When without argument in Kotlin - Mini Project: Build & Apply

Using When Without Argument in Kotlin

📖 Scenario: You are building a simple program that checks different conditions about a number and prints messages accordingly.

🎯 Goal: Learn how to use the when expression without an argument to check multiple conditions in Kotlin.

📋 What You'll Learn

Create a variable called number with the value 15

Create a variable called isEven that checks if number is even

Use a when expression without argument to check conditions on number and isEven

Print the correct message based on the conditions

💡 Why This Matters

🌍 Real World

Using <code>when</code> without argument helps you check multiple conditions clearly and cleanly, which is useful in many programs like games, calculators, or decision-making apps.

💼 Career

Understanding <code>when</code> expressions is important for Kotlin developers to write readable and efficient code that handles different cases without complicated if-else chains.

1

Create the number variable

Create a variable called number and set it to 15.

Kotlin

# Create a variable called number and set it to 15
# Your code here

Hint

Use val number = 15 to create the variable.

2

Create the isEven variable

Create a variable called isEven that is true if number is even, otherwise false. Use number % 2 == 0.

Kotlin

val number = 15
# Create a variable called isEven that checks if number is even
# Your code here

Hint

Use the modulus operator % to check if the number is even.

3

Use when without argument to check conditions

Write a when expression without argument that checks these conditions in order:
1. If number is less than 10, print "Number is less than 10".
2. If isEven is true, print "Number is even".
3. Otherwise, print "Number is odd and 10 or more".
Use when { ... } syntax with conditions as branches.

Kotlin

val number = 15
val isEven = number % 2 == 0
# Use when without argument to check conditions and print messages
# Your code here

Hint

Use when { condition -> action } to check each condition.

4

Print the result

Run the program to print the correct message based on the when expression.

Kotlin

val number = 15
val isEven = number % 2 == 0
when {
    number  println("Number is less than 10")
    isEven -> println("Number is even")
    else -> println("Number is odd and 10 or more")
}
# Run the program to see the output

Hint

The output should be Number is odd and 10 or more because 15 is odd and greater than 10.

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