Re: [Haskell-beginners] Using findIndex and then splitAt in Data.List

Ah yeah, I messed up a bit. Another way you can deal with this is using the maybe function. If you are absolutely positive that it can never fail, it doesn't hurt to choose a default index of 0. let gradeItemIndex = ... let twoListsTuple = splitAt (maybe 0 id gradeItemIndex) However if the code around it changes your assumption may become invalidated in the future, and your code will explode. On Fri, Feb 27, 2015 at 4:41 PM, Geoffrey Bays wrote:

David: Thanks for putting me on the right track. This works:

let twoListsTuple = case findIndex (\g -> (itemName g) == (itemName gradeItemP)) gradeItemList of Nothing -> (gradeItemList, []) Just x -> splitAt x gradeItemList

I was not sure if the splitAt function would take a Just Int in place of a regular Int without complaint, but that seems to be the case. As it turns out, I can be certain from the context that the item will definitely be found in the list, but the case statement allows for the unpacking of the Maybe.

Thanks,

Geoffrey

On Fri, Feb 27, 2015 at 4:16 PM, David McBride wrote:

...

You will probably do a case match on the result of findIndex to find out whether you want to split there. You will have to deal with the possibility that the item you are searching for is not in the list. Untested:

let twoListsTuple = case findIndex (\g -> (itemName g) == (itemName gradeItemP)) gradeItemList of Nothing -> (gradeItemList,[]) Just newlists -> newlists

On Fri, Feb 27, 2015 at 3:51 PM, Geoffrey Bays wrote:

...

Hi. An elementary question here about two functions in Data.List: how to use a value from findIndex which returns a Maybe Int, and then use that result in splitAt which takes a regular Int? This is in an IO() do

Like so: (This is in an IO() do block)

let gradeItemIndex = findIndex (\g -> (itemName g) == (itemName gradeItemP)) gradeItemList let twoListsTuple = splitAt gradeItemIndex gradeItemList

// does not compile obviously

Many Thanks,

Geoffrey

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