Rubber duck debugging

By Marc Littlemore
3 min read

I wrote video games for about 20 years and for a long time I made games for the Sony PlayStation consoles.

Sony were famous for using rubber ducks in their console announcement demos. Here's some old footage of the PlayStation 3 announcement from the wonderful Phil Harrison:

At Sony's Studio Liverpool, we had hundreds of rubber ducks in the offices. They became part of our debugging process.

Yes, I used to talk to them when I had coding problems. šŸ¦†

Writing computer software is a very precise skill. You have to be explicit in your code and clear in the logic to get the results you want. Humans are often not so good at this. Requirements can seem ambigiously or perhaps we don't have enough details before we start coding a solution. It's easy to become blind to errors that we've created. Our code can be incorrect but it can be difficult to understand why.

The idea of rubber duck debugging is that when you encounter a problem in your code, you talk through your code to a rubber duck. It doesn't have to be a duck. It can be another inanimate object, your children, another member of your team, your partner, or especially in these times of working from home, it could be your cat, dog, or hamster. šŸ¹

So what do you need to do?

1. Read through your code out loud

Explain your code at a high level to your duck friend. Tell it what the code is meant to do if it was working. This helps you to set the understanding for the problem at hand.

2. Explain each line and functions

Next you should try and explain each line on its own. Describe the flow of your function and what each line does. Say how it changes any variable values or state within the function. Don't skip any of the details. Ducks like details.

3. Discuss any state or global variables

If your code isn't purely functional, you may rely on state or global variables from outside of this code. Describe what this is, where it is set, and what its values should be.

At this point you should have your "DOH!" moment. Talking about the code out loud will help you to realise the assumptions you've made which might not be true. Maybe you'll spot the typo where you've misnamed a variable. Or perhaps you'll see that your logic is incorrect.

Your duck will have helped you to find your problem! Aren't ducks great? šŸ¤

Why does rubber duck debugging work?

You'll be amazed that this type of debugging actually works. I've often explained a problem to one of my team and as I explain each line, I end up discovering the issue. While writing new code, you can easily enter a state of flow, also known as "being in the zone". In this state, your brain is thinking quickly and your hands are busy typing. You think you understand what you need to do but you might miss the details.

Once you start explaining a problem to your rubber duck, your brain slows down to read and understand what's going on. You think a lot faster than you talk. Talking out loud helps you to focus on what's gone wrong.

Try out rubber duck debugging when you next have a problem. You'll find that you quickly gain clarity on what your code does rather than what you think it does. And don't forget to bring some bread to feed your duck. šŸ¦†

Marc Littlemore avatar

I'm Marc Littlemore.

Iā€™m a Senior Software Engineering Manager who works with high performing development teams and loves to help to grow other software leaders and engineers.


Want to read more?