Kotlin REPL and script mode - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of variable reassignment in Kotlin REPL

What is the output of the following Kotlin REPL session?

Kotlin

var x = 5
x = x + 3
println(x)

A 8

B 5

C Compilation error

D Runtime exception

💡 Hint

Remember that 'var' allows reassignment in Kotlin.

The variable x is initially 5, then increased by 3, so the output is 8.

❓ Predict Output

intermediate

Script mode output with top-level function

What will be printed when running this Kotlin script file?

Kotlin

fun greet(name: String) {
    println("Hello, $name!")
}
greet("Alice")

A No output

B greet

C Compilation error: function must be inside a class

D Hello, Alice!

💡 Hint

Kotlin scripts allow top-level functions and execute statements immediately.

The script defines a function and calls it immediately, printing the greeting.

❓ Predict Output

advanced

REPL session with shadowing and type inference

What is the output of this Kotlin REPL session?

Kotlin

val x = 10
val x = "Ten"
println(x)

A Ten

B 10

C Compilation error: val x already defined

D Runtime exception

💡 Hint

In Kotlin REPL, you can shadow previous variables by redeclaring them.

The second declaration shadows the first, so x is a String "Ten" when printed.

❓ Predict Output

advanced

Script mode with top-level statements and variable scope

What will be the output of this Kotlin script?

Kotlin

val a = 5
fun printA() {
    val a = 10
    println(a)
}
printA()
println(a)

A Compilation error: variable a redeclared

B

5
10

C

10
5

D

10
10

💡 Hint

Local variables inside functions shadow outer variables.

Inside printA(), local a=10 is printed. Outside, top-level a=5 is printed.

🧠 Conceptual

expert

Difference between Kotlin REPL and script mode execution

Which statement correctly describes a key difference between Kotlin REPL and Kotlin script mode?

A Kotlin REPL does not support variable reassignment, but script mode does.

B Kotlin REPL executes code line-by-line interactively, while script mode runs the entire file as a single unit.

C Kotlin REPL compiles to JVM bytecode, but script mode runs only interpreted code.

D Kotlin REPL requires all code inside functions, but script mode allows top-level statements.

💡 Hint

Think about how REPL and scripts handle code execution timing.

REPL executes each entered line immediately and interactively; scripts run the whole file at once.

Practice

1. What is the main purpose of Kotlin REPL?

easy

A. To run Kotlin code interactively line by line

B. To compile Kotlin code into Java bytecode

C. To create Android applications

D. To manage Kotlin project dependencies

Solution

1. Step 1: Understand REPL meaning

   Kotlin REPL stands for Read-Eval-Print Loop, which runs code interactively.

2. Step 2: Identify REPL use

   It lets you type and run Kotlin code line by line without compiling a full program.

3. Final Answer:

   To run Kotlin code interactively line by line -> Option A

4. Quick Check:

   Kotlin REPL = Interactive code running [OK]

Hint: REPL runs code line by line interactively [OK]

Common Mistakes:

• Confusing REPL with full project compilation
• Thinking REPL is for app deployment
• Assuming REPL manages dependencies

2. Which file extension is used for Kotlin script files?

easy

A. .kts

B. .skr

C. .kotlin

D. .kt

Solution

1. Step 1: Recall Kotlin file types

   Kotlin source files usually use .kt extension for normal programs.

2. Step 2: Identify script file extension

   Kotlin scripts use .kts extension to run code without full compilation.

3. Final Answer:

   .kts -> Option A

4. Quick Check:

   Kotlin script files = .kts [OK]

Hint: Scripts use .kts extension, not .kt [OK]

Common Mistakes:

• Choosing .kt which is for normal Kotlin files
• Assuming .kotlin or .skr are valid extensions
• Mixing script files with compiled files

3. What is the output of this Kotlin REPL session?

val x = 5
val y = 10
x + y

medium

A. 510

B. Error: undefined variables

C. x + y

D. 15

Solution

1. Step 1: Evaluate variable assignments

   x is assigned 5, y is assigned 10 in REPL environment.

2. Step 2: Calculate expression result

   x + y equals 5 + 10 which is 15.

3. Final Answer:

   15 -> Option D

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add values assigned to variables for output [OK]

Common Mistakes:

• Concatenating numbers as strings (510)
• Printing expression text instead of result
• Assuming variables are undefined in REPL

4. Identify the error in this Kotlin script file example.kts:

val name = "Alice"
println(Name)

medium

A. Script files cannot use println

B. Variable 'Name' is not defined (case sensitive)

C. Missing semicolon after println statement

D. String literals must use single quotes

Solution

1. Step 1: Check variable usage

   Variable declared as 'name' but used as 'Name' which differs in case.

2. Step 2: Understand Kotlin case sensitivity

   Kotlin variables are case sensitive, so 'Name' is undefined causing error.

3. Final Answer:

   Variable 'Name' is not defined (case sensitive) -> Option B

4. Quick Check:

   Variable names are case sensitive [OK]

Hint: Check variable name case carefully [OK]

Common Mistakes:

• Thinking semicolons are mandatory
• Believing println is disallowed in scripts
• Using single quotes for strings in Kotlin

5. You want to quickly test a Kotlin function without creating a full project. Which approach is best?

hard

A. Write the function in Java and call it from Kotlin

B. Create a full Kotlin project and add the function

C. Use Kotlin REPL to define and test the function interactively

D. Write the function in a .kts script file and run it

Solution

1. Step 1: Compare quick testing methods

   Creating a full project is slow; writing Java adds complexity.

2. Step 2: Choose between script and REPL

   REPL lets you define and test functions interactively without saving files.

3. Step 3: Understand REPL advantage

   REPL is fastest for quick tests; scripts require saving and running files.

4. Final Answer:

   Use Kotlin REPL to define and test the function interactively -> Option C

5. Quick Check:

   REPL = fastest interactive testing [OK]

Hint: Use REPL for fastest interactive function testing [OK]

Common Mistakes:

• Assuming scripts are faster than REPL
• Thinking full projects are needed for tests
• Confusing Java code with Kotlin quick tests