Setting values in 2D Arrays-CodePudding

When trying to set a value in a 2D array like this:

let a = Array.make  5 (Array.make 5 0);;
a.(0).(0) <- 4;

It will for some reason put 4 at index j in every array contained in the 2D array a.

Why is this, and how do i get it to only set a[i][j] to 4?

CodePudding user response：

The problem is with how you initialize the matrix. You're creating one inner array that is then assigned to every cell of the outer array instead of creating a new array for each cell. What you're doing is equivalent to this:

let inner = Array.make 5 0 in
let outer = Array.make 5 inner in
inner.(0) <- 4

You could use Array.init to create the outer array instead, which behaves exactly like Array.make except it takes an initialization function instead of the value to insert directly:

Array.init 5 (fun _ -> Array.make 5 0)

But even more convenient for creating a 2-dimensional array is Array.make_matrix:

Array.make_matrix 5 5 0;;

CodePudding user response：

As an addendum to glennsl's answer, you will see this issue anytime you store a mutable value in an array.

Consider:

let arr =
  let x = ref 5 in
  Array.make 5 x

If we assign a new value to that int ref value with arr.(1) := 7 or arr.(0).contents <- 7, arr` is now:

[|{contents = 7}; {contents = 7}; {contents = 7}; 
  {contents = 7}; {contents = 7}|]

The same does not apply if we fill our array with immutable data.

let arr = 
  let x = 5 in
  Array.make 5 x

Now if we update with arr.(0) <- 7, arr is:

[|7; 5; 5; 5; 5|]