p5js week 03

This week, let's make an automatic toaster in code that can toast 5, 10, or 1,000 pieces of toast for us! After you do that, your goal is to fill your screen with robots, wolves, or flowers.

(toaster robot by Nehal Siksik https://www.artstation.com/artwork/DALome )

To do that, first we need to learn some things about JavaScript:

  1. reassigning variables
  2. incrementing
  3. for-loops
  4. the remainder operator

Then let's use that in p5js to fill the screens.

It's a lot to study, so let's get started!

First, let's talk about these JavaScript concepts.

How could we make a lot of toast? How could we make things change for each piece of toast? What if we wanted to have more and more butter on each slice, or one extra slice of strawberry on each one?

(image from https://www.maxpixel.net/Eat-Delicious-Bread-Toast-Sandwich-Food-Breakfast-4115627 , Creative Commons license)

Butter and strawberries might be kind of hard for us to code just now. Let's think about our rainbow toast. We know how to do random colors, but how could we make each piece of toast change color a little bit, slowly going from one color to another? We know from last time that we can represent color with numbers, like RGB. If we slowly change one or more of these numbers, we could slowly change the color.

We're going to need to understand how to increment variables. “Increment” (https://en.wikipedia.org/wiki/Increment_and_decrement_operators) means to increase something by a regular amount.

First, remember how variables work. Let's declare a variable:

let toastRed = 0

The let keyword protects our variable. toastRed is the variable name. The equal sign is used to assign a value to the variable. Remember, a variable is like a box to keep things in. Do you remember the bread box from before? We can set (assign) this variable to a new number by using the same variable name and a different value:

toastRed = 20

Notice that we don't use 'let' when we reassign the variable. Often times, the new value is related to the old value. In that case, we can use the variable name to refer to the old value.

toastRed = toastRed + 1

The variable name on the left followed by the equal sign means we are assigning it. Since we assigned it before, that means we are reassigning it now. The value on the right is the old value plus one. In other words, we are adding one to the current value and then storing that new value in the same box (variable). Try evaluating this line multiple times and see what happens.

In the same way, we can decrement a number. “Decrement” is the opposite of “increment”. It means to make a number go down by steps.

toastRed = toastRed - 1

JavaScript gives us a special operator for incrementing. We can use it this way.

toastRed = ++toastRed

The ++ operator increases a value by one. Try evaluating this line multiple times and see what happens.

If you want to increment by another value, you have to use the previous method which is shown above.

toastRed = toastRed + 5

In all of the cases above, we assigned a new value to the variable by first writing the variable name on the left and then the new value after writing an equal sign. However, there's one more way!

toastRed += 10

That adds to the value. You can use this one to subtract:

toastRed -= 5

We can use this with a for-loop, which we are going to learn below, to do that gradual color change. When we get to a value of 255 for red (and 0 for the others), we'll get red toast!

(image from https://commons.wikimedia.org/wiki/File:Ajvar_toast.jpg)

Now we want to start learning a really important concept: for-loops. Above, we were incrementing numbers. It might be OK to repeat that code three times, but repeating it five times would be a little annoying. Imagine if you had to do it 10 times, or 100 times. What if you had to do it 1,000 times? Can you imagine toasting 1,000 pieces of toast?

(photo by Rohit https://pixahive.com/portfolio/rohit/)

Take a look at this function.

 function toaster (n) {
  for (let i = 0; i < n; i++) {
    console.log("have a piece of toast! " + i)

Try calling it in the console, just like you would do to any function:


Maybe you think 10 times is easy. What about 1000?


It contains a for-loop. A for-loop has a particular structure: the conditions for the for-loop are in parentheses and the body of the for-loop is in curly braces:

for () {}

In the parentheses, you need three parts (kind of like butter, jam, and a cup of coffee for your toast):

  1. a starting value stored in a variable, sometimes called a counter
  2. a stopping value which evaluates to a boolean (true or false; we talked about booleans last time)
  3. a statement which is executed on every loop, usually to increment the variable with the starting value/counter

In the curly braces, you put the things that you want to happen on each loop. Remember, the loop is going to run until the stopping value is false.

In the for loop above, notice these things:

  1. the starting value is 0, and it's stored in the variable called i
  2. the stopping value is 10
  3. the third value which is the result of the incrementing operator ++

What this means is that the for-loop will start the variable i at 0 and increase it by 1 every time. It will continue to run as long as i is less than 10.

Also notice that the variable i is used in the body of the for-loop in the call of console.log.

There are some details about the conditions for the for loop that we should think about. The first item in the conditions is the counter.

The first point to be careful about is using the let statement. We'll talk about “let” more in the future. For now, you should know that “let” is a JavaScript keyword, like “function” or “if”, that makes variables safer to use. If you forget to use “let”, it's going to be easy for a particular kind of bug to happen in your program. A bug is, according to Wikipedia, “… an error, flaw or fault in a computer program or system that causes it to produce an incorrect or unexpected result, or to behave in unintended ways.”

The second point is that the name of the variable that we use as a counter in the for-loop is up to you to choose. Just like you can name arguments “banana” or “chocolate”, you can also name the counter in the for-loop anything you want. However, it's very common for programmers to call the counter “i”, which can represent the ideas of “integer” or “index” or “iteration”. Personally, I think that 'iteration' is the most useful to think about for a for-loop. The Oxford Dictionary defines “iteration” as “repetition of a mathematical or computational procedure applied to the result of a previous application”. In that case, the i means which repetition or time we are at.

The third point is that we can then use this counter variable inside the for-loop any way we want. In this function, we are going to collect numbers in a variable and then add the value of i to it on each loop. Notice how i increases, and then the value of the numberCollector variable also increases.

function buildNumber (n) {
  let numberCollector = 0;
  for (let i = 0; i < n; i++) {
    console.log("hi! this is the value of i: " + i);
    numberCollector = numberCollector + i
    console.log("The current value of the numberCollector is " + numberCollector)
  console.log("The final value of the numberCollector is " + numberCollector)
  return numberCollector

Now call buildNumber and see what happens:


The fourth point is that you can start the counter of the for-loop at any value you want, but it's customary to start at 0. Just so that you can see how it works, let's start a for loop at a different value. Notice that it works the same as 0; we just have to be careful about setting the stopping point. The n argument still controls how many times we loop, but also notice that the value of i is different from our old repeater function.

function repeaterFrom10 (n) {
  for (let i = 10; i < n+10; i++) {
    console.log("hi! " + i)

Call it like this:


As always, be careful about the syntax: put the conditions of the for-loop in parentheses first, and then the body of the for-loop in curly brackets.

You have to be careful about where you put the return statement. If the return is inside the body of the for-loop, the loop will stop as soon as the return is evaluated. That means if you do some computation and then return the result, the loop stops immediately! It's like stopping the toaster before our bread is toasted.

The following function does that. Even when you ask it to loop 1,000 times, it stops on the very first loop.

 function returnsWrong (n) {
  let numberCollector = 0;
  for (let i = 0; i < n; i++) {
    console.log("adding this to the numberCollector: " + i);
    numberCollector = numberCollector + i;
    return numberCollector

Try it:


The key is to put the return statement after the for-loop, like this:

 function returnsRight (n) {
  let numberCollector = 0;
  for (let i = 0; i < n; i++) {
    console.log("adding this to the numberCollector: " + i);
    numberCollector = numberCollector + i;
  return numberCollector

Try this one:


There's another kind of mistake that some are making with the conditions in the for-loop. Have a look at this wrong function:

function wordPrinter (word1, n) {
  for (let b = 0; b < n; i++) {
  console.log(word1 + b)
  return (word1)

Here, the conditions for the for-loop are wrong. The conditions of the for-loop can be found in the parentheses after “for”. In this case, it's:

for (let b = 0; b < n; i++) // this is wrong!

Remember what the three parts mean:

  1. starting point
  2. stopping point
  3. how to increment

That means that in this case, the count is measured with b. However, b is not incremented. Only i is incremented. “i” will continue to go up, but this increasing value is never used later in the function. Maybe it's more important to notice that this means that the loop will never stop. Eventually it will crash the computer! Don't do this!

The conditions should be like this:

for (let b = 0; b < n; b++)

It's OK to use b, i, cherries, or any other variable we want; we just have to be sure that the same one is used for all three parts. If we want a traditional counter for our for-loop. i is usually the best choice because people are used to it.

for (let i = 0; i < n; i++)

(photo from https://www.piqsels.com/en/public-domain-photo-zkfbs)

OK, now let's try to use what we've learned for drawing in p5js.

  1. using a for-loop to gradually change colors
  2. filling the screen with earths using our earth function
  3. learning the remainder operator to get even more earths on the screen
  4. filling the screen with your rainbow toast

Look at this example. We'll use a for-loop to increase the value of a variable, like we talked about above.

Notice that we increase the toastRed variable, and we decrease the toastBlue variable.

Last time we put many earths on the screen, we had to write out our earth function call several times. One of the cool things about programming is that we can let the computer do the boring work like that for us. Let's use a for-loop to do it instead. Look at lines 17-19.

That for-loop only gives us one row of earths. What if we want to fill the whole screen? To do that, let's learn another tool: the remainder operator. It's the % sign. An operator is a symbol we put between two items to do something to them, like + - * and /.

Try this in the console:


The percent sign (%) in JavaScript is the remainder operator. The remainder is the amount that is left over after you divide one integer by another. When you divide 4 by 2, there's nothing left, so it returns 0. How about this one?


With five divided by two, the remainder is one. And this one?


What about this one?


By now, I think you can guess this one…


We can use it to produce a looping series of numbers (or to keep numbers within a certain range) when we have another number that is increasing or decreasing.

In this code, we use the remainder operator and Math.floor to get several rows of earths. Study this code and see if you can figure out how it works.

OK, you've learned a lot today. Now it's time for a more serious challenge. Can you fill your screen with your rainbow toast? Try to use the things that we've learned above to fill the screen with rainbow toast just like we did with the earth.

After you've finished that, then try to fill the screen with your wolves, robots, or flowers! Share your work with the class on your wiki page and screenshots in our chat rooms. We want to see what kind of art you've coded!

  • p5js-week-03.txt
  • Last modified: 12 months ago
  • by reina.chen