CODEWITHSUNDEEP
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colinfang
colinfang

Reputation: 21767

how to work with .net array?

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

Upvotes: 0

Views: 147

Answers (3)

pad
pad

Reputation: 41290

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

Upvotes: 5

Ken Wayne VanderLinde
Ken Wayne VanderLinde

Reputation: 19347

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).

Upvotes: 2

kvb
kvb

Reputation: 55185

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.

Upvotes: 4

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