I am trying to write some f# code to do some image processing. And I am trying to get the hang of the best way to initialize and modifiy some structures in a functional way using immutable types.

So far I have the following code listed below. My question is whether there is a better way to initialize the
regions. I use the Array2.iter but I would like to use something like Array2.fold_left as it seems expensive to keep creating a new region in each iteration, but this construct only seems to be available for
1-dimensional arrays. The second question is there a more functional way to replace the for loop where I calculate the mean and standard deviation for each region.

#light

open System.Drawing

type Pixel = { Label : int; X : int; Y : int; V : int}
type Region = { Sum : int; Sq_Sum : int; Mean : float; Variance : float; N : int;}

let random = new System.Random()
let img = Bitmap.FromFile(@"C:\Documents and Settings\jeffb\Desktop\232b\elephant.gif")
let bmp = img :?> Bitmap
let xmax = bmp.Width
let ymax = bmp.Height

let k = 2

let init = { Sum=0; Sq_Sum =0; Mean=0.; Variance=0.; N=0; }
let regions = Array.create k init

let pixels = Array2.init xmax ymax (fun i j -> { Label=random.Next(k); X=i; Y=j; V=int(bmp.GetPixel(i,j).B); } )

let _ = Array2.iter (fun p -> regions.[p.Label] <- { Sum = regions.[p.Label].Sum + p.V;
N = regions.[p.Label].N + 1;
Sq_Sum = regions.[p.Label].Sq_Sum + p.V*p.V;
Variance=0.; Mean=0.;}) pixels

for i = 0 to k-1 do
regions.[i] <- { Sum=regions.[i].Sum; Sq_Sum=regions.[i].Sq_Sum;
Mean=float(regions.[i].Sum)/float(regions.[i].N);
Variance=float(regions.[i].Sq_Sum)/float(regions.[i].N) -
float(regions.[i].Sum)/float(regions.[i].N)*float(regions.[i].Sum)/float(regions.[i].N);
N=regions.[i].N; }