);
}
function smiley(x , y)
{
print("smiley");
ellipse(x, y, 100, 100); // head
ellipse(x - 20, y - 10, 10, 15); // left eye
ellipse(x + 20, y - 10, 10, 15); // right eye
arc(x, y, 60, 60, radians(20), radians(180-20)); // mouth
}
This example shows how to write functions with return values. When a return value is used, a value is passed back to where the function was called. We can also imagine that the function call, in this case calculateSquare(4) is replaced with the returned value, i.e. 16. You will typically use a return value if your function is providing the answer to something.
function setup()
{
const result = calculateSquare(4); // assign variable to call a function
console.log(result);
print(calculateSquare(10)); // call function directly
}
function calculateSquare(x) {
return x * x;
}
There is another special type of function when working with p5.js: an event function. Unlike the previous examples, where you wrote both the function call and the function definition, the function call is happening automatically in response to an event such as a keyboard or mouse interaction. Events in p5.js use specific predefined names, which you can find in the reference. And here is our first interactive program:
function setup(){
createCanvas(300,300);
background(0);
stroke(255);
strokeWeight(10);
}
function draw(){
point(mouseX, mouseY);
}
function mousePressed(){
print("x:" + mouseX + ", y:" + mouseY);
clear();
background(0);
}
Write a sketch where a unique shape follows the mouse position. When you click the mouse, the shape should change in some way (form, colour, size).