Online compiler, debugger and visualizer
Code visualizer
Press Visualize and your program is recorded as it runs. Then move forward and backward through every step and see frames, variables, arrays and objects drawn as boxes and arrows.
- Free
- No sign-up
- Real compilers
- Works on phones
def fib(n): a, b = 0, 1 for _ in range(n): a, b = b, a + b return a n = int(input("How many? "))print([fib(i) for i in range(n)])What you can do
Every step recorded
Move forward and back through the whole run, or jump to any step with the slider.
Memory drawn for you
Lists, arrays, dictionaries and objects appear as boxes with arrows between references.
Call stack
See each function call and its local variables, which makes recursion easy to follow.
Line by line
The editor highlights the line that just ran and the one that runs next.
Ask AI about a step
Not sure why a value changed? Ask the assistant about the step you are looking at.
How it works
- 1
Open the visualizer and write a short program.
- 2
Press Visualize.
- 3
Use the arrows or slider to move through the steps.
Questions and answers
Which languages can be visualized?
Java and Python.
Is it free?
Yes. Running, debugging, tests and live sessions are free, with no sign-up.
Where are my projects saved?
In your browser, on your device, automatically as you type. Nothing is lost when you close the tab. They stay until you delete them.
Is my code safe to run?
Every program runs in its own isolated sandbox with no internet access and strict limits on time, memory and processes, and is deleted when it finishes.