將更改列表應用於可變向量的元素

Question

給出的是我要應用於Vector元素的更改列表。 突變如何實現？ 到目前為止，我當前的代碼如下：

import Control.Monad.ST
import qualified Data.Vector.Mutable as M
import qualified Data.Vector as V

main :: IO ()
main = do
    let v = V.fromList [8,7,4,1,5] :: V.Vector Integer
    -- The elements in changes are pairs of vector index and change to apply.
    let changes = [(0, (+1)), (3, (*3)), (2, (/2))]
    let v' = applyChanges changes v
    print $ V.toList v'

applyChanges changes v = runST $ do
    mV <- V.thaw v
    -- apply (+1) to element 0 -> [9,7,4,1,5]
    -- apply (*3) to element 3 -> [9,7,4,3,5]
    -- apply (/2) to element 2 -> [9,7,2,3,5]
    V.freeze mV

Answer 1

使用mapM_ ，您可以執行

apply mvec (idx, f) = do
    val <- M.read mvec idx
    M.write mvec idx $ f val

applyChanges :: [(Int, Integer -> Integer)] -> V.Vector Integer -> V.Vector Integer
applyChanges changes vec = runST $ do
    mV <- V.thaw v
    mapM_ (apply mV) changes
    V.freeze mV

main = do
    let v = V.fromList [8,7,4,1,5] :: V.Vector Integer
    let changes = [(0, (+1)), (3, (*3)), (2, (/2))]
    print $ V.toList $ applyChanges changes v

我實際上所做的就是編寫一個函數，該函數接受一個矢量和一個要應用的更改，然后將其映射到列表中的所有更改。 必要的步驟是M.read ， M.write和mapM_ 。