Rain, column by column

Challenge 15 of 16

One counter, one moving number, and a multiplication between them gives every column its own rate of fall. The left-hand one barely moves and the right-hand one races.

Set the step, the gap and the length of a drop.

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Set the step, the gap and the length of a drop.

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0 / 40 frames · 400×400

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Lesson Loops that Move

You have two kinds of number now, and this topic is about what happens where they meet.

The loop counter says which shape you are drawing. It goes 0, 1, 2, 3 and back to 0 on the next frame, over and over, and it never remembers anything.

A number kept outside everyFrame is the other kind. It is made once, above everyFrame, and every frame adds to it, so it carries on from where it got to:

var x = 0;

function everyFrame(){
backgroundColour(BLACK);
x = x + 3;
for (var i = 0; i < 4; i = i + 1) {
circle(30 + x + i * 70, 200, 44);
}
}

The counter spaces the circles out and x carries all four along together. Take x out and you have Topic 2's row. Take the loop out and you have Course 3's single mover. Together they are a row that moves.

Where you put x is the whole of it. In the across sum the row drifts sideways; in the down sum it falls; in a size every shape swells at once.

And you can multiply. Write x * i and every shape gets its own speed, because the counter is bigger further along the row — the first one stands still and the last one races away. That one line is the difference between a row that slides and a row that fans out, and it is worth trying both.

Two numbers can move at two rates. Two loops, two variables, two speeds, and the rows drift apart.

The loop's stop can read the frame number, and then the row gains a shape every frame and draws itself across the canvas.

One thing to know about the number kept outside. It has to live above everyFrame, not inside it. Put var x = 0 inside and it is made afresh every frame, so it goes back to 0 every time and nothing ever moves. That is the commonest way an animation comes out frozen.

Next topic is the last one, and the loops come out. You will write the headers, the bodies and the sums yourself, from a comment and a picture.