CODEWITHSUNDEEP
arraysf#
Marta
Marta

Reputation: 1162

F# add a string to an array of strings

I have a string in F#:

 let name = "Someone"

I also have an array of strings:

 let mutable arraysOfNames : string[] = [||]

I want to add the string name to the array arrayOfNames. How do I do that? It doesn't have to be an array, it can be a Sequence or any other collection that I can check then if it is empty or not.

Upvotes: 7

Views: 10284

Answers (4)

Hasan A Yousef
Hasan A Yousef

Reputation: 25008

You can use Array.append with mutable arrays:

let mutable my_array = [||]
for i in 0 .. array_size do
    my_array  <- [|i|] |> Array.append my_array 
    // my_array  <- Array.append my_array [|i|]
printfn "my array is: %A" my_array

Upvotes: 0

phoog
phoog

Reputation: 43076

It is not possible to add an item to a zero-length array. All you can do is create a new array that holds the item. The currently accepted answer, using Array.append, does not modify the input arrays; rather, it creates a new array that contains the elements of both input arrays. If you repeatedly call append to add a single element to an array, you will be using memory extremely inefficiently.

Instead, in f#, it makes much more sense to use F#'s list type, or, for some applications, ResizeArray<'T> (which is f#'s name for the standard .NET List). If you actually need the items in an array (for example, because you have to pass them to a method whose parameter is an array type), then do as Steve suggests in his comment. Once you have built up the collection, call List.toArray or Array.ofList if you're using an F# list, or call the ToArray() instance method if you're using a ResizeArray.

Example 1:

let mutable listOfNames : string list = []
listOfNames <- "Alice" :: listOfNames
listOfNames <- "Bob" :: listOfNames
//...
let names = listOfNames |> List.toArray

Example 2:

let resizeArrayOfNames = ResizeArray<string>()
resizeArrayOfNames.Add "Alice"
resizeArrayOfNames.Add "Bob"
let names = resizeArrayOfNames.ToArray()

Note that in the first example, you'll get the names in reverse order; if you need them in the same order in which they were added, you'd need

let names = listOfNames |> List.rev |> List.toArray

Upvotes: 7

Petr
Petr

Reputation: 4280

For any Seq which is IEnumerable<T> alias in F# you can write this function:

let addToSequence aseq elm =  Seq.append aseq <| Seq.singleton elm

And use it this way:

let withSomeone = addToSequence [||] "Someone"

You can use Seq.toArray or Seq.toList after you get a result sequence

Upvotes: 4

oopbase
oopbase

Reputation: 11415

Take a look at Array.append.

// Signature: Array.append : 'T [] -> 'T [] -> 'T []

// Usage: Array.append array1 array2

So in your case, you can use it like this:

let name = "someone"
let mutable arrayOfNames = Array.append [|"test"; "abc"|] [|name|]
printfn "%A" arrayOfNames
//[|"test"; "abc"; "someone"|]

So you simply need to transform your string into an array (by using [|string|]). Since Array contains the append function, you can append a string this way to an array.

Upvotes: 2

Related Questions