Thank you for the reply, but it looks like this is a different
function. Here is the interactive output of my function vs. the one
you give (my code is in Table/IO.hs):
Prelude> :load Table.IO
*Table.IO> maxTableColumnWidths (replicate 10 ["hello", "world"])
[5,5]
*Table.IO> let maxTableColumnWidths2 = map (maximum . map length)
*Table.IO> maxTableColumnWidths2 (replicate 10 ["hello", "world"])
[5,5,5,5,5,5,5,5,5,5]
-Keith
On Sat, Feb 21, 2009 at 5:29 PM, 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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners