--- title: "Run and walk through code | DuckyHelper" url: "https://duckyhelper.com/features/code/" description: "DuckyHelper runs your code against tests and plays it back step by step, every variable as it changes, so you see why it gives the wrong answer." updated: "2026-10-09" --- Mac app # Run and walk through code Your loop gives 45 and it should give 55. Ask DuckyHelper why, and it runs your code against tests, each one ticking in or showing expected against what it got. Then it plays your code back one step at a time, every variable as it changes, until you see it: the loop stopped one short. You fix it, and both tests pass. [Download for Mac](https://duckyhelper.com/download/)[Get the Mac app](https://duckyhelper.com/download/) Tests ## Expected, and what it got. Say run my code against these tests, or why does my loop give 45, not 55? Ducky runs it against tests. Each one ticks in green or opens to show what was expected and what your code returned, so you know exactly which case breaks. [Guide: Learn to code](https://docs.duckyhelper.com/study/code/) A sandbox with no internet Your code runs in a sandbox with no internet, so nothing it does can reach your files or the web. Your editor, any site On a Mac, ask about the code on your screen, in VS Code, a coding site or a notebook. In Eddy, paste it or ask Eddy to write the starting code with you. Step through ## Watch it run, one line at a time. Ask show me what this loop does and a player opens: the line that runs next is highlighted, and every variable, array and pointer is drawn as it changes. Play it, pause it, drag to any step. Watching total climb and i stop at 9 tells you more than any explanation. In Eddy ## A Code window on Eddy's desk. In Eddy, code opens in a Code window on the desk that runs: add_up, one line at a time, with the variables beside it and the output below. It is in the Mac app's Eddy tab. Write it with Ducky ## Walk me through writing this. On a Mac, ask Ducky to walk you through writing a piece of code. It says what the next line does, then ghosts that small chunk at your cursor in your own editor. Type it, and the next step follows. You end up with code you wrote, one chunk at a time. [The typing coach](https://duckyhelper.com/features/typing-coach/) ## Run it. Watch it. Fix it. How a code lesson goes. 1. 1. Ask why it is wrong Press fn on your editor, or paste it into Eddy. 2. 2. Run the tests Each one passes, or shows expected against got. 3. 3. Step through Watch the variables until you see it. 4. 4. Fix it yourself Run again until every test passes. ## See it run, then fix it. Seeing a program run one step at a time makes the invisible part of code, what the computer is holding at each moment, something you can look at. That is where most bugs become obvious. The learning science behind it: [Show a worked example first](https://duckyhelper.com/research/principles/worked-examples/). [Computer science with DuckyHelper](https://duckyhelper.com/subjects/computer-science/) ## How to use it ## Frequently asked questions ### Is there an AI coding tutor that explains my code? DuckyHelper runs your code against tests, shows expected against what it got, then plays it back step by step with every variable as it changes, so you see where it goes wrong. It explains out loud as it goes, and leaves the fix to you. ### Which languages can it run? It runs Python and JavaScript against tests, and the step-through player shows how Python runs. For code in other languages, DuckyHelper can still read it with you, explain it line by line and walk you through writing it. ### Is there a code visualizer for learning? Yes. Ask show me what this loop does and a step-through player opens: the line that runs next is highlighted, and variables, arrays and pointers are drawn as they change. Play, pause, or drag to any step. ### Can it help me practice for coding interviews? Yes. Talk through a problem out loud, run your solution against tests, and step through it when a case fails. On a Mac it works in any editor or coding site. [Coding interviews with DuckyHelper](https://duckyhelper.com/for/coding-interviews/) has a plan. ### Is my code safe? Your code runs in a sandbox with no internet, so it cannot reach your files or the web. On a Mac, Ducky reads code from your screen only while the pill is open. ## More features ### Ask out loud and hear the answer: press fn on a Mac, or talk to Eddy in the app's Eddy tab Ask DuckyHelper out loud and it answers out loud while it shows you the steps. On your Mac, press fn in any app, or talk to Eddy in its tab. ### Write your working and get it checked line by line, or answer a quick check on Eddy's whiteboard Write your working and DuckyHelper checks it: the first wrong line marked with why on a Mac, and Eddy's whiteboard for a quick drawing or check. ### Say quiz me: tap, type math or answer true or false, and every miss comes with why Say quiz me and DuckyHelper writes a quick quiz on what you are learning: tap, type math or answer true or false, and every miss comes with why. ### Worked steps, one move each, that you can keep, and graphs you can hover, zoom and slide Ask DuckyHelper to work it out and get worked steps, one move each, plus graphs you can hover, zoom and change with sliders. Keep the steps as a note. ### Small sandboxes you run yourself: drag the ball, move a slider, read the numbers Ask DuckyHelper to show you how a pendulum, a projectile or a circuit works and get a small sandbox to run yourself: drag, change a slider, read the numbers. ### It asks back and gives the next hint before an answer, so you do the thinking DuckyHelper asks back and gives the next hint before it gives an answer, so you do the thinking. Why that matters, and how it works on a Mac and in Eddy. ## Keep reading ### Computer science: run it, step through it, and write it yourself An AI computer science tutor: run your code against tests, step through it line by line to see the variables change, and get hints instead of the solution. ### Coding interviews: talk it through, run it against tests, step through the bug Practice coding interviews out loud: talk through your plan, get one hint at a time, run your code against tests and step through where it breaks. ### Grey ghost letters at your cursor in any Mac app; type over them and fix the slips yourself DuckyHelper's typing coach puts grey ghost letters at your cursor in any Mac app. Type over them; a slip is marked in red so you can fix it. ### Every DuckyHelper feature: what it does, where it runs, and a short film of each What DuckyHelper for Mac does: Eddy teaches out loud on a blackboard, and Ducky sees your screen, draws on it and walks you through your own apps. ## Find the bug yourself Press fn on your editor on a Mac, or open the Eddy tab, and ask why it gives the wrong answer. [Download for Mac](https://duckyhelper.com/download/) [See what it does](https://duckyhelper.com/features/) The Mac app needs Apple silicon and macOS 14 Sonoma or later. There is no Chromebook, Windows or web version right now.