# Review: Basic line

This code from Arranging a Line of Items draws a line of shapes. It uses the derivation strategy, to calculate a value for x directly from i each time through the loop.

void setup() {
size(600, 600);
}

void draw() {
for (int i = 10; i < 20; i++) {
float x = 10 + 50 * i;
float y = 100;
circle(x, 100, 40);
}
}

Note: From here on, the definition of setup() is not shown. Each of the following code samples assumes that the sketch also contains a setup() function:

void setup() {
size(600, 600);
}

# Using sin and cos

We can use the sin() or cos() functions to make the items swing between the top and bottom of the band, as the index value progresses.

void draw() {
for (int i = 0; i < 10; i++) {
float x = 20 + 50 * i;
float y = 100 + 50 * sin(i);
circle(x, y, 50);
}
}
void draw() {
for (int i = 0; i < 10; i++) {
float x = 20 + 50 * i;
float y = 100 + 50 * cos(i);
circle(x, y, 50);
}
}

Note: The sketches in this section end up computing sin(0), sin(1), sin(2), all the way to sin(9). The Processing sin() function actually takes radians, so typically a sketch would take much smaller steps. These large steps happen to draw sine waves anyway for complicated math-y reasons (because 1 and $pi$ are incommensurate), and this is just a step towards a sketch that will do things correctly anyway. So use the code in the section Items in a circle to copy from, not the sketches in this section.

Trying to use sin (or cos) for both x and y arranges the items on a diagonal line. This is true any time that x and y use the same equation, because then they will have the same value.

void draw() {
for (int i = 0; i < 10; i++) {
float x = 100 + 50 * sin(i);
float y = 100 + 50 * sin(i);
circle(x, y, 50);
}
}

The solution is to use cos for x and sin for y. These functions are cleverly designed so that using them together this way makes a circle.

Let’s look at just using sin for y (instead of x), and compare that to the code a couple of sketches above that uses cos for x.

void draw() {
for (int i = 0; i < 10; i++) {
float x = 100 + 50 * cos(i);
float y = 100 + 50 * i;
circle(x, y, 50);
}
}

Combining cos for x with sin for y creates a circle.

void draw() {
for (int i = 0; i < 10; i++) {
float x = 100 + 50 * cos(i);
float y = 100 + 50 * sin(i);
circle(x, y, 50);
}
}

There’s two things that may be wrong with this circle, depending on your design intent: (1) it is too small relative to the items that are placed along its edge; and, the items are wrapped around the circle more than once.

The first problem can be addressed by changing the 50 in 50 * cos(i) and 50 * sin(i) to a larger number. We will also change the 100 in 100 + 50 * cos(i) to a larger number, to move the center of the arrangement circle right so that the whole circle stays on the screen.

# Items in a circle

By default, sin and cos repeat every $2\pi$. We need to insure that the number that we putting into them varies from 0 to $2\pi$ by the time the loop is done. Since i varies from 0 to (just under) 10, i * TWO_PI / 10 varies from 0 to (one step under) TWO_PI.

void draw() {
for (int i = 0; i < 10; i++) {
float angle = i * TWO_PI / 10;
float x = 250 + 150 * cos(angle);
float y = 300 + 150 * sin(angle);
circle(x, y, 50);
}
}

Note: We could also have used map(i, 0, 10, 0, TWO_PI) instead of i * TWO_PI / 10.

Here is another way of doing this. angleMode(DEGREES) causes sin and cos to repeat every 360, instead of every $2\pi$.

void draw() {
for (int i = 0; i < 10; i++) {
float angle = radians(i * 360 / 10);
float x = 250 + 150 * cos(angle);
float y = 300 + 150 * sin(angle);
circle(x, y, 50);
}
}

Yet another way to do this is to increment the angle directly instead of computing it from the index position. This is useful if we know the number of degrees we want to leave between items, instead of the number of items.

void draw() {
for (float angle = 0; angle < 360; angle += 10) {
float x = 250 + 150 * cos(radians(angle));
float y = 300 + 150 * sin(radians(angle));
circle(x, y, 50);
}
}

# Drawing some items differently

We can use a conditional in order to draw some items differently from others.

void draw() {
for (int i = 0; i < 360; i += 10) {
float x = 250 + 150 * cos(angle);
float y = 300 + 150 * sin(angle);

if (i == 180) {
fill(0);
} else {
fill(255);
}
circle(x, y, 50);
}
}
void draw() {
for (int i = 0; i < 360; i += 10) {
float x = 250 + 150 * cos(angle);
float y = 300 + 150 * sin(angle);

if (i == 180) {
fill(0);
} else {
fill(255);
}
if (i == 90) {
strokeWeight(5);
} else {
strokeWeight(1);
}
circle(x, y, 50);
}
}
void draw() {
for (int i = 0; i < 360; i += 10) {
float x = 250 + radius * cos(angle);
float y = 300 + radius * sin(angle);

if (i % 40 == 0) {
fill(0);
} else {
fill(255);
}
if (i == 90) {
strokeWeight(5);
} else {
strokeWeight(1);
}
circle(x, y, 50);
}
}

# Animating the circle

Incorporate the time (millis()) into the equation, in order to animate the circle.

void draw() {
background(200);
for (int i = 0; i < 360; i += 10) {
float angle = radians(i + millis() / 15);
float x = 250 + 150 * cos(angle);
float y = 300 + 150 * sin(angle);

if (i % 40 == 0) {
fill(0);
} else {
fill(255);
}
if (i == 90) {
strokeWeight(5);
} else {
strokeWeight(1);
}
circle(x, y, 50);
}
}

# From circle to spiral

The circle sketches place each object the same distance from the center. sin and cos are defined to produce x, y pairs that are all a distance of 1.0 from the origin; multiplying them both by 150 makes everything a distance of 150 pixels from the origin. Instead of multiplying each sin and cos by 150, multiple them by larger and larger numbers as the angle increases. This creates a spiral.


void draw() {
for (int i = 0; i <= 360; i += 10) {
float radius = map(i, 0, 360, 10, 200) + 100;
float x = 250 + radius * cos(angle);
float y = 300 + radius * sin(angle);

if (i % 40 == 0) {
fill(0);
} else {
fill(255);
}
if (i == 90) {
strokeWeight(5);
} else {
strokeWeight(1);
}
circle(x, y, 80);
}
}

Increasing the angle to 720 degrees to wrap around the circle twice ($2 * 360$).

void setup() {
size(600, 600);
}

void draw() {
for (int i = 0; i < 720; i += 10) {
float radius = map(i, 0, 360, 10, 200);
float x = 250 + radius * cos(angle);
float y = 300 + radius * sin(angle);

if (i % 40 == 0) {
fill(0);
} else {
fill(255);
}
if (i == 90) {
strokeWeight(5);
} else {
strokeWeight(1);
}
circle(x, y, 50);
}
}

Original p5.js version ©2020–2022 by Oliver Steele. Updated for Processing by Margaret Minsky.