Counter-based while loop in Python - Step-by-Step Execution

Concept Flow - Counter-based while loop

Initialize counter = 0

↓

Check: counter < limit?

No→EXIT

Yes↓

Execute loop body

↓

Update counter (e.g., counter += 1)

↩Back to Check

Start with a counter, check a condition before each loop, run the loop body if true, then update the counter and repeat until the condition is false.

Execution Sample

Python

counter = 0
while counter < 3:
    print(counter)
    counter += 1

Prints numbers 0, 1, 2 by increasing counter until it reaches 3.

Execution Table

Step	counter	Condition (counter < 3)	Action	Output
1	0	True	Print 0, counter = 1	0
2	1	True	Print 1, counter = 2	1
3	2	True	Print 2, counter = 3	2
4	3	False	Exit loop

💡 counter reaches 3, condition 3 < 3 is False, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
counter	0	1	2	3	3

Key Moments - 3 Insights

Why does the loop stop when counter equals 3?

Because the condition counter < 3 becomes False at step 4 (see execution_table row 4), so the loop exits.

Why do we update the counter inside the loop?

Updating counter (counter += 1) inside the loop changes its value each time, eventually making the condition False to stop the loop (see execution_table actions).

What would happen if we forget to update the counter?

The condition would always be True, causing an infinite loop because counter never changes (no progress in execution_table).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of counter at Step 3?

A 1

B 3

C 2

D 0

💡 Hint

Check the 'counter' column in execution_table row 3.

At which step does the loop condition become False?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look at the 'Condition' column in execution_table to find when it is False.

If we change the condition to counter < 5, how many times will the loop run?

A 5 times

B 3 times

C 4 times

D 6 times

💡 Hint

The loop runs while counter is less than the limit; see variable_tracker for how counter changes.

Concept Snapshot

Counter-based while loop syntax:
counter = start_value
while counter < limit:
    # loop body
    counter += 1

Loop runs while condition is True.
Update counter inside loop to avoid infinite loop.

Full Transcript

This visual trace shows how a counter-based while loop works in Python. We start by setting counter to zero. Before each loop run, we check if counter is less than 3. If yes, we print the counter and increase it by one. This repeats until counter reaches 3, when the condition becomes False and the loop stops. The execution table tracks each step, showing counter values, condition checks, actions, and outputs. The variable tracker shows how counter changes from 0 to 3. Key moments explain why the loop stops, why updating counter is important, and what happens if we forget to update it. The quiz questions help check understanding by asking about counter values at specific steps, when the loop ends, and how changing the condition affects loop runs.