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

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