How to Find a Bug: A Simple Method for Beginners

When code fails, most beginners start changing things and hoping. It sometimes works, but you learn nothing, and you may add a second bug. Engineers follow a method.

Read the error message

Read all of it, slowly. It usually tells you the type of error, the file and the line. Many students skip this step and lose an hour.

Make the bug happen every time

Find the exact steps that cause it. If you cannot repeat a bug, you cannot be sure that you fixed it.

Cut the problem in half

Check the data halfway through the program. If it is right there, the bug is later. If it is wrong, the bug is earlier. Keep halving until only a few lines are left.

Check what you assumed

Print the real values, or use a debugger. Does the variable hold what you think it does? Is that function even being called? Bugs often hide in the things you were sure about.

Change one thing at a time

If you change three things and it works, you will not know which one fixed it. Change one, test, and undo it if it did not help.

Understand the cause before you move on

Ask why it happened, and whether the same mistake is elsewhere. If AI suggests a fix, make sure you can explain it first. You can practise this kind of thinking on real situations in the Engineering Lab.