Challenge type: Python Puzzlers · Difficulty: ★★★☆☆ Medium · Topic: nested loops
Read this Python program carefully. Don’t run it yet! Can you work out exactly what it prints?
for row in range(3):
line = ""
for col in range(4):
line = line + "o"
print(line)Choose an answer
| Choice | Output |
|---|---|
| A | oooo oooo oooo |
| B | ooo ooo ooo ooo |
| C | oooooooooooo |
| D | ooooo ooooo ooooo |
Need a hint?
Pretend you are the computer. Go through the program one line at a time, and write down every variable’s value on paper as it changes. Programmers call this a trace.
Grown-ups: Reading code and predicting what it does is a key programming skill. Once your child has chosen an answer, run the code in IDLE together to check.
For teachers and grown-ups
Skills practised: Code reading, tracing variables, and understanding Python syntax.
The big idea: Predicting a program's output before running it builds a solid mental model of how code works.
Questions to ask:
- What is the value of each variable after every line?
- Which line decides the output?
- What small change would make it print something different?
Take it further: Children use a trace table on paper for the code, then run it in IDLE to check, and change one value to make a new puzzle for a friend.