Using OCaml for Scientific Computing  1. Setup & Conventions
Intro
Use Owl in Toplevel
load Owl in utop with the following commands. owltop is Owl’s toplevel library which will automatically load several related libraries (including owlzoo, owlbase, and owl core library) to set up a complete numerical environment.
#require "owltop";;
open Owl;;
Use Owl in Notebook
opam install jupyter
jupyter kernelspec install name ocamljupyter "$(opam config var share)/jupyter"
For the above, if installing confzmq is a problem, try brew install zeromq
.
Once notebook is opened, run the following OCaml code in the first input field to load Owl environment:
#use "topfind";;
#require "owltop, jupyter.notebook";;
Alternatively, just open a new notebook of ocaml default.
If jupyter couldn’t find “topfind”, consider editing ~/.ocamlinit
to automatically load path (see here), then do reinitialize opam with opam init
Conventions
Pure vs. Impure
Pure Functions
(a.k.a immutable functions) refer to those which do not modify the passed in variables but always return a new one as result.
Impure functions
(a.k.a mutable functions) refer to those which modifies the passedin variables in place.
 : make it harder to reason the correctness of the code.
 +: help avoid expensive memory allocation and deallocation operations, this can significantly improve the runtime performance of a numerical application especially when large ndarrays and matrices involved.
Middel Ground
Can we write functional code and achieve memory efficiency at the same time? Yes. Use lazy evaluation with Owl’s computation graph, where Owl takes care of allocating and managing memory.
Notation
Many pure functions in Ndarray module have their corresponding impure version.
 pure:
Arr.sin;;
 impure:
Arr.sin_;;
example
Arr.add_ x y
 signature:
val Arr.add_ : ?out:Arr.arr > Arr.arr > Arr.arr > unit
 the output can be specified by an optional out parameter. If out is missing in the inputs, then Owl will try to use first operand (i.e. x) to store the final result. Because the binary operators in Owl support broadcasting operations by default, this further indicates when using impure functions every dimension of the first argument x must not be smaller than that of the second argument y. In other words, impure function only allows broadcasting smaller y onto x which is big enough to accomodate the result.
Shorthand operators
============== ============== ==============
Function Name Pure Impure
============== ============== ==============
add `+` `+=`
sub `` `=`
mul `*` `*=`
div `/` `/=`
add_scalar `+$` `+$=`
sub_scalar `$` `$=`
mul_scalar `*$` `*$=`
div_scalar `/$` `/$=`
============== ============== ==============
Arr.(x + y)
returns the result in a new ndarrayArr.(x += y)
adds up x and y and saves the result into x.
Ndarray vs. Scalar
There are three types of ndarray operations:
 map
 scan
 reduce
reduction operations
Such as Arr.sum
, Arr.prod
, Arr.min
, Arr.mean
, Arr.std
, and etc.
All the reduction functions in Owl has a name parameter called axis. When you apply these reduction operations on a multidimensional array, there are two possible cases:
 if axis is explicitly specified, then Owl reduces along the specified axis
 if axis is not specified, then Owl flattens the ndarray into a vector first and reduce all the elements along the axis 0
example:
let x = Arr.sequential [3;3;3];;
let a = Arr.sum ~axis:1 x;;
let b = Arr.sum x;;
retrieve result
Use get function:
let c = Arr.get b [0] in c +. 10.;;
signature of get:
Arr.arr > int array > float = <fun>
two versions
Every reduction operation has two versions:

one allows you to reduce along the specified axis, or reduce all the elements, but always returns an ndarray;
Arr.sum;; Arr.min;; Arr.prod;; Arr.mean;; Arr.std;;

one only reduces all the elements and always returns a scalar value.
Arr.sum';; Arr.min';; Arr.prod';; Arr.mean';; Arr.std';;
Technically,
Arr.sum'
is equivalent to the following code:let sum' x = let y = Arr.sum x in Arr.get y [0]