Kotlin REPL and script mode - Mini Project: Build & Apply

Kotlin REPL and Script Mode Basics

📖 Scenario: You are learning how to quickly test Kotlin code snippets using the Kotlin REPL (Read-Eval-Print Loop) and how to write simple Kotlin scripts.

🎯 Goal: Build a simple Kotlin script that defines variables, uses a helper variable, performs a calculation, and prints the result. This will help you understand how Kotlin code runs interactively and as a script.

📋 What You'll Learn

Create variables with exact names and values

Add a helper variable for configuration

Use a calculation with the variables

Print the final result exactly

💡 Why This Matters

🌍 Real World

Kotlin REPL and script mode let you quickly test ideas and write small programs without setting up a full project.

💼 Career

Many Kotlin developers use REPL and scripts to prototype code, debug, or automate tasks efficiently.

1

Create initial variables

Create two variables called width and height with values 5 and 10 respectively.

Kotlin

// Create variables width and height with values 5 and 10
// Your code here

Hint

Use val to create variables that do not change.

2

Add a helper variable

Add a variable called scale and set it to 2.

Kotlin

val width = 5
val height = 10
// Add a variable scale with value 2
// Your code here

Hint

Use val scale = 2 to create the helper variable.

3

Calculate scaled area

Create a variable called scaledArea that multiplies width, height, and scale.

Kotlin

val width = 5
val height = 10
val scale = 2
// Calculate scaledArea by multiplying width, height, and scale
// Your code here

Hint

Use the * operator to multiply the variables.

4

Print the scaled area

Print the value of scaledArea using println.

Kotlin

val width = 5
val height = 10
val scale = 2
val scaledArea = width * height * scale
// Print scaledArea
// Your code here

Hint

Use println(scaledArea) to show the result.

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