When with multiple conditions in Kotlin - Time & Space Complexity

Time Complexity: When with multiple conditions

O(n)

Understanding Time Complexity

We want to see how the time it takes to run a Kotlin when statement changes as we add more conditions.

How does checking multiple conditions affect the program's speed?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fun checkNumber(num: Int): String {
    return when (num) {
        1, 2, 3 -> "Small number"
        4, 5, 6 -> "Medium number"
        7, 8, 9 -> "Large number"
        else -> "Unknown"
    }
}
    

This code checks a number against several groups of values using multiple conditions in each when branch.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Checking the input number against each condition group in the when statement.
• How many times: The program compares the input to each condition until it finds a match or reaches else.

How Execution Grows With Input

As the number of condition groups grows, the program checks more groups one by one until it finds a match.

Input Size (number of condition groups)	Approx. Operations (checks)
3	Up to 3 checks
10	Up to 10 checks
100	Up to 100 checks

Pattern observation: The number of checks grows directly with the number of condition groups.

Final Time Complexity

Time Complexity: O(n)

This means the time to find the matching condition grows in a straight line as you add more condition groups.

Common Mistake

[X] Wrong: "The when statement checks all conditions at once, so time stays the same no matter how many conditions there are."

[OK] Correct: The when checks conditions one by one until it finds a match, so more conditions mean more checks in the worst case.

Interview Connect

Understanding how multiple conditions affect time helps you explain code efficiency clearly and shows you can think about how programs grow with input size.

Self-Check

"What if we replaced the multiple conditions in each when branch with a data structure lookup? How would the time complexity change?"

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