Range operator (..) and in operator in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the range operator .. do in Kotlin?

The range operator .. creates a range of values from a start to an end, including both ends. For example, 1..5 means numbers 1, 2, 3, 4, and 5.

beginner

How do you check if a value is inside a range in Kotlin?

You use the in operator. For example, 3 in 1..5 checks if 3 is between 1 and 5 (inclusive). It returns true if yes, false if no.

beginner

What will this code print?

for (i in 1..3) {
    println(i)
}

It will print:
1
2
3
Because the loop goes through the range from 1 to 3, including both ends.

intermediate

Can the in operator be used with characters in Kotlin?

Yes! You can check if a character is inside a range of characters. For example, 'b' in 'a'..'d' returns true because 'b' is between 'a' and 'd'.

intermediate

What does 5 !in 1..4 mean in Kotlin?

It means 5 is NOT in the range from 1 to 4. The !in operator checks if a value is outside the range. Here, it returns true because 5 is not between 1 and 4.

What does 1..5 represent in Kotlin?

A A list of numbers from 1 to 4

B A range including numbers 1 to 5

C A range excluding 5

D A function call

1..5 creates a range including both 1 and 5.

What is the result of 3 in 1..5?

A false

B Error

C true

D null

3 is inside the range 1 to 5, so it returns true.

Which operator checks if a value is NOT in a range?

A in

B ..

C !=

D !in

The !in operator checks if a value is outside a range.

What will this print?
for (c in 'a'..'c') println(c)

A a b c

B a b

C b c

D a c

The loop prints characters from 'a' to 'c' inclusive.

Is 10 in 1..9 true or false?

A false

B true

C Error

D Depends on context

10 is not in the range 1 to 9, so it returns false.

Explain how the range operator .. works in Kotlin and give an example.

Think about counting numbers from one to another.

Describe how to use the in operator to check if a value is inside a range.

It's like asking if something is inside a box.

Practice

1. What does the Kotlin expression 1..5 represent?

easy

A. An error because ranges need parentheses

B. A list containing numbers 1 and 5 only

C. A function that returns 1 or 5

D. A range of numbers from 1 to 5 inclusive

Solution

1. Step 1: Understand the range operator

   The .. operator creates a range including both start and end values.

2. Step 2: Interpret 1..5

   This means all numbers from 1 up to 5, including 1 and 5.

3. Final Answer:

   A range of numbers from 1 to 5 inclusive -> Option D

4. Quick Check:

   1..5 = numbers 1 to 5 [OK]

Hint: Remember: .. includes both ends in Kotlin ranges [OK]

Common Mistakes:

• Thinking it creates a list instead of a range
• Assuming it excludes the last number
• Confusing it with a function call

2. Which of the following is the correct syntax to check if variable x is between 10 and 20 inclusive in Kotlin?

easy

A. if (x between 10 and 20) { }

B. if (x in 10..20) { }

C. if (10 <= x <= 20) { }

D. if (x inside 10..20) { }

Solution

1. Step 1: Recall Kotlin's in operator syntax

   Kotlin uses in to check if a value is inside a range.

2. Step 2: Match syntax with options

   Only if (x in 10..20) correctly uses in and the range operator ...

3. Final Answer:

   if (x in 10..20) { } -> Option B

4. Quick Check:

   Use in with .. for range checks [OK]

Hint: Use 'in' with '..' for range checks in Kotlin [OK]

Common Mistakes:

• Using 'between' or 'inside' which are not Kotlin keywords
• Trying chained comparisons like 10 <= x <= 20
• Missing the range operator '..'

3. What is the output of this Kotlin code?

val x = 7
if (x in 1..5) println("Inside") else println("Outside")

medium

A. Outside

B. Compilation error

C. Inside

D. No output

Solution

1. Step 1: Understand the range check

   The range 1..5 includes numbers 1, 2, 3, 4, 5.

2. Step 2: Check if x=7 is in the range

   7 is not between 1 and 5, so the condition is false.

3. Final Answer:

   Outside -> Option A

4. Quick Check:

   7 not in 1..5 means print "Outside" [OK]

Hint: Check if value lies within range boundaries [OK]

Common Mistakes:

• Assuming 7 is inside 1..5
• Confusing inclusive ranges with exclusive
• Expecting a syntax error

4. Identify the error in this Kotlin code snippet:

for (i in 1..5)
    if (i in 3..1) println(i)

medium

A. The for loop syntax is incorrect

B. The range 3..1 is invalid because start is greater than end

C. No error, code runs fine

D. The if condition should use 'not in' instead of 'in'

Solution

1. Step 1: Analyze the range 3..1

   In Kotlin, a range with start greater than end (3..1) creates an empty range, which is valid.

2. Step 2: Verify code execution

   The for loop runs for i=1 to 5, but i in 3..1 is always false for any i, so nothing prints. No syntax or runtime errors occur.

3. Final Answer:

   No error, code runs fine -> Option C

4. Quick Check:

   Empty ranges (start > end) are valid in Kotlin [OK]

Hint: Ranges with start > end are empty but valid [OK]

Common Mistakes:

• Thinking reversed ranges like 3..1 are invalid
• Assuming the for loop syntax is wrong
• Expecting a syntax error or crash with empty range

5. You want to print all even numbers between 2 and 10 inclusive using a Kotlin for loop and range. Which code snippet correctly does this?

hard

A. for (i in 2..10 step 2) println(i)

B. for (i in 2..10) if (i % 2 != 0) println(i)

C. for (i in 2 until 10 step 2) println(i)

D. for (i in 2..10 step 3) println(i)

Solution

1. Step 1: Understand the range and step usage

   2..10 step 2 creates numbers 2, 4, 6, 8, 10.

2. Step 2: Check each option

   for (i in 2..10 step 2) println(i) prints 2,4,6,8,10 directly. for (i in 2..10) if (i % 2 != 0) println(i) checks i % 2 != 0 and prints odds 3,5,7,9. for (i in 2 until 10 step 2) println(i) uses until excluding 10 (2,4,6,8). for (i in 2..10 step 3) println(i) steps by 3 (2,5,8).

3. Final Answer:

   for (i in 2..10 step 2) println(i) -> Option A

4. Quick Check:

   Use step 2 with inclusive range for even numbers [OK]

Hint: Use 'step 2' with '..' to get even numbers directly [OK]

Common Mistakes:

• Using wrong modulo condition like % 2 != 0
• Using 'until' which excludes the end value
• Using wrong step size like 3