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Seq.iter<'T> Function (F#)

Applies the given function to each element of the collection.

Namespace/Module Path: Microsoft.FSharp.Collections.Seq

Assembly: FSharp.Core (in FSharp.Core.dll)

// Signature:
Seq.iter : ('T -> unit) -> seq<'T> -> unit

// Usage:
Seq.iter action source

action

Type: 'T -> unit

A function to apply to each element of the sequence.

source

Type: seq<'T>

The input sequence.

Exception

Condition

ArgumentNullException

Thrown when the input sequence is null.

This function is named Iterate in compiled assemblies. If you are accessing the function from a language other than F#, or through reflection, use this name.

The following example illustrates the use of Seq.iter.

printf "Seq.iter: "
Seq.iter (fun (a,b) -> printf "(%d, %d) " a b) (seq { for i in 1..5 -> (i, i*i) })
Seq.iter: (1, 1) (2, 4) (3, 9) (4, 16) (5, 25)

The following example illustrates the use of Seq.iter to work with CSV (Comma-Separated Value) files.

// Write a test file
System.IO.File.WriteAllLines(@"test.csv", [| "Desmond, Barrow, Market Place, 2"; 
                                             "Molly, Singer, Band, 12" |]);

/// This function builds an IEnumerable<string list> object that enumerates the CSV-split 
/// lines of the given file on-demand  
let CSVFileEnumerator(fileName) = 

    // The function is implemented using a sequence expression
    seq { use sr = System.IO.File.OpenText(fileName)
          while not sr.EndOfStream do 
             let line = sr.ReadLine() 
             let words = line.Split [|',';' ';'\t'|] 
             yield words }

// Now test this out on our test file, iterating the entire file 
let test = CSVFileEnumerator(@"test.csv")  
printfn "-------Enumeration 1------";
test |> Seq.iter (string >> printfn "line %s");
// Now do it again, this time determining the numer of entries on each line. 
// Note how the file is read from the start again, since each enumeration is  
// independent.
printfn "-------Enumeration 2------";
test |> Seq.iter (Array.length >> printfn "line has %d entries");
// Now do it again, this time determining the numer of entries on each line. 
// Note how the file is read from the start again, since each enumeration is  
// independent.
printfn "-------Enumeration 3------";
test |> Seq.iter (Array.map (fun s -> s.Length) >> printfn "lengths of entries: %A")
-------Enumeration 1------
line System.String[]
line System.String[]
-------Enumeration 2------
line has 8 entries
line has 7 entries
-------Enumeration 3------
lengths of entries: [|7; 0; 6; 0; 6; 5; 0; 1|]
lengths of entries: [|5; 0; 6; 0; 4; 0; 2|]

Windows 8, Windows 7, Windows Server 2012, Windows Server 2008 R2

F# Core Library Versions

Supported in: 2.0, 4.0, Portable

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