Passing array into Function

Question

I'm trying to write a simple F# function that I can pass an array into and then print the values, but I'm having trouble. Here is what I have so far:

let a = [| a; b; c; d |];;
let f arrayFunction (string[] array) = function
 for b=0 to array.Length
 Console.WriteLine(array.[]);;

Answer 1

The F# syntax for defining parameters is backwards from the C# syntax; in F#, the name of the parameter comes first, then the type (with a colon to separate the two).

You also don't need the function keyword here, just the normal let binding -- function is for creating anonymous pattern-matching functions. You do, however, need to add a do at the end of the line in your for loop. Finally, the value after the to in an F# for loop is inclusive -- so you need to subtract one from the array length or you'll end up raising an IndexOutOfRangeException.

Your function should be written like this:

let a = [| a; b; c; d |];;
let f arrayFunction (array : string[]) =
 for b = 0 to array.Length - 1 do
 Console.WriteLine (array.[b]);;

• John Oriely: Wow That works perfect. Thank you so much for your help and explaination!

Answer 2

Jack's answer is exactly correct however there are built-in functions in F# to do these kinds of tasks. In this instance we can send the array to Array.iter which will iterate over each item and pass the item into a string -> unit function.

So an example might look like this:

let a = [| "a"; "b"; "c"; "d" |];;
let f arrayFunction (array : string[]) =
 array |> Array.iter arrayFunction;;
a |> f Console.WriteLine;;

• John Oriely: oh great, thanks. And to call this function I can use: arraryFunction(a);; ?
• Gary.S: @JohnOriely arrayFunction (as in your example) is a parameter, in this case it is a parameter that is a function. I call the f function passing Console.WriteLine as the arrayFunction parameter in the last line of the example.
• Jack P.: @Gary.S If you know the input is an array, you should use Array.iter, not Seq.iter. The latter is slower because it uses an enumerator to traverse the array instead of a loop.
• Gary.S: @JackP. Thanks, not sure why I didn't do that to begin with.

Answer 3

In addition to the other answers - you didn't need to specify the type of the array argument explicitly. Type inference normally handles it fine (depending on the wider context). So for example this works:

let a = [| "a"; "b"; "c"; "d" |]
let f arrayFunction array =
 array |> Seq.iter arrayFunction
let printme s = printfn "%s" s
f printme a