Decisions

What this is

Everything before this runs the same way every time, whatever the values are. This is where a program learns to decide: to ask a question, and do something only when the answer is yes.

Four units take that from a single test to a chain of them, to keeping an answer in a box for later, to joining questions so one test can ask about several things at once.

Who it’s for

Anyone who has finished Working It Out. The questions a program asks are comparisons between values, so you need to be comfortable with values first.

The shape

• Making a choice — IF, and the questions =, < and >.
• One or the other — ELSE, and a chain of questions with one answer.
• A yes you can keep — TRUE and FALSE as values, and the flag that carries an answer from where it was found to where it is needed.
• Asking two things at once — AND, OR and NOT, and brackets that say what to work out first.

Next comes Repeating. If your tests are getting tangled, Truth Tables is a short side-path from here that shows you how to be sure what a test does.