While and do-while loops in Kotlin - Step-by-Step Execution

Concept Flow - While and do-while loops

Initialize counter

↓

Check condition

↓

Execute body

↓

Update counter

↩Back to Check

The loop starts by checking a condition. If true, it runs the code inside, updates variables, then checks again. If false, it stops.

Execution Sample

Kotlin

var i = 1
while (i <= 3) {
  println(i)
  i++
}

var j = 1
do {
  println(j)
  j++
} while (j <= 3)

This code prints numbers 1 to 3 using a while loop and then again using a do-while loop.

Execution Table

Step	Variable i	Condition i <= 3	Action	Output
1	1	true	Print i=1, i++	1
2	2	true	Print i=2, i++	2
3	3	true	Print i=3, i++	3
4	4	false	Exit loop

💡 i reaches 4, condition 4 <= 3 is false, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	1	2	3	4	4
j	1	2	3	4	4

Key Moments - 2 Insights

Why does the while loop sometimes not run at all?

Because the condition is checked before the loop body. If false at start (see Step 4 in execution_table), the loop body never runs.

Why does the do-while loop always run at least once?

Because the loop body runs first, then the condition is checked. So even if condition is false initially, body runs once.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of i at Step 3?

A 4

B 2

C 3

D 1

💡 Hint

Check the 'Variable i' column at Step 3 in execution_table

At which step does the while loop stop running?

A Step 3

B Step 4

C Step 2

D Step 1

💡 Hint

Look at the 'Condition i <= 3' column and see when it becomes false

If the initial value of i was 5, what would happen to the while loop?

A It does not run at all

B It runs once

C It runs three times

D It runs infinitely

💡 Hint

Refer to key_moments about condition checked before loop body

Concept Snapshot

while (condition) {
  // code runs while condition is true
}

- Condition checked before loop body
- May run zero or more times

do {
  // code runs at least once
} while (condition)

- Condition checked after loop body
- Runs one or more times

Full Transcript

This lesson shows how while and do-while loops work in Kotlin. The while loop checks the condition first. If true, it runs the code inside and updates variables, then checks again. If false, it stops immediately. The do-while loop runs the code first, then checks the condition. This means do-while always runs at least once. We traced a simple example printing numbers 1 to 3. The variable i starts at 1 and increases each loop. The while loop stops when i becomes 4 because 4 <= 3 is false. Key points: while loop may not run if condition is false at start; do-while runs once no matter what. The quiz asks about variable values and loop stopping points to reinforce understanding.