[Haskell-cafe] Dividing integers?

[Matias Hernandez]

Hello there!

Although you might not believe it since I'm going to ask a very very very basic question, I have been using Haskell for educational purposes for a couple semesters now. Nevertheless, I don't think I have ever worked with non-Int number types! :)

I don't understand why I get this error. I mean, apparently "/" is not defined for integers but I don't know how to "cast" the result of the length function into a Double...

Prelude> (length [1,2]) / 3
<interactive>:1:
No instance for (Fractional Int)
arising from use of `/' at <interactive>:1
In the definition of `it': it = (length [1, 2]) / 3

Besides, a simple integer division works:

Prelude> 2 / 3
0.6666666666666666

But I would guess that's because '2' and '3' are parsed as Floats because they are used in a divide function.

Any help will be appreciated!

Best regards,
- Matías

[Keith Wansbrough]

Post by Matias Hernandez
Hello there!
Although you might not believe it since I'm going to ask a very very
very basic question, I have been using Haskell for educational
purposes for a couple semesters now. Nevertheless, I don't think I
have ever worked with non-Int number types! :)
I don't understand why I get this error. I mean, apparently "/" is
not defined for integers but I don't know how to "cast" the result
of the length function into a Double...

fromInteger or fromIntegral. (the former converts an Integer to any
numeric type, the latter converts any integral type to any numeric type).

--KW 8-)

[Arjan van IJzendoorn]

Post by Matias Hernandez
I don't understand why I get this error. I mean, apparently "/" is not

defined for integers but I don't know

Post by Matias Hernandez
how to "cast" the result of the length function into a Double...

Use div for dividing integers with loss of precision:
3 `div` 2 (or: div 3 2) == 1

"Casting" an Int to a Float/Double can be done using fromIntegral:

fromIntegral (length [1,2]) / 3 == 0.6666666666666666

Post by Matias Hernandez
Prelude> 2 / 3
0.6666666666666666

This is not an integer division. Writing down a literal (like 2 or 3)
denotes a number, not necessarily an Int. The context will influence what
type it will be. Something I find really confusing myself, too (which is why
it is not in Helium :-).

Greetings, Arjan

[Ferenc Wagner]

Post by Matias Hernandez
I mean, apparently "/" is not defined for integers but I
don't know how to "cast" the result of the length function
into a Double...
Prelude> (length [1,2]) / 3

Prelude> fromIntegral (length [1,2])/3
0.666666666666667

Post by Matias Hernandez
Prelude> 2 / 3
0.6666666666666666

Literal 2 means fromIntegral 2, but length::[a]->Int.

[Jon Fairbairn]

Post by Ferenc Wagner

Post by Matias Hernandez
I mean, apparently "/" is not defined for integers but I
don't know how to "cast" the result of the length function
into a Double...
Prelude> (length [1,2]) / 3

Literal 2 means fromIntegral 2, but length::[a]->Int.

Note also

Data.List.genericLength :: Num a => [b] -> a