Le 30/10/07, Dupont Corentin a écrit :

There must be a way to divide a real by an integer?? My integer is eventually a "number of elements" as in the exemple of Dan. How would you compute a mean then?

No, you can't "divide a real by an integer", but you can introduce the integer in the reals and divide by this. divideByI :: (Fractional a, Integral b) => a -> b -> a divideByI a b = a / fromIntegral b -- Jedaï

--- Dupont Corentin wrote:

There must be a way to divide a real by an integer?? My integer is eventually a "number of elements" as in the exemple of Dan.

As you can see below it's impossible to divide a Float by an Integer directly. Haskell is "strongly-typed" . But there exist a conversion function: fromInt :: Int->Float Prelude> (10.2::Float)/(2::Integer) ERROR - Type error in application *** Expression : 10.2 / 2 *** Term : 10.2 *** Type : Float *** Does not match : Integer Prelude> (10.2::Float)/(2::Int) ERROR - Type error in application *** Expression : 10.2 / 2 *** Term : 10.2 *** Type : Float *** Does not match : Int Solution: Prelude> (10.2::Float)/fromInt(2::Int) 5.1 References: Simon Thompson, Craft of Functional Programming, second edition, page 44 - basic types and definitions.

How would you compute a mean then?

On 10/29/07, Dan Popa wrote:

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Why don't you try :

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map (/5.0) [0.0 .. 10.0]

let m = 5.0 map (/m) [0.0 .. 10.0]

I have a similar problem trying a divison by a

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of a list. Finaly I have to build a version of length called len:

len [] = 0.0 len (h:t) = 1.0 + len t

Dan

--- Dupont Corentin wrote:

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Salut, J'ai vraiment du mal à mettre au point un petit programme... Il me sort souvent des problèmes du style :

No instance for (Fractional Integer) arising from use of `/' at <interactive>:1:4-7 Possible fix: add an instance declaration for (Fractional Integer)

Par exemple sous ghci si je fait:

map (/5) [0..10]

tout se passe bien. Mais si je me dit "tiens je voudrais bien

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le 5":

let m = 5 map (/m) [0..10]

il me sort: No instance for (Fractional Integer) arising from use of `/' at <interactive>:1:4-7 Possible fix: add an instance declaration for (Fractional Integer) In the first argument of `map', namely `(/ m)' In the expression: map ((/ m)) ([0 .. 10]) In the definition of `it': it = map ((/ m)) ([0 .. 10])

Pourtant les 2 codes me sembles assez équivalents!!! Je peux utiliser des "fromInteger", ce qui résout temporairement le problème.

Je me retrouve par la suite avec des (de mémoire) Infered type : Int Expected type : Integer

Ce qui me laisse dans le flou...

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