For loop with ranges in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a for loop with a range in Kotlin?

A for loop with a range in Kotlin repeats a block of code for each number in a sequence defined by a range, like 1..5, which means from 1 to 5 inclusive.

beginner

How do you write a for loop in Kotlin that counts from 1 to 5?

You write:

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

This prints numbers 1 through 5, one per line.

beginner

What does the range 1..5 mean in Kotlin?

It means all numbers starting at 1 and going up to 5, including both 1 and 5.

intermediate

How can you loop backwards from 5 down to 1 in Kotlin?

Use the downTo keyword:

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

This counts down from 5 to 1.

intermediate

What does the step keyword do in a Kotlin for loop?

It changes how much the loop variable increases or decreases each time. For example, for (i in 1..10 step 2) counts 1, 3, 5, 7, 9.

What does for (i in 1..3) do in Kotlin?

A Loops with i = 1, 2, 3

B Loops with i = 1, 2

C Loops with i = 0, 1, 2, 3

D Loops with i = 3, 2, 1

The range 1..3 includes 1, 2, and 3, so the loop runs with i equal to those values.

How do you write a loop that counts down from 5 to 1?

A for (i in 5..1)

B for (i in 1..5)

C for (i in 5 downTo 1)

D for (i in 1 downTo 5)

The downTo keyword counts backwards from 5 down to 1.

What does step 3 do in a Kotlin for loop?

A Ends the loop at 3

B Increases the loop variable by 3 each time

C Starts the loop at 3

D Runs the loop 3 times

The step keyword changes how much the loop variable increases each time.

Which of these is a valid Kotlin range?

A 1..5

B 5..1

C 1...5

D 1-5

The correct syntax for a range is start..end, like 1..5.

What will this code print?
for (i in 1..5 step 2) println(i)

A 1 2 3 4 5

B 5 3 1

C 2 4

D 1 3 5

The loop counts from 1 to 5, increasing by 2 each time, so it prints 1, 3, and 5.

Explain how to use a for loop with a range in Kotlin to count from 1 to 10.

Think about how to write the loop header and what the range means.

Describe how to loop backwards from 10 to 1 using Kotlin ranges.

Remember Kotlin has a special keyword for counting down.

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