Names for Your Numbers

Try It — Count the Damage

Open your Project 8 program — the big one with all four missions in it. Scroll through it slowly.

Now break it

That last question is the real problem. If you missed one, what would happen — and how would you find out?

Learn It — Give the Number a Name

A variable is a name that holds a value. You set it once at the top of your program, then use the name everywhere instead of the number.

Here is the whole idea in one line:

int arm = 0;

Comments to code: three small edits

You have been writing your positions as comments since Project 7, in the name = number format. There was a reason for that. Turning a comment into a variable takes three changes:

• Delete the two slashes.
• Add int at the front.
• Add a semicolon at the end.

Before — comments

int main ()
{
	// arm  = 0
	// up   = 230
	// down = 1234

	...
}

After — variables

int main ()
{
	int arm  = 0;
	int up   = 230;
	int down = 1234;

	...
}

Then use the names

Before

set_servo_position(0, 1234);
msleep(500);
set_servo_position(0, 230);

motor(0, 50);
motor(3, 50);

After

set_servo_position(arm, down);
msleep(500);
set_servo_position(arm, up);

motor(left, 50);
motor(right, 50);

Read both versions out loud. One is a list of numbers. The other tells you what the robot is doing.

Five reasons this is worth the trouble

1. You do not have to remember which port is the arm and which is the claw — the computer remembers for you.
2. You do not have to remember which number is up, down, open, or closed.
3. Your program is easier to read.
4. Your program is easier to debug.
5. If a port or a position changes, you change one line.

Name things properly

Short names are fine if they are obvious. If they are not obvious, explain them:

// L is my left wheel
int L = 0;
// R is my right wheel
int R = 3;

A name nobody can decode is barely better than a bare number.

Variables can change

Most of your variables hold one value forever — a port is always the same port. But a variable is called a variable because it can vary.

int position = 250;

// Now 260
position = position + 10;
// Back to 250
position = position - 10;

Adding or subtracting one is so common it has its own shorthand:

You will meet the real use for these in Project 10, once you have a loop to put them in.

Do It — Clean It Up, Then Score

Step 1 — Convert one small program first

Open your Wave program from Project 7. Turn the three position comments into variables, then replace the numbers in the set_servo_position() lines with the names.

Compile. Run. It should behave exactly as before.

• Wave runs the same with variables as it did with numbers

Step 2 — Trigger the yellow banner on purpose

Add a variable you do not use anywhere:

int notused = 999;

Compile and look at the banner colour.

Delete the line. Get your green banner back.

Step 3 — Convert your big Project 8 program

This is the real job. Declare all your variables at the top, then work down the program replacing bare numbers with names.

• My whole Project 8 program uses variables and still scores the same

Step 4 — Now do the claw change again

Same problem as Try It. Your claw’s closed position drops by 200. Change it.

Compare those two numbers with the ones you wrote in Try It.

Step 5 — Try changing a variable while the program runs

Small experiment. Add this to a test program and watch the arm.

int arm = 0;
int position = 250;

set_servo_position(arm, position);
enable_servos();
msleep(500);

// Change the variable
position = position + 300;
set_servo_position(arm, position);
msleep(500);

// Change it again
position = position + 300;
set_servo_position(arm, position);
msleep(500);

disable_servos();

The arm moved in three hops instead of one jump. What would you have to do to make it move in thirty small hops?

Hold that thought. Project 10 gives you the tool that makes it one line instead of thirty.

Step 6 — Mission 5 — Top Shelf Delivery for 20 points

Where do your red cubes end up after your Mission 2 push? Does that help or hurt you here?

• One Small Red Cube is ON TOP OF the Large Red Cube
• Both Small Red Cubes are up there at the same time

The bonus needs both cubes up there at the same time. Why does that make the second one harder than the first?

Step 7 — Run it five times

Score It — Checkpoint

My score

Write the variable

Turn each comment into a proper variable declaration.

Read the code

Given int arm = 0; and int down = 1234; — what does each line do?

Can you do it again?

• I can declare a variable with int, a name, a value, and a semicolon
• I know variables go inside the block, at the top of main
• I can turn a // name = number comment into a variable in three edits
• I use variable names inside motor() and set_servo_position() instead of bare numbers
• My variable names make sense to someone else on my team
• I know what a yellow warning banner means and what usually causes it
• I can change a variable’s value part-way through a program

Think about it

Nothing your robot does changed in this project — it drives, grabs, and stacks exactly as it did before. So what actually got better?

Variables fixed the problem of one number appearing in many places. But your program still has the six-step grab sequence typed out four separate times. Would variables fix that too? Why or why not?

Your teammate opens your program for the first time. Which version would they be able to work on — the one from Project 8, or this one? What does that tell you about who you are really writing code for?

Next

Your robot still cannot tell when it has arrived anywhere. It drives for a length of time and hopes.

In Project 10 — Feeling for Things, it gets its first sense: a switch that knows when it has touched something — and the loop that keeps checking.

KIPR · Botball Explorer — Discovery Projects · © KISS Institute for Practical Robotics 1997–2026