A common way to debug, especially early on, is to stare at the code, guess what might be wrong, change something, and re-run it to see if the problem went away. This works sometimes, but it's slow, and it doesn't get much faster with experience — because it isn't really a method, it's trial and error.
A more reliable process looks less like guessing and more like narrowing down a search space:
- Reproduce it reliably first. Before trying to fix anything, find the smallest, most consistent way to trigger the bug. A bug you can reproduce on demand is far easier to fix than one that "sometimes happens."
- Form a specific hypothesis. Instead of "something's wrong with the login," aim for "I think the password check is comparing the wrong two values." A vague hypothesis leads to vague, scattered changes.
- Check the hypothesis directly. Print the values involved, use a debugger to pause and inspect them, or write a small isolated test — whatever confirms or rules out your specific guess, rather than changing code and hoping.
- Change one thing at a time. If you fix several suspected issues at once, you won't know which change actually mattered, and you risk introducing a new problem alongside the fix.
- Once it's fixed, ask why it happened. The same category of mistake tends to repeat. A minute spent understanding the root cause often prevents several future bugs of the same shape.
This process is slower than a lucky guess, but far faster than an unlucky streak of them — and unlike guessing, it gets more effective the more you practice it.