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Create a histogram of a sequence using map

Creates a map containing each item and its frequency as a key/value pair. Then sort by value in reverse order before printing each item.

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let histogram =
    Seq.fold (fun acc key ->
        if Map.containsKey key acc
        then Map.add key (acc.[key] + 1) acc
        else Map.add key 1 acc
    ) Map.empty
    >> Seq.sortBy (fun kvp -> -kvp.Value)

"Testing shows the presence, not the absence of bugs"
|> histogram |> Seq.iter (printfn "%A")

val histogram : (string -> seq<System.Collections.Generic.KeyValuePair<char,int>>)

Full name: Script.histogram

module Seq

from Microsoft.FSharp.Collections

val fold : folder:('State -> 'T -> 'State) -> state:'State -> source:seq<'T> -> 'State

Full name: Microsoft.FSharp.Collections.Seq.fold

val acc : Map<char,int>

val key : char

Multiple items
module Map

from Microsoft.FSharp.Collections

--------------------
type Map<'Key,'Value (requires comparison)> =
  interface IEnumerable
  interface IComparable
  interface IEnumerable<KeyValuePair<'Key,'Value>>
  interface ICollection<KeyValuePair<'Key,'Value>>
  interface IDictionary<'Key,'Value>
  new : elements:seq<'Key * 'Value> -> Map<'Key,'Value>
  member Add : key:'Key * value:'Value -> Map<'Key,'Value>
  member ContainsKey : key:'Key -> bool
  override Equals : obj -> bool
  member Remove : key:'Key -> Map<'Key,'Value>
  ...

Full name: Microsoft.FSharp.Collections.Map<_,_>

--------------------
new : elements:seq<'Key * 'Value> -> Map<'Key,'Value>

val containsKey : key:'Key -> table:Map<'Key,'T> -> bool (requires comparison)

Full name: Microsoft.FSharp.Collections.Map.containsKey

val add : key:'Key -> value:'T -> table:Map<'Key,'T> -> Map<'Key,'T> (requires comparison)

Full name: Microsoft.FSharp.Collections.Map.add

val empty<'Key,'T (requires comparison)> : Map<'Key,'T> (requires comparison)

Full name: Microsoft.FSharp.Collections.Map.empty

val sortBy : projection:('T -> 'Key) -> source:seq<'T> -> seq<'T> (requires comparison)

Full name: Microsoft.FSharp.Collections.Seq.sortBy

val kvp : System.Collections.Generic.KeyValuePair<char,int>

property System.Collections.Generic.KeyValuePair.Value: int

val iter : action:('T -> unit) -> source:seq<'T> -> unit

Full name: Microsoft.FSharp.Collections.Seq.iter

val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn

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More information

Link:	http://fssnip.net/n5
Posted:	9 years ago
Author:	Bjørn Bæverfjord
Tags:	histogram , map , seq , fold , composition , >>