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模式匹配允許您“將數(shù)據(jù)與邏輯結構或結構進行比較,將數(shù)據(jù)分解為組成部分,或以各種方式從數(shù)據(jù)中提取信息”。
換句話說,它提供了一種更靈活和更強大的方法來根據(jù)一系列條件測試數(shù)據(jù),并根據(jù)滿足的條件執(zhí)行一些計算。
從概念上講,它就像一系列if ... then語句。
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)
當你編譯和執(zhí)行程序,它產生以下輸出
Fibonacci 1: 1 Fibonacci 2: 1 Fibonacci 3: 2 Fibonacci 4: 3 Fibonacci 5: 5 Fibonacci 6: 8 Fibonacci 7: 13 Fibonacci 8: 21 Fibonacci 9: 34 Fibonacci 10: 55
您也可以串聯(lián)多個條件,其中返回相同的值在一起。例如
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"
當你編譯和執(zhí)行程序,它產生以下輸出
Winter Spring Autumn Rainy
F#允許你寫的模式匹配使用function關鍵字功能
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")
當你編譯和執(zhí)行程序,它產生以下輸出
10 20.5 21 8.75 15
let sign = function | 0 -> 0 | x when x < 0 -> -1 | x when x > 0 -> 1 printfn "%d" (sign -20) printfn "%d" (sign 20) printfn "%d" (sign 0)
當你編譯和執(zhí)行程序,它產生以下輸出
-1 1 0
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 var2 compareInt (11,25) compareInt (72, 10) compareInt (0, 0)
當你編譯和執(zhí)行程序,它產生以下輸出
11 is less than 25 72 is greater than 10 0 equals 0
下面的示例演示元組匹配模式
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"))
當你編譯和執(zhí)行程序,它產生以下輸出
Hello, Zara Hello, Raman from the department of Computer Sc. You are not registered into the system
下面的例子演示了記錄模式匹配
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 yVal evaluatePoint { 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 }
當你編譯和執(zhí)行程序,它產生以下輸出
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).
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