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Array2 with dictionary<int,string>
f#

Hi

The following interactive session has confused me.

> open System.Collections.Generic;;
> let eim = [| for i in 0..1 -> new Dictionary<int,string>() |];;
val eim : Dictionary<int,string> array
> eim.[0].[25] <- "TwentyTwo";;
val it : unit = ()
> eim.[1].ContainsKey(25);;
val it : bool = false
> eim.[0].ContainsKey(25);;
val it : bool = true
> eim.[0].ContainsKey(24);;
val it : bool = false
> let tim = Array2.create 2 2 (new Dictionary<int,string>() );;
val tim : Dictionary<int,string> [,]
> tim.[0,0].[25] <- "TwentyFive";;
val it : unit = ()
> tim.[0,0].ContainsKey(25);;
val it : bool = true
> tim.[1,1].ContainsKey(25);;
val it : bool = true

I would have the last line to return false.

So the 'problem' is with the 2d array, where it seems they share the dictionary. It's most likely me do not understand the Array2.create, but in the mean time I'll use another dictionary<(int*int), Dictionary<int,string>() ) instead of the array (like one does with python)

.

Hi,

In the line 

1

let eim = [| for i in 0..1 -> new Dictionary<int,string>() |];;

new Dictionary<int,string>() is executed twice: once for i = 0 and another one for i = 1.
So every element for eim has a different dictionary.

In 

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let tim = Array2.create 2 2 (new Dictionary<int,string>() );;

new Dictionary<int,string>() is executed once, and then passed as a parameter for Array2.create, which proceeds to fill the entire 2x2 array with this value.
Think of this as:

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let tim = 
  let d = new Dictionary<int,string>() in
  Array2.create 2 2 d;;

You should use Array2.init instead:

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let tim = Array2.init 2 2 (fun _ _ -> new Dictionary<int,string>() );;

Hope this helps,
Friendly,
Dmitry

By on 4/29/2008 6:33 PM ()

@Dmitry

Thanks, that did it. (there should be a warning not to use .create with mutable data : ) )

By on 4/30/2008 1:41 PM ()

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