The signal — visuco

Challenge 7 of 16

A speed with three bands makes a signal: stop, slow down, or go. One circle, three possible colours, and only the chain decides which of them you see.

Set the two band numbers so this speed shows amber.

Your code JavaScript

Set the two band numbers so this speed shows amber.

Console print() output

0 / 30 frames · 400×400

Target

Yours

Yours over Target — ghosted shapes are in the target but missing from yours

Results

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Penalties: none this run

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Lesson Choosing from Several

An if with one else can tell two things apart, and that is all. A mark of 72 is not a fail and not top of the class, so two outcomes are not enough.

else if is an else with a question of its own.

if (mark >= 80) {
shapeColour(YELLOW);
} else if (mark >= 50) {
shapeColour(GREEN);
} else {
shapeColour(RED);
}

Read it from the top. Is the mark 80 or more? If so, gold, and the sketch walks straight out of the chain — nothing below is even looked at. Otherwise, is it 50 or more? And if none of the questions said true, the else on the end catches whatever is left.

THE FIRST TRUE ONE WINS. This is the whole idea, and it is what makes a chain different from a row of separate ifs. A mark of 90 is over 80 and over 50 as well, and both questions would say true if they were asked — but the second one never is.

That is also why order matters so much. Put the widest band at the top and it swallows everything meant for the narrower ones underneath it:

if (mark >= 20) {
shapeColour(RED);
} else if (mark >= 80) {

Every mark of 20 or more comes out red, and the gold band is never reached at all. Nothing errors. The picture is simply wrong. Ask about the narrowest band first and work outwards.

The else on the end is optional. Leave it off and a value that fits no band means nothing happens at all — which is Topic 2's grey badge again.

The last thing here is the one to be careful about for the rest of your life.

if (mark = 40)

That is one equals sign, and it does not ask anything. It puts 40 into mark and then treats the box it has just filled as an answer, which always comes out true. Nothing errors, nothing complains, and the number you were comparing has been quietly overwritten. Two equals signs ask. One puts a value in a box. If a chain is behaving as though the first band always wins, count the equals signs first.