Write a multiline function in ocaml

Question

I'm having trouble understanding how to write functions in ocaml as I've only written recursive functions that don't require multiple lines separated by a ;.

I'm trying to create a function that given a int number n returns a matrix full of zeroes and just ones in the diagonal, so an identity matrix of size n.
I'm new to functional programming and ocaml so I was wodering if it was possible to write this in a imperative way. For example in Java I'd write:

public int[][] identity(int size) {
    int[][] matrix;
    matrix = new int[size][size];
    //fill the matrix with zeroes
    for( int i=0; i<size; i++){
        for( int j=0; j<size; j++){
            matrix[i][j] = 0;
        }
    }
    //diagonal values at 1
    for (int i=0; i<size; i++){
        matrix[i][i] = 1;
    }
    return matrix;
}

How can I achieve this in ocaml?

Answer 1

You can write OCaml code in an imperative way, yes. OCaml has matrices (represented as arrays of arrays), matrices are mutable, and it has a for statement. So you can write code that looks very similar to what you wrote in Java.

It seems a shame to learn OCaml only to write imperative code, however!

Here are some code fragments that you can put together:

# let m = Array.make_matrix 3 3 0;;
val m : int array array = [|[|0; 0; 0|]; [|0; 0; 0|]; [|0; 0; 0|]|]

# m.(1).(1) <- 1;;
- : unit = ()

# m;;
- : int array array = [|[|0; 0; 0|]; [|0; 1; 0|]; [|0; 0; 0|]|]

# for i = 0 to 9 do Printf.printf " %d" i done;;
 0 1 2 3 4 5 6 7 8 9- : unit = ()

There is nothing special about a multi-line function in OCaml. Here is a function that does three things. You can write it on one line or on many lines:

let f x =
    Printf.printf "I will now write the value of x\n";
    Printf.printf "Here is the value of x: %d\n" x;
    Printf.printf "I just wrote the value of x\n"

In OCaml, ; is used to separate expressions that should be evaluated in order. I.e., there's no ; after the last expression.