how to work with .net array?

Question

I read about that when deal with array we should use like myArray.[i], however, from my experience myArray[i] compiles too.

However, when I want to write to an array in .Net (so it is mutable), this gives an error let myArray.[i] = 3 but let myArray[i] =3 works.

What is the best practice to deal with such thing?

Also, should I use Array.get or use .[]?

How do I set value to a jagged array e.g. let myArray.[i].[j] = 5

• t0yv0: let setJagged (a: 'T[][]) (i: int) (j: int) (v: 'T) = a.[i].[j] <- v and let set2D (a: 'T[,]) (i: int) (j: int) (v: 'T) = a.[i,j] <- v.

Answer 1

1) If you want to assign a value to an array cell, use the assignment operator <-:

myArray.[i] <- 3 

2) let myArray[i] = 3 compiles because the compiler understands it as myArray function with a list as its argument and returns 3. If you read the warning and the type signature, you will see you're doing it wrong.

3) Array.get is a single call to .[]. Array.get is convenient in some cases for function composition and avoiding type annotation. For example you have

let mapFirst arrs = Array.map (fun arr -> Array.get arr 0) arrs 

vs.

let mapFirst arrs = Array.map (fun (arr: _ []) -> arr.[0]) arrs

Answer 2

Neither of your approaches is correct. Assuming that you have some definitions like

let myArray = [| 1; 7; 15 |]
let i = 2

what you actually want is something like this:

myArray.[i] <- 3

When you write

let myArray[i] = 3

you are actually defining a function myArray, which takes an integer list and returns an integer. This is not what you want at all.

Answer 3

The problem here is that you're trying to embed an array assignment inside of a let expression. The expression let myArray[3] = 2 is not an assignment into an array. Rather, it's a function definition. See what happens in fsi:

let myArray[i] = 3;;
val myArray : int list -> int

(There's actually a warning in there as well). Formatting it differently also reveals this fact: let myArray [3] = 2.

As the others have pointed out, .[] is for array access, and to assign to an array, you use myArray.[i] <- 3 (not inside a let expression).