I’d be interested in a tutorial on elementary use of the mysterious Bigarray–or even simple examples of use. The OCaml manual chapter on Bigarray and the ocamldoc pages are pretty opaque.
I don’t actually want to interface with C or Fortran or anything else. (Other folks have done that part.)
At this point, I would just like to be able to create a Genarray or Array2 with values that I have specified. This is non-trivial! Too much mysterious type information is required, and I don’t know how to provide it.
In the long run, it might be useful if someone made available a more thorough introduction to Bigarray somewhere.
(I had also checked the two books I have, RWO (just some passing references), and Développement d’applications avec Objective Caml from 2000 (nothing–maybe Bigarray was developed later).)
One thing that confused me is that I initially tried to make an integer array, and discovered that normal ints are neither Bigarray.int32 nor Bigarray.int64. So I assumed that floats would not be Bigarray.float32 or Bigarray.float64. Yet I can assign them into either kind of float Array2.
(I must be very tired. I just tried to end a sentence with “;;”.)
The first type float is the type used to store and retrieve values from the array. If you look at the type returned for a float64 array you’ll see that it is also float:
# let my_second_big_array =
Bigarray.Array2.create Bigarray.float64 Bigarray.c_layout 20 20;;
val my_second_big_array :
(float, Bigarray.float64_elt, Bigarray.c_layout) Bigarray.Array2.t =
<abstr>
It’s true, on my 64-bit machine, the store/retrieve type for a 32-bit int array is int32:
# let my_third_big_array =
Bigarray.Array2.create Bigarray.int32 Bigarray.c_layout 20 20;;
val my_third_big_array :
(int32, Bigarray.int32_elt, Bigarray.c_layout) Bigarray.Array2.t =
<abstr>
Presumably this is so the code will also work on a 32-bit machine. (On a 32-bit machine a plain OCaml int has 31 bits.)
@jeffsco, thanks for explaining the meaning of the first type in the tuple. And I guess the second one is the internal type of the array. That’s very helpful. It was the hint that allowed me to get a little bit further.
I don’t think the problem with storing ints into an Array2 has anything to do with the machine type:
My machine should be 64 bits, but I’ll try both 64-bit and 32-bit integers just to be sure:
# let int32_array = Bigarray.Array2.create Bigarray.int32 Bigarray.c_layout 20 20;;
val int32_array :
(int32, Bigarray.int32_elt, Bigarray.c_layout) Bigarray.Array2.t = <abstr>
# Bigarray.Array2.set int32_array 0 0 22;;
Error: This expression has type int but an expression was expected of type
int32
# let int64_array = Bigarray.Array2.create Bigarray.int64 Bigarray.c_layout 20 20;;
val int64_array :
(int64, Bigarray.int64_elt, Bigarray.c_layout) Bigarray.Array2.t = <abstr>
# Bigarray.Array2.set int64_array 0 0 22;;
Error: This expression has type int but an expression was expected of type
int64
So I can’t put an int in either kind of Array2.
Then I discovered that there is an Int64 module with an of_int function:
The problem here is that we still don’t have a surfaceable Bigarray tutorial merged into the official docs or website, we just have another search result.
My observation is just that on a 64-bit machine it would be most convenient to use a plain int as the store/retrieve type for a 32-bit integer bigarray. int32’s are a little cumbersome and are boxed. I would assume using int32 is for compatibility with 32-bit machines where you can’t use a plain int. They’re not big enough to hold 32 bits.