A loop inside a loop

Challenge 1 of 16

A loop can hold another loop. The inner one runs all the way through, start to finish, for every single turn of the outer one, so three turns of five make fifteen circles.

Set the gap across and the gap down.

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Set the gap across and the gap down.

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Lesson Loops inside Loops

A loop can hold another loop. That is all a nest is, and the whole of the difficulty is in one sentence: the inner loop runs all the way through, from its start to its stop, for every single turn of the outer one.

for (var row = 0; row < 3; row = row + 1) {
for (var col = 0; col < 5; col = col + 1) {
circle(50 + col * 76, 90 + row * 110, 44);
}
}

Three turns of five is fifteen circles, out of four lines. The counters go 0-0, 0-1, 0-2, 0-3, 0-4, then 1-0, 1-1 and on. Notice what col does when row moves on: it starts over. It always starts over, which is exactly why this draws a grid and not a diagonal.

The names matter more than they did. With one counter, i was fine. With two, row and col tell you which is which at a glance, and mixing them up is the mistake to expect: put the row counter in the across sum and you get a picture that is nothing like the one you wanted.

Which counter goes where is the whole design. The one that changes every turn, col, spaces things out along a row. The one that only moves when a whole row has finished, row, moves you down to the next one. Put a counter in a size instead and it is the same choice again: a size from row makes every row a different size, and a size from col makes every column one.

Both counters can go into the same sum. Add a little of row into the across sum and each row starts further along than the one above, so the grid leans.

The inner loop's stop can read the outer counter. Write col < row + 1 and the first row draws one shape, the second two, and you have a triangle instead of a rectangle.

And a nest needn't look like a nest. Put the inner loop in a function of its own and the outer loop just calls it once per row — same fifteen shapes, and often easier to read.

Next topic these grids start moving. The frame number joins the two counters, and everything you have built here can drift, grow or spin.