Add imputeExpr function

[fserucas]

This change adds a imputeExpr function which behaves like impute but accepts Col Expr. #45

[fserucas]

It was a challenge understanding this, and i am still not sure about this.
Could you guide me.
Tks

[mchav]

Col a is just a typed reference to a column. It sort of looks like: data Expr = Col String | .... As written, a === columnName. You probably want to interpret the expression, get the resulting column then zip it with the result of getColumn.

[adityakaldate21-dev]

-- src/DataFrame/Operations/Transformations.hs

{-# LANGUAGE OverloadedStrings #-}

module DataFrame.Operations.Transformations (imputeExpr) where

import qualified Data.Map as M
import DataFrame.Types
import DataFrame.Exceptions
import DataFrame.Utils (getColumn, setColumn, zipWithDefault)

-- Simplified expression type
data Expr = Col String | Lit Value

-- Evaluate an expression to a column of values
evalExpr :: Expr -> DataFrame -> [Value]
evalExpr (Col name) df =
case M.lookup name (columnIndices df) of
Just _ -> getColumn name df
Nothing -> error $ "Column " ++ name ++ " not found in expression"
evalExpr (Lit v) _ = repeat v

-- Impute missing values in a column using values from an expression
imputeExpr :: ColumnName -> Expr -> DataFrame -> DataFrame
imputeExpr columnName expr df =
case M.lookup columnName (columnIndices df) of
Nothing -> throw $ ColumnNotFoundException columnName "imputeExpr" (map fst $ M.toList $ columnIndices df)
Just (OptionalColumn _) ->
let targetCol = getColumn columnName df
sourceCol = evalExpr expr df
imputedCol = zipWithDefault targetCol sourceCol
in setColumn columnName imputedCol df
Just _ -> throw $ InvalidColumnTypeException columnName "Expected OptionalColumn"

-- Replace Nothing in targetCol with corresponding value from sourceCol
zipWithDefault :: [Value] -> [Value] -> [Value]
zipWithDefault = zipWith (\t s -> case t of { Nothing -> s; Just _ -> t })