Variables: Name a Number, and Let the Player Type It

Last lesson ended with a bug waiting to happen.

Your wall needed the number 10 in two places — once in the inner repeat, and once in the move back that walks the Agent home. Change one and forget the other, and the wall comes out leaning like a staircase. Nothing warns you. Nothing goes red.

Today that stops being possible.

A variable is a number with a name

A variable is a box with a name on it, and a number inside.

You put a number in the box once, at the top of the program. Everywhere you would have typed the number, you use the name instead. The Agent looks in the box each time it needs the value.

So the number exists in one place. Change that one line and everything that uses the name changes with it — because there was only ever one number to begin with.

The second half: let the player fill the box

Here is the better trick. If the number lives in one place, why should that place be inside your code at all?

The on chat command block has a small (+) on it. Click it and the command grows a number you type in the chat box. Type wall 12 and the program receives 12. Type wall 30 and it receives 30.

You never open Code Builder again. The program asks; the player answers.

One idea, wearing two hats

That is why these two things are one lesson and not two:

	Who puts the number in the box?
a variable you set	you, in the program, once
a chat number	the player, when they run it

Both are a named box holding a number. The only difference is who fills it. In fact the chat number is a variable — it arrives called num1, and you can rename it to whatever you like.

Warm-up — the recipe

A recipe says serves 4 at the top, then lists amounts all the way down. To cook for eight, a good recipe makes you change one number at the top. A bad one makes you double thirty lines and hope you did not miss any.

Your wall last lesson was the bad recipe. Today you write the good one.

What you will be able to do

1. Make a variable, give it a name, and put a number in it.
2. Use that name anywhere a number used to be.
3. Resize a whole build by editing one line.
4. Add a number to a chat command so the player chooses the size without touching the code.

Follow along on your own computer. Everyone should finish this part with the same working program.

Step 1 — Find the number that repeats

Open your wall program from Lesson 4:

Two tens. They are not two numbers that happen to match — they are the same number written twice, and that is the problem.

Step 2 — Make a variable

1. Press C and open your project.
2. Click the Variables drawer — it is dark red.
3. Click Make a Variable…, type size, and click OK.

Three new blocks appear in the drawer, including set size to 0.

Step 3 — Fill the box once

Drag set size to 0 to the very top of your program, above the teleport, and change the 0 to 10.

It goes outside every loop, for the same reason the teleport does: setting it once is the whole point.

Step 4 — Use the name instead of the number

Now drag the rounded size block out of the Variables drawer and drop it on top of each 10 — once in the inner repeat, once in the move back.

A tower to order.

Your tower from Lesson 3 is always the same height. Make the player choose.

1. Press C and start a new chat command called tower.
2. Click the (+) on the command block and rename the number it gives you to height.
3. Build a tower that is height blocks tall, using the name — not a number — wherever the height is needed.
4. Bring the Agent back to the ground at the end, whatever the height. You solved this exact problem in Lesson 3; the same solution still works.
5. Go back to the world, press T, and type tower 8. Then tower 20. Then tower 3.

It works if: typing tower followed by a number builds a tower of exactly that many blocks, three different numbers give three different towers, and there is no number typed into the program anywhere that decides the height.

A room to order.

Lesson 4's room was 5 by 5. Now the player decides.

1. Press C and start a new chat command called room.
2. Click the (+) and rename the number to side.
3. Build the hollow room from Lesson 4 — four walls, no roof, no floor — but 2 blocks tall this time, with side deciding how long each wall is.
4. Think carefully about which loops should use side and which should keep a plain number. Not every number in the program is the same number.
5. Type room 4, then room 6, then room 8.

Keep the number you type at 8 or less. Four walls of 8, two levels high, is 64 blocks — the whole slot. Ask for more and the room will stop halfway round, and now you know why.

It works if: three different numbers give three different hollow rooms, each one square with all four walls closed, and the height stays 2 whatever you type.

Two numbers, one command.

One number was useful. Two is a building you can describe out loud.

1. Press C and start a new chat command called hut.
2. Click the (+) twice. You get two numbers. Rename them side and height.
3. Build a hollow hut with four closed walls, where side decides how long each wall is and height decides how many levels tall it is.
4. Before you test it, work out the largest side and height you can safely ask for, and write it down. Four walls, side long, height high — and 64 blocks in the slot.
5. Type hut 4 3. Then try hut 6 2. Then try the biggest pair you worked out in step 4.

It works if: hut 4 3 and hut 6 2 build two visibly different huts, each with four closed walls, and you can say out loud which typed number controls which part of the building.