List.fold<'T,'State> 函式 (F#)
將函式 f 套用至集合的每一個項目，以透過計算建立累計值引數的執行緒。fold 函式會接受第二個引數，並且將函式 f 套用至這個引數和清單的第一個項目。然後將這個結果與第二個項目一併輸入函式 f，依此類推。它會傳回最終結果。如果輸入函數為 f 而且項目為 i0...iN，則這個函式會計算 f (... (f s i0) i1 ...) iN。
**命名空間/模組路徑：**Microsoft.FSharp.Collections.List
組件：FSharp.Core (在 FSharp.Core.dll 中)
// Signature:
List.fold : ('State -> 'T -> 'State) -> 'State -> 'T list -> 'State
// Usage:
List.fold folder state list
參數
folder
型別：'State -> 'T -> 'State根據指定的輸入項目更新狀態的函式。
state
型別：'State初始狀態。
list
型別：'Tlist輸入清單。
傳回值
最終狀態值。
備註
這個函式在已編譯的組件中名為 Fold。如果您是透過 F# 以外的語言，或是透過反映來存取函式，請使用這個名稱。
範例
下列範例說明 List.fold 的用法。
let data = [("Cats",4);
("Dogs",5);
("Mice",3);
("Elephants",2)]
let count = List.fold (fun acc (nm,x) -> acc+x) 0 data
printfn "Total number of animals: %d" count
下列程式碼範例說明 List.fold 的其他用途。請注意，程式庫函式存在已封裝以下實作的功能。例如，List.sum 可用於加總清單中的所有項目。
let sumList list = List.fold (fun acc elem -> acc + elem) 0 list
printfn "Sum of the elements of list %A is %d." [ 1 .. 3 ] (sumList [ 1 .. 3 ])
// The following example computes the average of a list.
let averageList list = (List.fold (fun acc elem -> acc + float elem) 0.0 list / float list.Length)
// The following example computes the standard deviation of a list.
// The standard deviation is computed by taking the square root of the
// sum of the variances, which are the differences between each value
// and the average.
let stdDevList list =
let avg = averageList list
sqrt (List.fold (fun acc elem -> acc + (float elem - avg) ** 2.0 ) 0.0 list / float list.Length)
let testList listTest =
printfn "List %A average: %f stddev: %f" listTest (averageList listTest) (stdDevList listTest)
testList [1; 1; 1]
testList [1; 2; 1]
testList [1; 2; 3]
// List.fold is the same as to List.iter when the accumulator is not used.
let printList list = List.fold (fun acc elem -> printfn "%A" elem) () list
printList [0.0; 1.0; 2.5; 5.1 ]
// The following example uses List.fold to reverse a list.
// The accumulator starts out as the empty list, and the function uses the cons operator
// to add each successive element to the head of the accumulator list, resulting in a
// reversed form of the list.
let reverseList list = List.fold (fun acc elem -> elem::acc) [] list
printfn "%A" (reverseList [1 .. 10])
Output
平台
Windows 8 中， Windows 7， Windows Server 2012 上， Windows Server 2008 R2
版本資訊
F# 核心程式庫版本
支援版本:2.0， 4.0，可攜式執行檔 (PE)。