While loop execution flow in Python - Step-by-Step Execution

Concept Flow - While loop execution flow

Initialize variable

↓

Check condition

↓

Execute loop body

↓

Update variable

↩Back to Check condition

The while loop starts by initializing a variable, then checks a condition. If true, it runs the loop body, updates the variable, and repeats. If false, it exits.

Execution Sample

Python

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

This code prints numbers 0, 1, and 2 by looping while i is less than 3.

Execution Table

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

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

Variable Tracker

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

Key Moments - 2 Insights

Why does the loop stop when i is 3 even though the code inside the loop updates i?

The condition i < 3 is checked before each loop iteration. When i becomes 3, the condition is False, so the loop exits before running the body again (see execution_table step 4).

What happens if we forget to update i inside the loop?

If i is not updated, the condition i < 3 stays True forever, causing an infinite loop. The execution_table shows i changing each step to avoid this.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of i at step 3?

A 2

B 3

C 1

D 0

💡 Hint

Check the 'i value' column in execution_table row 3.

At which step does the condition become false and the loop stops?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Condition' column in execution_table where it shows False.

If we change the condition to i < 5, how would the variable_tracker change?

A i would stop at 3

B i would go up to 5

C i would stay at 0

D i would decrease

💡 Hint

Variable i increases each loop until condition is False; changing condition to i < 5 means i goes up to 5.

Concept Snapshot

while condition:
    # loop body

- Checks condition before each loop
- Runs body only if condition True
- Updates variables inside loop to avoid infinite loops
- Stops when condition becomes False

Full Transcript

This visual shows how a while loop works in Python. We start with a variable i set to 0. The loop checks if i is less than 3. If yes, it prints i and adds 1 to i. This repeats until i reaches 3. Then the condition is false and the loop stops. The execution table tracks each step, showing i's value, the condition check, the action taken, and output printed. The variable tracker shows how i changes after each loop. Key moments explain why the loop stops and what happens if we forget to update i. The quiz tests understanding of i's value at steps, when the loop ends, and effects of changing the condition.

Practice

1.

What does a while loop do in Python?

easy

A. Stops the program immediately

B. Runs code only once

C. Repeats code as long as a condition is true

D. Runs code a fixed number of times

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs repeatedly while its condition is true.

2. Step 2: Compare options with this behavior

   Only Repeats code as long as a condition is true describes repeating code while a condition is true.

3. Final Answer:

   Repeats code as long as a condition is true -> Option C

4. Quick Check:

   While loop = repeat while true [OK]

Hint: While loops run repeatedly while condition is true [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statement
• Believing while loops run only once

2.

Which of the following is the correct syntax to start a while loop in Python?

?

easy

A. while x > 0:

B. while (x > 0)

C. while x > 0 {}

D. while x > 0 then

Solution

1. Step 1: Recall Python while loop syntax

   Python uses a colon after the condition and no parentheses or braces.

2. Step 2: Match options with correct syntax

   while x > 0: uses 'while x > 0:' which is correct syntax.

3. Final Answer:

   while x > 0: -> Option A

4. Quick Check:

   Colon ends while condition line [OK]

Hint: Use colon after condition, no braces or 'then' [OK]

Common Mistakes:

• Adding braces {} like other languages
• Using 'then' keyword
• Putting condition in parentheses unnecessarily

3.

What will be the output of this code?

count = 3
while count > 0:
    print(count)
    count -= 1
print("Done")

medium

A. Done

B. 3 2 1 Done

C. 3 2 1 0 Done

D. 3 2 1 2 Done

Solution

1. Step 1: Trace the while loop iterations

   count starts at 3, prints 3, then decreases to 2, prints 2, then 1, then stops when count is 0.

2. Step 2: Note the final print after loop

   After loop ends, "Done" is printed.

3. Final Answer:

   3 2 1 Done -> Option B

4. Quick Check:

   Prints 3,2,1 then Done [OK]

Hint: Count decreases before loop ends, stops at zero [OK]

Common Mistakes:

• Expecting 0 to print inside loop
• Forgetting to decrease count
• Thinking loop runs forever

4.

Find the error in this code snippet:

i = 1
while i < 5
    print(i)
    i += 1

medium

A. Missing colon after while condition

B. Variable i is not initialized

C. Indentation error in print statement

D. Infinite loop because i never changes

Solution

1. Step 1: Check while loop syntax

   The while line must end with a colon (:).

2. Step 2: Identify the missing colon

   The code misses ':' after 'while i < 5', causing syntax error.

3. Final Answer:

   Missing colon after while condition -> Option A

4. Quick Check:

   While line needs colon [OK]

Hint: Always put colon after while condition [OK]

Common Mistakes:

• Forgetting colon after while
• Misindenting inside loop
• Not updating loop variable

5.

Consider this code:

n = 5
result = 1
while n > 1:
    result *= n
    n -= 1
print(result)

What does this code calculate?

hard

A. Sum of numbers from 1 to 5

B. Infinite loop with no output

C. Product of numbers from 1 to 4

D. Factorial of 5 (5!)

Solution

1. Step 1: Understand the loop's multiplication

   result multiplies by n each time, starting at 5 down to 2.

2. Step 2: Recognize factorial pattern

   Multiplying 5*4*3*2*1 equals 5 factorial (5!).

3. Final Answer:

   Factorial of 5 (5!) -> Option D

4. Quick Check:

   Multiplying down to 1 = factorial [OK]

Hint: Multiplying down from n to 1 calculates factorial [OK]

Common Mistakes:

• Thinking it sums numbers instead of multiplies
• Stopping multiplication at 2 instead of 1
• Assuming infinite loop