When expression as powerful switch in Kotlin - Step-by-Step Execution

Concept Flow - When expression as powerful switch

Start

↓

Evaluate expression

↓

Match case 1?

No→Match case 2?

↓

Execute case 1

↓

Return result

↓

End

The when expression evaluates a value and matches it against multiple cases, executing the first matching case or else the default else case, then returns the result.

Execution Sample

Kotlin

val x = 2
val result = when (x) {
  1 -> "One"
  2 -> "Two"
  3 -> "Three"
  else -> "Unknown"
}

This code checks the value of x and returns a string matching the number or "Unknown" if no match.

Execution Table

Step	Expression Value	Case Checked	Match?	Action	Result
1	2	1	No	Check next case
2	2	2	Yes	Return "Two"	"Two"
3		3	Skipped	Skipped	"Two"
4		else	Skipped	Skipped	"Two"

💡 Match found at case 2, returned "Two", execution stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
x	2	2	2	2
result	undefined	undefined	"Two"	"Two"

Key Moments - 3 Insights

Why does the when expression stop checking cases after finding a match?

Because when acts like a switch, it executes only the first matching case and then returns immediately, as shown in execution_table row 2.

What happens if none of the cases match and there is no else branch?

The when expression will throw an exception if no else branch is provided and no case matches. The else branch acts as a default to avoid this.

Can when expression return a value?

Yes, when is an expression that returns the value of the matched case, as shown by the 'result' variable in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table, what is the result after step 2?

A "Two"

B "One"

C "Three"

D "Unknown"

💡 Hint

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

At which step does the when expression stop checking further cases?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Match?' column and see where 'Yes' appears first in execution_table.

If x was 5, which case would be executed?

A Case 1

B Case 2

C Case 3

D Else case

💡 Hint

Refer to the concept_flow and execution_table logic for unmatched values.

Concept Snapshot

when (value) {
  case1 -> result1
  case2 -> result2
  ...
  else -> defaultResult
}

- Evaluates value once
- Matches cases top to bottom
- Executes first matching case
- Returns the matched case's result
- else is mandatory if cases don't cover all values

Full Transcript

The when expression in Kotlin works like a powerful switch statement. It evaluates a value once and checks it against multiple cases in order. When it finds the first matching case, it executes that case and returns its result immediately, skipping the rest. If no cases match, the else branch runs as a default. This makes when concise and expressive for branching logic. In the example, x is 2, so when matches case 2 and returns "Two". Variables track the value of x and the result returned. The execution table shows step-by-step how each case is checked and when the match happens. This helps beginners see how when stops checking after the first match and how the result is assigned. Remember to always include else if your cases don't cover all possibilities to avoid errors.