For loop with ranges in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple for loop with range

What is the output of this Kotlin code?

Kotlin

fun main() {
    for (i in 1..3) {
        print(i)
    }
}

A 0123

B 1 2 3

C 123

D 3 2 1

💡 Hint

Remember that the range 1..3 includes both 1 and 3, and print() does not add spaces or newlines.

The for loop iterates over 1, 2, and 3. The print function outputs each number without spaces or newlines, so the output is '123'.

❓ Predict Output

intermediate

For loop with step in range

What is the output of this Kotlin code?

Kotlin

fun main() {
    for (i in 0..6 step 2) {
        print(i)
    }
}

A 135

B 0123456

C 246

D 0246

💡 Hint

The step keyword skips numbers by the given step size.

The loop starts at 0 and goes up to 6, stepping by 2 each time, so it prints 0, 2, 4, 6 without spaces.

❓ Predict Output

advanced

For loop with downTo and step

What is the output of this Kotlin code?

Kotlin

fun main() {
    for (i in 10 downTo 2 step 3) {
        print(i)
    }
}

A 1074

B 108642

C 10374

D 109642

💡 Hint

downTo counts backwards, step skips numbers by the step size.

The loop starts at 10, then 7, then 4, then stops because next step 1 is less than 2. So output is '1074'.

🔧 Debug

advanced

Identify the error in the for loop with range

What error does this Kotlin code produce?

Kotlin

fun main() {
    for (i in 1..5 step) {
        print(i)
    }
}

A SyntaxError: Expecting an expression after 'step'

B No error, prints 12345

C TypeError: step must be an integer

D RuntimeException: Invalid range

💡 Hint

Check the syntax of the step keyword in the range expression.

The 'step' keyword requires a number after it. Missing the number causes a syntax error.

🧠 Conceptual

expert

Number of iterations in a for loop with range and step

How many times does this Kotlin for loop run?

Kotlin

fun main() {
    var count = 0
    for (i in 5 downTo 0 step 2) {
        count++
    }
    println(count)
}

A 5

B 3

C 4

D 6

💡 Hint

List the numbers generated by the range and count them.

The loop runs for i = 5, 3, 1. It stops before reaching -1. So it runs 3 times.

Practice

1. What does the following Kotlin code do?
for (i in 1..5) { println(i) }

easy

A. Prints numbers from 1 to 4

B. Prints numbers from 1 to 5 inclusive

C. Prints numbers from 0 to 5

D. Prints numbers from 5 down to 1

Solution

1. Step 1: Understand the range operator

   The operator 1..5 creates a range including numbers 1, 2, 3, 4, and 5.

2. Step 2: Loop through the range

   The for loop runs from 1 to 5, printing each number.

3. Final Answer:

   Prints numbers from 1 to 5 inclusive -> Option B

4. Quick Check:

   Range 1..5 includes 5 [OK]

Hint: .. means inclusive range from start to end [OK]

Common Mistakes:

• Thinking .. excludes the last number
• Confusing range with step or downTo
• Assuming it counts down automatically

2. Which of the following is the correct syntax for a for loop counting down from 5 to 1 in Kotlin?

easy

A. for (i in 5 downTo 1) { println(i) }

B. for (i in 1..5 step -1) { println(i) }

C. for (i in 5..1) { println(i) }

D. for (i in 1 downTo 5) { println(i) }

Solution

1. Step 1: Identify correct descending range syntax

   Kotlin uses downTo to count down from a higher number to a lower number.

2. Step 2: Check each option

   for (i in 5 downTo 1) { println(i) } uses 5 downTo 1, which is correct. for (i in 5..1) { println(i) } uses an invalid range (5..1). for (i in 1..5 step -1) { println(i) } tries to use negative step incorrectly. for (i in 1 downTo 5) { println(i) } reverses the range incorrectly.

3. Final Answer:

   for (i in 5 downTo 1) { println(i) } -> Option A

4. Quick Check:

   Use downTo for descending loops [OK]

Hint: Use downTo to count down in Kotlin loops [OK]

Common Mistakes:

• Using .. for descending ranges
• Trying negative step with .. (not allowed)
• Swapping start and end in downTo

3. What is the output of this Kotlin code?

var sum = 0
for (i in 1..5 step 2) {
  sum += i
}
println(sum)

medium

A. 6

B. 15

C. 9

D. 10

Solution

1. Step 1: Understand the loop range and step

   The loop runs from 1 to 5, stepping by 2: values are 1, 3, 5.

2. Step 2: Calculate the sum of these values

   sum = 0 + 1 + 3 + 5 = 9

3. Final Answer:

   9 -> Option C

4. Quick Check:

   1 + 3 + 5 = 9 [OK]

Hint: Add numbers in steps to find sum quickly [OK]

Common Mistakes:

• Adding all numbers 1 to 5 instead of stepping
• Using step as a multiplier
• Confusing step with downTo

4. Find the error in this Kotlin code snippet:

for (i in 10..1 step 2) {
  println(i)
}

medium

A. println cannot be used inside for loop

B. step cannot be used with ranges

C. Variable i is not declared

D. Range 10..1 is invalid for counting down

Solution

1. Step 1: Analyze the range 10..1

   The range 10..1 tries to count up from 10 to 1, which is invalid because 10 is greater than 1.

2. Step 2: Understand correct descending syntax

   To count down, use downTo instead of .., e.g., 10 downTo 1 step 2.

3. Final Answer:

   Range 10..1 is invalid for counting down -> Option D

4. Quick Check:

   Use downTo for descending ranges [OK]

Hint: .. always counts up; use downTo to count down [OK]

Common Mistakes:

• Using .. for descending ranges
• Thinking step fixes descending range
• Ignoring Kotlin's range direction rules

5. You want to print all even numbers from 2 to 20 inclusive using a for loop in Kotlin. Which code snippet correctly does this?

hard

A. for (i in 2..20 step 2) { println(i) }

B. for (i in 2..19) { if (i % 2 == 0) println(i) }

C. for (i in 2 downTo 20 step 2) { println(i) }

D. for (i in 1..10 step 2) { println(i * 2) }

Solution

1. Step 1: Identify the range and step for even numbers

   Even numbers from 2 to 20 can be generated by starting at 2 and stepping by 2 up to 20.

2. Step 2: Check each option

   for (i in 2..20 step 2) { println(i) } uses 2..20 step 2, which prints 2,4,6,...,20 correctly. for (i in 2..19) { if (i % 2 == 0) println(i) } ends at 19 so prints evens up to 18, missing 20. for (i in 2 downTo 20 step 2) { println(i) } counts down incorrectly. for (i in 1..10 step 2) { println(i * 2) } prints 2,6,10,..., not all even numbers.

3. Final Answer:

   for (i in 2..20 step 2) { println(i) } -> Option A

4. Quick Check:

   Use step 2 from 2 to 20 for evens [OK]

Hint: Use step 2 starting at 2 to print even numbers [OK]

Common Mistakes:

• Using downTo instead of .. for ascending
• Forgetting to step by 2
• Multiplying inside loop incorrectly