Instructions a computer follows exactly

You are telling someone over the phone how to draw a simple house: a square with a triangle roof. They can't see your paper. They can't ask questions. Whatever you say, they draw. How do you make sure the drawing comes out right?

Every program is in the same position. The computer is the person on the other end of the phone.

To a person, "draw a house" is enough. To a computer, those words mean nothing. It needs the small steps, in order:

• draw a square, 4 cm on each side
• on top of the square, draw a triangle the same width
• done

The computer is fast, and it never gets tired. It takes every word as it is written, and it never fills a gap on its own.

"Put on socks, then shoes" and "put on shoes, then socks" use the same words and give very different results. Steps run top to bottom, one at a time. If you swap two lines, the result can change a lot. The computer won't warn you. It just does line 1, then line 2, then line 3.

Here are directions to a friend's place. One step is too vague for someone who takes every word literally. Which one is it, and how would you fix it?

1. Walk to the end of the street.
2. Turn toward the tall building.
3. Go to the third door.

Answer: step 2. "Toward the tall building" assumes the person can see the building and knows which one you mean. An exact step would be "turn left." Everything you write for a computer has to pass the same test.

A computer follows exact steps, in order, with no guessing. When you write an algorithm, your one job is to leave it no gaps and no chance to read you wrong. A vague step breaks sooner or later. Aim for exact, even if it is boring.