When without argument in Kotlin - Time & Space Complexity

Time Complexity: When without argument

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a Kotlin when expression without an argument changes as we add more conditions.

How does the number of checks grow when we add more cases?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

fun checkNumber(num: Int) {
    when {
        num < 0 -> println("Negative")
        num == 0 -> println("Zero")
        num > 0 -> println("Positive")
    }
}
    

This code checks a number against several conditions without using a direct argument in when.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The when expression checks each condition one by one.
• How many times: It checks conditions until one matches or all are checked.

How Execution Grows With Input

As you add more conditions, the program checks more cases in order until it finds a match.

Number of Conditions (n)	Approx. 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 conditions.

Final Time Complexity

Time Complexity: O(n)

This means the time to find a matching condition grows linearly as you add more conditions.

Common Mistake

[X] Wrong: "The when without argument checks all conditions at once, so it is constant time."

[OK] Correct: Actually, it checks conditions one by one until it finds a match, so more conditions mean more checks.

Interview Connect

Understanding how when without argument works helps you reason about condition checks and efficiency in Kotlin code, a useful skill for clear and efficient programming.

Self-Check

"What if the conditions were reordered so the most common case is first? How would the time complexity be affected in practice?"

Practice

1. What does when without argument do in Kotlin?

easy

A. It matches a value against constants only.

B. It checks boolean conditions one by one until one is true.

C. It creates a loop that runs multiple times.

D. It declares a variable with multiple types.

Solution

1. Step 1: Understand when without argument

   When used without an argument, when evaluates each condition as a boolean expression.

2. Step 2: Compare with other uses

   Unlike when with argument, it does not match values but checks conditions one by one.

3. Final Answer:

   It checks boolean conditions one by one until one is true. -> Option B

4. Quick Check:

   when without argument = boolean checks [OK]

Hint: Remember: no argument means boolean conditions checked [OK]

Common Mistakes:

• Thinking it matches values like switch-case
• Confusing it with loops
• Assuming it declares variables

2. Which of the following is the correct syntax for when without argument in Kotlin?

easy

A. when {
condition1 -> action1
condition2 -> action2
else -> defaultAction
}

B. when(value) {
1 -> action1
2 -> action2
else -> defaultAction
}

C. when (condition) {
true -> action1
false -> action2
}

D. when (value) {
condition1 -> action1
condition2 -> action2
}

Solution

1. Step 1: Identify when without argument syntax

   The correct syntax omits the argument and uses boolean conditions directly inside curly braces.

2. Step 2: Check options

   when { condition1 -> action1 condition2 -> action2 else -> defaultAction } matches this pattern with conditions and an else branch; others use an argument or incorrect structure.

3. Final Answer:

   when { condition1 -> action1 condition2 -> action2 else -> defaultAction } -> Option A

4. Quick Check:

   Correct syntax = when { condition1 -> action1 condition2 -> action2 else -> defaultAction } [OK]

Hint: No argument means conditions directly inside braces [OK]

Common Mistakes:

• Adding argument inside parentheses
• Missing else branch
• Using values instead of conditions

3. What is the output of this Kotlin code?

val x = 10
when {
  x < 5 -> println("Less than 5")
  x in 5..10 -> println("Between 5 and 10")
  else -> println("More than 10")
}

medium

A. More than 10

B. Less than 5

C. Between 5 and 10

D. No output

Solution

1. Step 1: Evaluate conditions in order

   First condition: x < 5 is false (10 < 5 is false). Second condition: x in 5..10 is true (10 is in 5 to 10).

2. Step 2: Execute matching branch

   Since second condition is true, it prints "Between 5 and 10" and stops checking further.

3. Final Answer:

   Between 5 and 10 -> Option C

4. Quick Check:

   10 in 5..10 = true [OK]

Hint: Check conditions top to bottom, first true runs [OK]

Common Mistakes:

• Assuming x < 5 is true
• Ignoring range check
• Expecting else branch output

4. Find the error in this Kotlin code using when without argument:

val y = 3
val result = when {
  y > 5 -> println("Greater than 5")
  y < 5 -> println("Less than 5")
}

medium

A. Missing else branch for unmatched cases

B. Incorrect use of parentheses after when

C. Conditions must be constants, not expressions

D. Cannot use println inside when

Solution

1. Step 1: Check when branches

   The code has two conditions but no else branch to handle all other cases.

2. Step 2: Understand Kotlin requirement

   When used without argument, Kotlin requires an else branch to cover all possibilities or the compiler errors about non-exhaustive when.

3. Final Answer:

   Missing else branch for unmatched cases -> Option A

4. Quick Check:

   Always add else branch [OK]

Hint: Always include else branch in when without argument [OK]

Common Mistakes:

• Ignoring else branch requirement
• Adding parentheses after when
• Thinking conditions must be constants

5. You want to print a message based on temperature:

val temp = 25
when {
  temp <= 0 -> println("Freezing")
  temp in 1..15 -> println("Cold")
  temp in 16..25 -> println("Warm")
  temp > 25 -> println("Hot")
  else -> println("Unknown")
}

What will be printed and why is this when usage correct?

hard

A. Compilation error due to overlapping ranges

B. "Cold"; because 25 is in 1..15 range

C. "Hot"; because 25 is greater than 25

D. "Warm"; because conditions cover all ranges with else as fallback

Solution

1. Step 1: Check which condition matches temp = 25

   temp <= 0 is false; temp in 1..15 is false; temp in 16..25 is true (25 is included).

2. Step 2: Confirm correctness of when usage

   All possible values are covered by conditions and else branch ensures fallback; no overlap causes error.

3. Final Answer:

   "Warm"; because conditions cover all ranges with else as fallback -> Option D

4. Quick Check:

   25 in 16..25 = true, else covers all [OK]

Hint: Check ranges carefully; else handles unexpected values [OK]

Common Mistakes:

• Misreading range boundaries
• Assuming 25 > 25 is true
• Thinking else is optional