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F# - Nested loops in a functional style
f#em algorithmnested loops

I'm a scientist using languages such as F#, Python, R, Matlab in my research, so I'm not a hardcore programmer. I've just recently started to look more deeply into functional programming and have been moving more towards F# from mainly Python when building more resource intense applications.

Currently I'm working on an EM-algorithm implementation for an aspect model in F#, but I'm having some problems translating my previous code to a more functional style.

I have a list (list1) of objects (the list is created in C# outside of my F# implementation), and each object contains a <Aspect,double> dictionary (dic1). Aspect refers to an object.

What I want to do is iterate through every object in list1 and apply a function to the values of dic1.

Using for loops I loop through the documents, then the dictionary values so it may look like:

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let mutable sum = 0
for obj in list1 do
  for elem in obj.dic1.Values do
    sum<-sum+elem*10

I can get the same result by:

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for obj in list1 do
  let sum = Seq.fold (fun (acc : double) v -> acc+10.0*v) 0.0 obj.dic1.Values

However, the dic1 dictionary contains a subset of, say, 100 aspects. So each dic1 can only contain at most 100 objects. The problem is I also need to check that the current aspect is also present in another objects dic1 dictionary (this object is outside of list1). Modifying the above for loop I can code this as:

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let mutable sum = 0
for obj in list1 do
  for key in obj.dic1.Keys do
    if extObj.dic1.ContainsKey(key) then
      sum<-sum+obj.dic1.[key]*10

Where extObj is some object not present in list1.

I'm having problems getting the key from the dictionary though. I've found code snippets that look something like:

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let sum = List.fold (fun acc (KeyValue(k,v))->acc+v*10) list1.dic1

So that I can check for the occurence of the key k in the external object.
But this creates an error where list1.dic1 is expected to be
System.Collections.Generic.KeyValuePair<'a,'b> list, and is actually an IDictionary.

Do you have any pointers what I should be doing to make this nested loop work in a "functional style"?

Appreciate any help!

.

I would probably do something like:

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list1
|> Seq.collect (fun obj -> obj.dic1)
|> Seq.filter (fun (KeyValue(k, _)) -> extObj.dic1.ContainsKey(k))
|> Seq.sumBy (fun (KeyValue(_, v)) -> v * 10.0)

(|KeyValue|) is used when enumerating a dictionary. The error is due to enumerating the Keys collection and not the dictionary itself.

By on 10/13/2011 6:46 PM ()

That code looks really neat!
But I get an error: "This expression should have type 'unit' but has type 'float'"

I get this error a lot in my journey converting to F#, which is probably a result of me not fully grasping types yet having come from R/Matlab/Python ;)

By on 10/14/2011 8:36 AM ()

Nevermind, I just hadn't assigned the value to anything..

Thanks for the answer Daniel!

By on 10/14/2011 9:56 AM ()

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