F＃模式匹配

模式匹配允许您“将数据与逻辑结构或结构进行比较，将数据分解为组成部分，或以各种方式从数据中提取信息”。
换句话说，它提供了一种更灵活和更强大的方法来根据一系列条件测试数据，并根据满足的条件执行一些计算。
从概念上讲，它就像一系列if ... then语句。

语法

match expr with| pat1 - result1| pat2 -> result2| pat3 when expr2 -> result3| _ -> defaultResult

let rec fib n = match n with | 0 -> 0 | 1 -> 1 | _ -> fib (n - 1) + fib (n - 2)for i = 1 to 10 do printfn "Fibonacci %d: %d" i (fib i)

当你编译和执行程序，它产生以下输出 

Fibonacci 1: 1Fibonacci 2: 1Fibonacci 3: 2Fibonacci 4: 3Fibonacci 5: 5Fibonacci 6: 8Fibonacci 7: 13Fibonacci 8: 21Fibonacci 9: 34Fibonacci 10: 55

您也可以串联多个条件，其中返回相同的值在一起。例如 

例2

let printSeason month = match month with | "December" | "January" | "February" -> printfn "Winter" | "March" | "April" -> printfn "Spring" | "May" | "June" -> printfn "Summer" | "July" | "August" -> printfn "Rainy" | "September" | "October" | "November" -> printfn "Autumn" | _ -> printfn "Season depends on month!"printSeason "February"printSeason "April"printSeason "November"printSeason "July"

当你编译和执行程序，它产生以下输出 

WinterSpringAutumnRainy

模式匹配函数

F＃允许你写的模式匹配使用function关键字功能

let getRate = function | "potato" -> 10.00 | "brinjal" -> 20.50 | "cauliflower" -> 21.00 | "cabbage" -> 8.75 | "carrot" -> 15.00 | _ -> nan (* nan is a special value meaning "not a number" *)printfn "%g"(getRate "potato")printfn "%g"(getRate "brinjal")printfn "%g"(getRate "cauliflower")printfn "%g"(getRate "cabbage")printfn "%g"(getRate "carrot")

当你编译和执行程序，它产生以下输出 

1020.5218.7515

向模式添加过滤器或保护

let sign = function | 0 -> 0 | x when x < 0 -> -1 | x when x > 0 -> 1printfn "%d" (sign -20)printfn "%d" (sign 20)printfn "%d" (sign 0)

当你编译和执行程序，它产生以下输出 

-110

例2

let compareInt x = match x with | (var1, var2) when var1 > var2 -> printfn "%d is greater than %d" var1 var2 | (var1, var2) when var1 < var2 -> printfn "%d is less than %d" var1 var2 | (var1, var2) -> printfn "%d equals %d" var1 var2compareInt (11,25)compareInt (72, 10)compareInt (0, 0)

当你编译和执行程序，它产生以下输出 

11 is less than 2572 is greater than 100 equals 0

模式匹配元组

下面的示例演示元组匹配模式 

let greeting (name, subject) = match (name, subject) with | ("Zara", _) -> "Hello, Zara" | (name, "English") -> "Hello, " + name + " from the department of English" | (name, _) when subject.StartsWith("Comp") -> "Hello, " + name + " from the department of Computer Sc." | (_, "Accounts and Finance") -> "Welcome to the department of Accounts and Finance!" | _ -> "You are not registered into the system"printfn "%s" (greeting ("Zara", "English"))printfn "%s" (greeting ("Raman", "Computer Science"))printfn "%s" (greeting ("Ravi", "Mathematics"))

当你编译和执行程序，它产生以下输出 

Hello, ZaraHello, Raman from the department of Computer Sc.You are not registered into the system

模式匹配记录

下面的例子演示了记录模式匹配 

type Point = { x: float; y: float }let evaluatePoint (point: Point) = match point with | { x = 0.0; y = 0.0 } -> printfn "Point is at the origin." | { x = xVal; y = 0.0 } -> printfn "Point is on the x-axis. Value is %f." xVal | { x = 0.0; y = yVal } -> printfn "Point is on the y-axis. Value is %f." yVal | { x = xVal; y = yVal } -> printfn "Point is at (%f, %f)." xVal yValevaluatePoint { x = 0.0; y = 0.0 }evaluatePoint { x = 10.0; y = 0.0 }evaluatePoint { x = 0.0; y = 10.0 }evaluatePoint { x = 10.0; y = 10.0 }

当你编译和执行程序，它产生以下输出 

Point is at the origin.Point is on the x-axis. Value is 10.000000.Point is on the y-axis. Value is 10.000000.Point is at (10.000000, 10.000000).