Swift 笔记（四）

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Making Decisions

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1、Consider this code:

let number = 10switch (number) {case let x where x % 2 == 0:    print("Even")default:    print("Odd")}  

This will print the following:

Even

This switch statement uses the let-where syntax, meaning the case will match only when a certain condition is true. In this example, you've designed the case to match if the value is even-that is, if the value modulo 2 equals 0.

The method by which you can match values based on conditions is known as pattern matching.

2、Another way you can use switch statements to great effect is as follows:

let coordinates: (x: Int, y: Int, z: Int) = (3, 2, 5)switch (coordinates) {case (0, 0, 0):    print("Origin")case (_, 0, 0):    print("On the x-axis.")case (0, _, 0):    print("On the y-axis.")case (0, 0, _):    print("On the z-axis.") default:    print("Somewhere in space")}  

You're using the underscore to mean that you don't care about the value. If you don't want to ignore the value, then you can use it in your switch statement, like this:

let coordinates: (x: Int, y: Int, z: Int) = (3, 2, 5)switch (coordinates) {case (0, 0, 0):    print("Origin")case (let x, 0, 0):    print("On the x-axis at x = \(x)")case (0, let y, 0):    print("On the y-axis at y = \(y)")case (0, 0, let z):    print("On the z-axis at z = \(z)")case (let x, let y, let z):    print("Somewhere in space at x = \(x), y = \(y), z = \(z)")  }  

You can use the same let-where syntax you saw earlier to match more complex cases. For example:

let coordinates: (x: Int, y: Int, z: Int) = (3, 2, 5)switch(coordinates) {case (let x, let y, _) where y == x:    print("Along the y = x line.")case (let x, let y, _) where y == x * x:    print("Along the y = x^2 line.")default:    break}