# Review: Fruitless functions (for effect)

Here's a couple of lines that draw a shape (a circle and a square).

```
function setup() {
createCanvas(windowWidth, windowHeight);
background(100);
}
function draw() {
circle(mouseX, mouseY, 20);
square(mouseX, mouseY, 30);
}
```

Extract them into a function.

```
function draw() {
squareAndCircle();
}
function squareAndCircle() {
circle(mouseX, mouseY, 20);
square(mouseX, mouseY, 30);
}
```

Parameterize the function.

```
function draw() {
squareAndCircle(mouseX, mouseY);
}
function squareAndCircle(x, y) {
circle(x, y, 20);
square(x, y, 30);
}
```

# Use `return`

to exit the function early

This function only *sometimes* draws the shape.

These are two different ways of doing the same thing.

```
function squareAndCircle(x, y) {
if (x < 300) return;
circle(x, y, 20);
square(x, y, 30);
}
```

```
function squareAndCircle(x, y) {
if (x >= 300) {
circle(x, y, 20);
square(x, y, 30);
}
}
```

# Fruitful Functions

`squareAndCircle()`

, above, is called *for effect*. Calling it has an effect on something (in this case, on what's in the canvas). `circle()`

and `square()`

are p5 functions that are also used for effect.

`sin()`

, in contrast, is called in order to use its value. The call to `sin()`

in this code doesn't do anything (except slow the program down slightly):

```
function draw() {
sin(10);
}
```

In order to use make use of `sin()`

, we need to do something with the value that it returns.

```
function draw() {
let d = sin(10);
circle(d, d, 10);
}
```

A sequence of statements that are executed for effect can be extracted into a fruitless function, that is called for effect. This is what we did with `circleAndSquare()`

, above.

An expression can be extracted into a fruitful function, that uses `return`

to send a value back to the code that calls it. The following two codes have the same effect.

```
function draw() {
for (let x = 0; x < width; x++) {
let y = map(sin(x / 100), -1, 1, 0, height);
square(x, y, 1);
}
}
```

```
function draw() {
for (let x = 0; x < width; x++) {
let y = computeY(x);
square(x, y, 1);
}
}
function computeY(x) {
return map(sin(x / 100), -1, 1, 0, height);
}
```

A fruitful function can contain several statements (just like a fruitless function), that are run one after another in order to compute a value. All of these functions are equivalent:

```
function computeY(x) {
return map(sin(x / 100), -1, 1, 0, height);
}
```

```
function computeY(x) {
let y = map(sin(x / 100), -1, 1, 0, height);
return y;
}
```

```
function computeY(x) {
let range = sin(x / 100);
let y = map(range, -1, 1, 0, height);
return y;
}
```

You can also include `if`

statements in a fruitful function (just like a fruitless function).

Here are two ways of drawing rectangles that are small on the left side of the screen and large on the right side of the screen, that don't use a separate function:

```
function draw() {
for (let x = 0; x < width; x++) {
if (x < 300) {
square(x, x, 1);
} else {
square(x, x, 10);
}
}
}
```

```
function draw() {
for (let x = 0; x < width; x++) {
let size;
if (x < 300) {
size = 1;
} else {
size = 10;
}
square(x, x, size);
}
}
```

And here are two ways to factor the size computation into a separate function:

```
function draw() {
for (let x = 0; x < width; x++) {
square(x, x, computeSize(x));
}
}
function computeSize(x) {
if (x < 300) {
return 1;
} else {
return 10;
}
}
```

```
function draw() {
for (let x = 0; x < width; x++) {
square(x, x, computeSize(x));
}
}
function computeSize(x) {
let size = 10;
if (x < 300) {
size = 1;
} else {
size = 10;
}
return size;
}
```

©2020–2022 by Oliver Steele.