Variables

Variables

Why anybody needs one

Long before there were computers, people had exactly this problem. A shepherd counting sheep through a gate, a trader counting sacks of grain, a builder counting days — none of them can hold the number in their head while they get on with the work. So they scratched a mark on a wall, cut a notch in a stick, or wrote a number on a piece of paper. The number lived outside the person, in a place they had agreed on, and they could go back to it, read it, and change it.

Better still, once the number is written down somebody else can use it. Watch these two: one of them counts and writes, the other never sees a single animal and simply reads the wall.

Neither Abby nor Ben is holding the number — the wall is. And notice what never happens: Ben does not ask Abby. He does not need to, because the count is not in her head. It is in a place they both agreed on, which is what makes it useful to more than one of them.

That is all a variable is. The robot cannot hold a number in its head either, so you give it a wall of its own, write a name at the top so everyone knows which wall is which — score, count, degree_turn — and the program can read what is on it and write something new. One part of the program writes; another part reads. Exactly Abby and Ben.

The paper, and the two things you can do to it

Say we are counting rotations of a motor. Before we start we write 0 on the paper. Every time the motor completes a turn we cross out what is there and write one more: 0 becomes 1, then 2, then 3. That is change — it has to read the old number to work out the new one.

set is the other thing you can do, and it is completely different: rub the whole paper out and write the number you want. It does not care what was there. Press the buttons and watch what happens to the crossings-out.

• change leaves a trail — every value follows from the one before it. This is what counting is.
• set wipes the sheet. Use it to start a count, never to continue one.
• Press set score to 0 after counting up a few times and watch the whole history vanish. That is what happens to a count when a set block ends up in the wrong place — and it is the commonest variable bug there is.

Blocks reference

set [count v] to (0)

change [count v] by (1)

(count)

Set, or change?

set replaces; change adds. Counting things needs change. Starting a count needs set. Both programs below have both blocks — the only difference is whether the set block is inside the loop or above it.

The count on the right is not broken; it is being told to start again on every pass. Each time round the loop it is wiped back to zero and then changed by one, so the honest answer is always 1 — while the motor cheerfully turns four times. A counter stuck at 1 almost always means a set block that has slipped inside the loop.

Anything oval is a number you can pick up

EV3 Classroom tells you what a block does by its shape, and once you have noticed that, a whole set of questions answers itself:

• Oval — reports a number. The blue degrees counted, the timer, a distance, your own variable.
• Pointed — reports true or false. These go in an if or a wait until, not in a variable.
• Block-shaped — does something. These stack up; they do not fit inside anything.

So when a slot is oval, any oval fits it — and it does not matter in the least where that number came from. You can take the motor’s own A degrees counted and keep it in a variable you named degree_turn, then compare that with a number later. Pick an oval below and watch the same one drop into all three kinds of slot.

This is what makes a variable more than a counter. A sensor reading is true only at the instant you read it; copying it into a variable freezes it, so the robot can compare where it is now against where it was when something happened:

when program starts :: events hat
[A v] reset degrees counted :: motors
set [degree_turn v] to ([A v] degrees counted :: sensors)
start moving [right: 30] :: movement
wait until <(([A v] degrees counted :: sensors) - (degree_turn)) > (400)>
stop moving :: movement

Read the condition aloud: how far the motor has gone now, minus where it was when we started, is more than 400. Both are ovals, so both can go into a subtraction, and the subtraction is an oval too — which is why it can go into a comparison. Ovals nest inside ovals as deep as you need.

Reset at the start, every time

A variable keeps its value after the program ends. Run the program again without setting it back and the second run begins where the first left off — the count starts at 14, the robot thinks it has already done the job. Every variable a program changes must be set to its starting value at the top.

Why it matters

A variable is the difference between a machine that repeats a fixed routine and one that responds to how things have gone — counting parts, tracking a score, remembering where it started.