Haskell 中的列表到列表

Question

我有一个 CSV 文件，有 2 列，每列有 32100 行。我需要导入这些数据并将每列分成一个列表列表，每个列表有一千个值。例如，让column2_data = [[1,2,3,4,], [3,4,6,7], ...]（注意：数据是float类型）。”

import Data.Csv
import qualified Data.ByteString.Lazy as BL
import qualified Data.Vector as V


parserCsv :: IO ()
parserCsv = do
    csvData <- BL.readFile "src/salary.csv"
    case decode NoHeader csvData of
        Left err -> putStrLn err
        Right v -> V.forM_ v $ \(x, y) ->
            putStrLn $ show (y :: Float) ++ " // " ++ show (y :: Float)

I need to import this data and separate each column into a list of lists with a thousand values each list. E.g., let column2_data = [[1,2,3,4,], [3,4,6,7], ...] (Note: the data is of type float).

Answer 1

我会分离逻辑以将其转换为列表，其中：

groupAt :: Int -> [a] -> [[a]]
groupAt n = go
  where go [] = []
        go xs = ys : go zs
            where ~(ys, zs) = splitAt n xs

然后用它来处理解码的结果：

parserCsv :: IO ([[Float]], [[Float]])
parserCsv = do
    csvData <- BL.readFile "src/salary.csv"
    case decode NoHeader csvData of
        Left err -> error err
        Right v -> (groupAt 1000 (toList fmap fst v), groupAt 1000 (toList fmap snd v))