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Two bus stops need the same three lines.

This track is Functions and Debugging. It has two halves. First you build a function. Then you find out why a program is wrong.

Debugging means finding and fixing bugs, the mistakes in a program. To hand a function a value means to give it one. To hand back means to give back.

What you will learn

• A name for steps. function gives a group of lines one name.
• What you hand it. A parameter starts empty. An argument fills it.
• The value it hands back. return is not the same as print.
• One call inside another. The inner call finishes first.
• A print in the middle. It shows what a box holds there.
• One line at a time. A table shows every box.
• Predict, then run. Work out the answer before the program does.
• Halve the hunt. Check the middle of the lines, then check again.

Need a quick recap?

• Typing it out — you type each line yourself.
• `let` and `const` — a box that can change, and one that cannot.
• Finding a bug — a bug is a mistake in a program. Read a line, then check what it did.