[Haskell-beginners] Prelude.(!!): index too large

[David McBride]

It just means you are trying to directly access a list element by its index
in a list that doesn't have that many elements.  It is probably somewhere
in your floatData function, which you did not list.  When you don't print
it out to screen, haskell does not bother to evaluate it and therefore it
never hits that error.  If you didn't use that operator, then it is used in
some library you are using.

Prelude> [1,2,3] !! 5
*** Exception: Prelude.(!!): index too large


On Thu, Apr 4, 2013 at 1:49 PM, Roger Mason <rmason at mun.ca> wrote:

> Hello,
>
> The following fragment compiles but causes a runtime error:
>
> Prelude.(!!): index too large
>
> If I comment out the line 'hPutStr outh ( show ncat)' the program runs
> fine (but there is no output, of course).
>
> Thanks for any help.
>
> Roger
>
> writeOutput infile outfile list fox mw nox ncat units = do
>           inh <- openFile infile ReadMode -- the file of analyses, one per
> line
>           outh <- openFile outfile WriteMode
>           mainloop inh outh list fox mw nox ncat units
>           hClose outh
>           hClose inh
>
> -- the calculations are here
> mainloop inh outh list fox mw nox ncat units = do
>              ineof <- hIsEOF inh
>              if ineof
>                      then do
>                          print list      -- For testing/debugging only
>                          return ()
>                      else do
>                    inpStr <- hGetLine inh
>                    let list' = inpStr:list
>                            let dat = floatData list' -- the analytical
> input data
>                            let ta = zipWith (/) dat units
>            -- convert all to wt%
>                            let moles = zipWith (/) ta mw
>                            let apo = zipWith (*) nox moles
>                            let factor = fox/(sum apo)
>                            let nanions= (map (* factor) apo)
>                            let catox = zipWith (/) ncat nox
>                            let ncat = zipWith (*) catox nanions
>                            hPutStr outh ( show ncat)
>                    mainloop inh outh list' fox mw nox ncat units
>
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