When with multiple conditions in Kotlin - Step-by-Step Execution

Concept Flow - When with multiple conditions

Start

↓

Evaluate expression

↓

Check first condition

| Yes↓

Execute first branch

↓

End

No↓

Check second condition

| Yes↓

Execute second branch

↓

End

No↓

Check multiple conditions combined

| Yes↓

Execute combined branch

↓

End

No↓

Execute else branch

↓

End

The 'when' expression evaluates an input and checks conditions one by one, including multiple conditions combined with commas, executing the matching branch and then ending.

Execution Sample

Kotlin

val x = 3
when (x) {
  1, 2 -> println("One or Two")
  3, 4 -> println("Three or Four")
  else -> println("Other")
}

This code checks the value of x and prints a message depending on which group of values x matches.

Execution Table

Step	Expression Value	Condition Checked	Condition Result	Branch Taken	Output
1	3	1, 2	False	No
2	3	3, 4	True	Yes	Three or Four
3	3	else	Skipped	No

💡 Condition '3, 4' is True, branch executed, when expression ends.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	3	3	3	3

Key Moments - 2 Insights

Why does the 'when' stop checking after the first true condition?

Because 'when' executes only the first matching branch and then exits, as shown in execution_table step 2 where it stops after matching '3, 4'.

How do multiple conditions work in one branch like '1, 2 ->'?

Multiple conditions separated by commas mean 'or'. If the expression matches any of them, that branch runs, as seen in step 1 checking '1, 2'.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 2?

A "One or Two"

B "Other"

C "Three or Four"

D No output

💡 Hint

Check the 'Output' column at step 2 in the execution_table.

At which step does the 'when' expression stop checking conditions?

A Step 2

B Step 3

C Step 1

D It checks all steps

💡 Hint

Look at the 'Branch Taken' column to see when a branch is executed and the rest skipped.

If x was 5, which branch would be taken according to the execution table logic?

A "Three or Four"

B "Other"

C "One or Two"

D No branch

💡 Hint

Consider that 5 does not match any listed conditions, so 'else' branch runs.

Concept Snapshot

when (value) {
  condition1, condition2 -> action1
  condition3, condition4 -> action2
  else -> defaultAction
}
- Checks conditions in order
- Multiple conditions separated by commas mean OR
- Executes first matching branch and stops
- else handles all unmatched cases

Full Transcript

The Kotlin 'when' expression evaluates a value and compares it against multiple conditions. Each condition can have multiple values separated by commas, meaning if the value matches any of them, that branch runs. The checking stops at the first true condition. If none match, the else branch runs. For example, if x is 3, it does not match '1, 2' but matches '3, 4', so it prints 'Three or Four' and stops checking further.

Practice

1.

What does the when expression do when multiple conditions are separated by commas in Kotlin?

easy

A. It throws an error if multiple conditions are used.

B. It checks if the value matches any of the listed conditions and executes the corresponding block.

C. It only checks the first condition and ignores the rest.

D. It requires all conditions to be true at the same time to execute the block.

Solution

1. Step 1: Understand how when handles multiple conditions

   In Kotlin, when multiple conditions are separated by commas, it means the when expression checks if the value matches any one of those conditions.

2. Step 2: Interpret the behavior of multiple conditions

   If the value matches any condition in the list, the corresponding block runs. It does not require all conditions to be true simultaneously.

3. Final Answer:

   It checks if the value matches any of the listed conditions and executes the corresponding block. -> Option B

4. Quick Check:

   Multiple conditions = any match triggers block [OK]

Hint: Multiple conditions mean OR logic, not AND [OK]

Common Mistakes:

• Thinking all conditions must be true at once
• Believing only the first condition is checked
• Assuming multiple conditions cause errors

2.

Which of the following is the correct syntax to check if a variable num is either 1, 2, or 3 using when with multiple conditions?

when (num) {
    ? -> println("Number is 1, 2, or 3")
    else -> println("Other number")
}

easy

A. 1, 2, 3

B. 1 || 2 || 3

C. 1 && 2 && 3

D. [1, 2, 3]

Solution

1. Step 1: Recall Kotlin when syntax for multiple conditions

   In Kotlin, multiple conditions in a when case are separated by commas without operators.

2. Step 2: Match the correct syntax

   1, 2, 3 uses commas to separate values: 1, 2, 3, which is correct. Other options use logical operators or list syntax, which are invalid here.

3. Final Answer:

   1, 2, 3 -> Option A

4. Quick Check:

   Multiple conditions use commas [OK]

Hint: Use commas, not operators, to separate conditions [OK]

Common Mistakes:

• Using || or && inside when conditions
• Trying to use list syntax like [1, 2, 3]
• Missing commas between values

3.

What will be the output of the following Kotlin code?

val x = 5
when (x) {
    1, 2, 3 -> println("Low")
    4, 5, 6 -> println("Medium")
    else -> println("High")
}

medium

A. Medium

B. High

C. Low

D. No output

Solution

1. Step 1: Identify the value of x

   The variable x is assigned the value 5.

2. Step 2: Match x with when conditions

   The second case checks if x is 4, 5, or 6. Since x is 5, this condition matches and prints "Medium".

3. Final Answer:

   Medium -> Option A

4. Quick Check:

   5 in 4,5,6 prints Medium [OK]

Hint: Check which group contains the value [OK]

Common Mistakes:

• Choosing the first matching group incorrectly
• Confusing else with matching values
• Assuming no output if multiple conditions exist

4.

Find the error in this Kotlin when expression:

val grade = 'B'
when (grade) {
    'A', 'B' -> println("Excellent")
    'C' || 'D' -> println("Good")
    else -> println("Try again")
}

medium

A. The variable grade should be an Int, not Char.

B. Using commas to separate 'A' and 'B' is wrong.

C. Missing parentheses around conditions.

D. Using '||' inside when conditions is invalid syntax.

Solution

1. Step 1: Review the when condition syntax

   In Kotlin, multiple conditions in a when case must be separated by commas, not logical operators like '||'.

2. Step 2: Identify the incorrect usage

   The line 'C' || 'D' uses '||', which is invalid syntax in when. It should be 'C', 'D'.

3. Final Answer:

   Using '||' inside when conditions is invalid syntax. -> Option D

4. Quick Check:

   Use commas, not logical operators [OK]

Hint: Never use || or && in when conditions [OK]

Common Mistakes:

• Using logical operators instead of commas
• Thinking commas are wrong separators
• Confusing variable types with syntax errors

5.

You want to write a Kotlin when expression that prints "Weekend" if the day is either "Saturday" or "Sunday", "Weekday" if the day is any other day of the week, and "Invalid" for any other input. Which code correctly implements this?

hard

A. when(day) {
in listOf("Saturday", "Sunday") -> println("Weekend")
else -> println("Weekday")
}

B. when(day) {
"Saturday" || "Sunday" -> println("Weekend")
else -> println("Weekday")
}

C. when(day) {
"Saturday", "Sunday" -> println("Weekend")
"Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> println("Weekday")
else -> println("Invalid")
}

D. when(day) {
"Saturday" && "Sunday" -> println("Weekend")
else -> println("Weekday")
}

Solution

1. Step 1: Check correct multiple condition syntax for weekend days

   when(day) { "Saturday", "Sunday" -> println("Weekend") "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> println("Weekday") else -> println("Invalid") } correctly uses commas to separate "Saturday" and "Sunday" to match either day.

2. Step 2: Verify handling of other days and else case

   when(day) { "Saturday", "Sunday" -> println("Weekend") "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> println("Weekday") else -> println("Invalid") } lists all weekdays explicitly and uses else for invalid inputs, matching the problem requirements.

3. Final Answer:

   when(day) { "Saturday", "Sunday" -> println("Weekend") "Monday", "Tuesday", "Wednesday", "Thursday", "Friday" -> println("Weekday") else -> println("Invalid") } -> Option C

4. Quick Check:

   Use commas for multiple matches and else for fallback [OK]

Hint: List all matches with commas; use else for others [OK]

Common Mistakes:

• Using || or && instead of commas
• Missing else for invalid inputs
• Not listing all weekdays explicitly