Swift REPL and Playgrounds - Mini Project: Build & Apply

Exploring Swift REPL and Playgrounds

📖 Scenario: You are learning how to use Swift's REPL (Read-Eval-Print Loop) and Playgrounds to quickly test and run Swift code snippets. This helps you experiment with Swift code in a simple and interactive way, just like trying out ideas on paper before writing a full program.

🎯 Goal: Build a simple Swift program step-by-step using REPL or Playgrounds. You will create variables, configure a helper value, write a loop to process data, and finally print the result. This will help you get comfortable with Swift syntax and the interactive coding environment.

📋 What You'll Learn

Create a dictionary with exact keys and values

Add a configuration variable with a specific value

Use a for loop with exact variable names to process the dictionary

Print the final result exactly as specified

💡 Why This Matters

🌍 Real World

Filtering data based on conditions is common in apps, like showing only available items or filtering search results.

💼 Career

Understanding how to quickly test and run Swift code in REPL or Playgrounds is useful for developers to prototype ideas and debug code efficiently.

1

Create a dictionary of fruits and their quantities

Create a dictionary called fruits with these exact entries: "apple": 3, "banana": 5, "orange": 2

Swift

# Create the dictionary called fruits with the exact entries
# Your code here

Hint

Use square brackets to create a dictionary. Use colons to separate keys and values.

2

Add a minimum quantity threshold

Create a variable called minQuantity and set it to 3

Swift

var fruits = ["apple": 3, "banana": 5, "orange": 2]
# Create the variable minQuantity and set it to 3
# Your code here

Hint

Use var to create a variable and assign the number 3.

3

Filter fruits with quantity at least the threshold

Create a variable called filteredFruits and use a for loop with variables fruit and quantity to iterate over fruits. Add fruits with quantity greater than or equal to minQuantity to filteredFruits dictionary.

Swift

var fruits = ["apple": 3, "banana": 5, "orange": 2]
var minQuantity = 3
# Create filteredFruits dictionary and use a for loop to add fruits with quantity >= minQuantity
# Your code here

Hint

Start with an empty dictionary. Use a for loop to check each fruit's quantity. Add only those that meet the threshold.

4

Print the filtered fruits dictionary

Write print(filteredFruits) to display the filtered fruits dictionary

Swift

var fruits = ["apple": 3, "banana": 5, "orange": 2]
var minQuantity = 3
var filteredFruits = [String: Int]()
for (fruit, quantity) in fruits {
    if quantity >= minQuantity {
        filteredFruits[fruit] = quantity
    }
}
# Print the filteredFruits dictionary
# Your code here

Hint

Use the print function to show the dictionary. The output should only include fruits with quantity 3 or more.

Practice

1. What is the main purpose of Swift Playgrounds?

easy

A. To provide an interactive environment to write and test Swift code easily

B. To compile Swift code into an executable file

C. To manage Swift project dependencies

D. To deploy Swift apps to the App Store

Solution

1. Step 1: Understand what Playgrounds offer

   Swift Playgrounds allow users to write and run Swift code interactively with immediate feedback.

2. Step 2: Compare options with Playgrounds' purpose

   Options B, C, and D describe other development tasks, not the main purpose of Playgrounds.

3. Final Answer:

   To provide an interactive environment to write and test Swift code easily -> Option A

4. Quick Check:

   Playgrounds = Interactive coding environment [OK]

Hint: Playgrounds = interactive coding + instant results [OK]

Common Mistakes:

• Confusing Playgrounds with full app deployment tools
• Thinking Playgrounds compile apps for release
• Mixing Playgrounds with dependency managers

2. Which of the following is the correct way to start the Swift REPL in a terminal?

easy

A. swift run

B. swift

C. swift repl

D. swift playground

Solution

1. Step 1: Recall the command to start Swift REPL

   Typing swift alone in the terminal starts the Swift REPL environment.

2. Step 2: Check other options

   swift run runs a Swift package, swift repl is not a valid command, and swift playground does not start REPL.

3. Final Answer:

   swift -> Option B

4. Quick Check:

   Start REPL with 'swift' command [OK]

Hint: Just type 'swift' to open REPL in terminal [OK]

Common Mistakes:

• Using 'swift run' which runs packages, not REPL
• Typing 'swift repl' which is invalid
• Confusing REPL with Playgrounds commands

3. What will be the output when you run this code in Swift REPL?

let x = 5
let y = 10
x + y

medium

A. 15

B. x + y

C. SyntaxError

D. 10

Solution

1. Step 1: Evaluate the variables and expression

   Variables x and y are assigned 5 and 10 respectively. The expression x + y sums them.

2. Step 2: Calculate the sum

   5 + 10 equals 15, so the REPL outputs 15.

3. Final Answer:

   15 -> Option A

4. Quick Check:

   5 + 10 = 15 [OK]

Hint: Add values step-by-step to find output [OK]

Common Mistakes:

• Expecting the expression to print the code itself
• Thinking REPL shows errors for valid expressions
• Confusing variable values

4. You wrote this code in a Swift Playground but it shows an error:

var greeting = "Hello"
greeting += 5

What is the problem?

medium

A. You must use '==' instead of '+='

B. Variable 'greeting' should be a constant

C. You cannot add an integer to a string directly

D. Strings cannot be modified in Playgrounds

Solution

1. Step 1: Analyze the code operation

   The code tries to add an integer (5) to a string variable using '+='.

2. Step 2: Understand Swift type rules

   Swift does not allow adding an Int directly to a String; types must match or be converted.

3. Final Answer:

   You cannot add an integer to a string directly -> Option C

4. Quick Check:

   String + Int without conversion causes error [OK]

Hint: Check data types before combining values [OK]

Common Mistakes:

• Thinking '+=' works with different types
• Assuming strings are immutable in Playgrounds
• Confusing assignment operators

5. You want to quickly test a function that calculates the square of a number using Swift Playgrounds. Which approach is best?

hard

A. Write the function in a text editor and run it later in the terminal

B. Write the function in a Swift file and compile it with Xcode without running

C. Use Swift REPL but avoid Playgrounds because it can't run functions

D. Write the function in Playgrounds and call it with test values to see results immediately

Solution

1. Step 1: Understand Playgrounds' interactive nature

   Playgrounds allow writing functions and immediately running them with test inputs to see results.

2. Step 2: Evaluate other options

   Write the function in a Swift file and compile it with Xcode without running compiles but does not run immediately; Use Swift REPL but avoid Playgrounds because it can't run functions is false because Playgrounds can run functions; Write the function in a text editor and run it later in the terminal delays testing.

3. Final Answer:

   Write the function in Playgrounds and call it with test values to see results immediately -> Option D

4. Quick Check:

   Playgrounds = quick function testing [OK]

Hint: Use Playgrounds for instant function testing [OK]

Common Mistakes:

• Thinking Playgrounds can't run functions
• Confusing compiling with running
• Delaying testing by using external editors