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Variables and Constants

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In Swift, variables and constants are fundamental concepts that allow you to store and manipulate data. Understanding how to use them effectively is crucial for any Swift programmer. This section will cover the following topics:

  1. Introduction to Variables and Constants
  2. Declaring Variables
  3. Declaring Constants
  4. Type Annotations
  5. Type Inference
  6. Mutability
  7. Practical Examples
  8. Exercises

  1. Introduction to Variables and Constants

Variables and constants in Swift are used to store values. The key difference between them is that the value of a variable can change, while the value of a constant cannot.

  • Variables: Use the var keyword to declare a variable.
  • Constants: Use the let keyword to declare a constant.

  1. Declaring Variables

To declare a variable, use the var keyword followed by the variable name and an optional type annotation.

var variableName: DataType = initialValue

Example:

var age: Int = 25

In this example, age is a variable of type Int with an initial value of 25.

  1. Declaring Constants

To declare a constant, use the let keyword followed by the constant name and an optional type annotation.

let constantName: DataType = initialValue

Example:

let pi: Double = 3.14159

In this example, pi is a constant of type Double with an initial value of 3.14159.

  1. Type Annotations

Type annotations explicitly specify the type of a variable or constant. This is useful for clarity and when the type cannot be inferred.

Example:

var name: String = "John"
let year: Int = 2023

  1. Type Inference

Swift can often infer the type of a variable or constant based on the initial value provided. This means you can omit the type annotation.

Example:

var city = "New York" // Swift infers that city is of type String
let temperature = 72.5 // Swift infers that temperature is of type Double

  1. Mutability

  • Variables: The value of a variable can be changed after it is initially set.

    var score = 10
score = 20 // score is now 20

  • Constants: The value of a constant cannot be changed once it is set.

    let maxScore = 100
// maxScore = 200 // This will cause a compile-time error

  1. Practical Examples

Example 1: Using Variables

var greeting = "Hello"
greeting = "Hi"
print(greeting) // Output: Hi

Example 2: Using Constants

let birthYear = 1990
// birthYear = 1991 // This will cause a compile-time error
print(birthYear) // Output: 1990

  1. Exercises

Exercise 1: Declare and Modify a Variable

  1. Declare a variable named height with an initial value of 180.
  2. Change the value of height to 185.
  3. Print the value of height.

Solution:

var height = 180
height = 185
print(height) // Output: 185

Exercise 2: Declare and Use a Constant

  1. Declare a constant named maxSpeed with a value of 120.
  2. Try to change the value of maxSpeed to 130 and observe the error.
  3. Print the value of maxSpeed.

Solution:

let maxSpeed = 120
// maxSpeed = 130 // This will cause a compile-time error
print(maxSpeed) // Output: 120

Common Mistakes and Tips

  • Mistake: Trying to change the value of a constant.
    • Tip: Use var for values that need to change and let for values that should remain constant.
  • Mistake: Omitting type annotations when the type cannot be inferred.
    • Tip: Always provide a type annotation if the type is not clear from the initial value.

Conclusion

In this section, you learned about variables and constants in Swift, including how to declare them, the difference between them, and how to use type annotations and type inference. Understanding these concepts is essential for managing data in your Swift programs. Next, you will learn about the different data types available in Swift.

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