Why For loop with ranges in Kotlin? - Purpose & Use Cases

The Big Idea

What if you could tell your program to count for you instead of writing every step yourself?

The Scenario

Imagine you want to count from 1 to 10 and print each number. Doing this by writing each number manually is like writing a list of numbers one by one on paper.

The Problem

Writing each step manually is slow and boring. If you want to count to 100 or 1000, it becomes impossible to do without mistakes or wasting a lot of time.

The Solution

Using a for loop with ranges lets you tell the computer to count for you automatically. You just say the start and end numbers, and the loop repeats the action for each number in that range.

Before vs After

✗ Before

println(1)
println(2)
println(3)
// ... up to 10

✓ After

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

What It Enables

This lets you easily repeat tasks many times without writing repetitive code, saving time and avoiding errors.

Real Life Example

For example, if you want to print a list of page numbers for a book from 1 to 50, a for loop with ranges does it quickly and cleanly.

Key Takeaways

Manually repeating code is slow and error-prone.

For loops with ranges automate repetition over a sequence of numbers.

This makes your code shorter, clearer, and easier to maintain.

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