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Using Dictionaries
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The type System.Collections.Generic.Dictionary<'key,'value> is an efficient hash table structure that is excellent for storing associations between values. The use of this collection from F# code requires a little care, because it must be able to correctly hash the key type. For simple key types such as integers, strings, and tuples, the default hashing behavior is adequate. Here is a simple example: > open System.Collections.Generic;; > let capitals = new Dictionary<string, string>();; val capitals : Dictionary <string, string> > capitals.["USA"] <- "Washington";; val it : unit = ()
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CHAPTER 4 INTRODUCING IMPERA TIVE PROGRA MMING
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> capitals.["Bangladesh"] <- "Dhaka";; val it : unit = () > capitals.ContainsKey("USA");; val it : bool : true > capitals.ContainsKey("Australia");; val it : bool : false > capitals.Keys;; val it : KeyCollection<string,string> = seq["USA"; "Bangladesh"] > capitals.["USA"];; val it : string = "Washington" Dictionaries are compatible with the type seq<KeyValuePair<'key,'value>>, where KeyValuePair is a type from the System.Collections.Generic namespace and simply supports the properties Key and Value. (From the F# perspective, we may have expected this type to be seq<('key * 'value)>, but tuples are not available in the .NET collection library design.) Armed with this knowledge, we can now use iteration to perform an operation for each element of the collection: > for kvp in capitals do printf "%s has capital %s\n" kvp.Key kvp.Value;; USA has capital Washington Bangladesh has capital Dhaka val it : unit = ()
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Using Dictionary s TryGetValue
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The Dictionary method TryGetValue is of special interest, because its use from F# is a little nonstandard. This method takes an input value of type 'Key and looks it up in the table. It returns a bool indicating whether the lookup succeeded: true if the given key is in the dictionary and false otherwise. The value itself is returned via a .NET idiom called an out parameter. From F# code there are actually three ways to use .NET methods that rely on out parameters: You may use a local mutable in combination with the address-of operator &. You may use a reference cell. You may simply not give a parameter, and the result is returned as part of a tuple. Here s how you do it using a mutable local: open System.Collections.Generic
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C HAPTE R 4 IN TRODUCING IMPERATIVE PROGRAM MIN G
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let lookupName nm (dict : Dictionary<string,string>) = let mutable res = "" let foundIt = dict.TryGetValue(nm, &res) if foundIt then res else failwithf "Didn t find %s" nm At the time of this writing, the previous code gives a warning in F# since the use of the address-of operator & may lead to unverifiable or even invalid .NET code. The use of a reference cell can be cleaner. For example: > let res = ref "";; val res: string ref > capitals.TryGetValue("Australia", res);; val it: bool = false > capitals.TryGetValue("USA", res);; val it: bool = true > res;; val it: string ref = { contents = "Washington" } Finally, here is the technique where you simply don t pass the final parameter, and instead the result is returned as part of a tuple: > capitals.TryGetValue("Australia");; val it: bool * string = (false, null) > capitals.TryGetValue("USA");; val it: bool * string = (true, "Washington") Note the value returned in the second element of the tuple may be null if the lookup failed when this technique is used. null values are discussed in the section Working with null Values at the end of this chapter.
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You can use dictionaries with compound keys such as tuple keys of type (int * int). If necessary, you can specify the hash function used for these values when creating the instance of the dictionary. The default is to use generic hashing, also called structural hashing, a topic covered in more detail in 8. If you want to indicate this explicitly, you do so by specifying Microsoft.FSharp.Collections.HashIdentity.Structural when creating the collection instance. In some cases, this can also lead to performance improvements because the F# compiler often generates a hashing function appropriate for the compound type. Here is an example where we use a dictionary with a compound key type to represent sparse maps:
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