F＃字典初始化，就像在Python中一样

Question

I'm starting to learn F# and I'm having some hard times trying to figure out simple things. I have a python code that I would like to convert to F#. The problem is the initialization of dictionaries in python that I don't really know how to convert to F#.

dicoOfItems = {'aaaaa': {'a': 2.5, 'b': 3.5, 'c': 3.0, 'd': 3.5, 'e': 2.5,'f': 3.0}, 'bbbbb': {'a': 3.0, 'b': 3.5}}

then there is a function

def sim_distance(prefs,person1,person2):
 si={} // I want the same in F#
  for item in prefs[person1]: 
    if item in prefs[person2]: si[item]=1

 // do stuff
return something

this function is called with the following parameters for example

sim_distance(dicoOfItems, 'aaaaa', 'bbbbb')

My question is how I could do the same in F# for obtaining a new dictionary si ?

The Python if .. in .. list syntax I tried to use with f# Seq.exists but then I didn't know how to initialize the new dictionary.

I've played with Seq.choose, Seq.map but with no success.

Answer 1

let test = dict [for x in [1..10] do
                     if x%2 = 0 then
                         yield x.ToString(),x] //note that this is returning a (string*int)
printfn "%d" test.["5"]

As John Palmer noted, the appropriate way to create a dictionary in a single statement in F# is to use the dict function which takes a sequence type and converts it to a dictionary.

dict;;
val it : (seq<'a * 'b> -> IDictionary<'a,'b>) when 'a : equality = <fun:clo@3>

Note that

[for x in [1..10] do
     if x%2 = 0 then
         yield x.ToString(),x]

creates a list ([] is list notation and lists are sequences), and then that list is the parameter to the function dict which converts it to a dictionary.

Your function would then look like this:

let sim_distance prefs person1 person2 =
    let si=dict [for item in prefs.[person1] do
                     if prefs.[person2].Contains(item) then 
                         yield item,1]
    something

Answer 2

So I think you want to use the System.Collections.Generic.Dictionary<_,_> which is mutable rather than the F# dict which is immutable. You would use it as follows:

let  sim_distance(prefs:System.Collections.Generic.IDictionary<_,_>,person1,person2) =
  let si= System.Collections.Generic.Dictionary<_,_>() 

  for KeyValue(k,v) in prefs.[person1] do 
    for KeyValue(k2,v2) in prefs.[person2] do if k2=k then si.Add(k,1)