[Haskell-beginners] A better way?

[Thomas Davie]

How about this:

maxTableColumnWidths :: [[String]] -> [Int]
maxTableColumnWidths = map (maximum . map length)

Bob


On 21 Feb 2009, at 22:35, Keith Sheppard wrote:

> Hello,
>
> I'm new to haskell and still getting used to working with lazy
> evaluation. I created a little function to calculate column widths for
> a 2D list of strings (a table) by iterating through the list and
> accumulating a max value for each column. My initial implementation
> ran out memory for tables with many rows because of lazy evaluation
> and here is how I dealt with it:
>
> {- |
> for a table, calculate the max width in characters for each column
> -}
> maxTableColumnWidths :: [[String]] -> [Int]
> maxTableColumnWidths [] = []
> maxTableColumnWidths table =
>    maxTableColumnWidthsInternal table []
>
> maxTableColumnWidthsInternal :: [[String]] -> [Int] -> [Int]
> maxTableColumnWidthsInternal [] prevMaxValues = prevMaxValues
> maxTableColumnWidthsInternal (row:tableTail) prevMaxValues
>    | seqList prevMaxValues = undefined
>    | otherwise = maxTableColumnWidthsInternal tableTail
> (maxRowFieldWidths row prevMaxValues)
>
> -- this little function is for making the list strict... otherwise
> -- we run out of memory
> seqList [] = False
> seqList (head:tail)
>    | head `seq` False = undefined
>    | otherwise = seqList tail
>
> maxRowFieldWidths :: [String] -> [Int] -> [Int]
> maxRowFieldWidths row prevMaxValues =
>    let colLengths = map length row
>        lengthOfRow = length row
>        lengthOfPrevMax = length prevMaxValues
>        maxPrefixList = zipWith max colLengths prevMaxValues
>    in
>        if lengthOfRow == lengthOfPrevMax then
>            maxPrefixList
>        else if lengthOfRow > lengthOfPrevMax then
>            maxPrefixList ++ (drop lengthOfPrevMax colLengths)
>        else
>            maxPrefixList ++ (drop lengthOfRow prevMaxValues)
>
>
> This works but it isn't very pretty (maybe also inefficient?). Is
> there a better way to deal with this kind of memory issue?
>
> Thanks!
> Keith
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