passing multiple statements in ocaml using "in" and "and" operations - ocaml

Question

I wanted to convert this C++ code to ocaml, but i'm getting syntax error
C++ code
int** matrix(int n,int **a,int**b)
{
t=n/2;
a11=new int*[t];
for(i=0;i<t;i++)
a11[i]=new int [t];
for(i=0;i<t;i++)
for(j=0;j<t;j++)
a11[i][j]=a[i][j];
a12=new int*[t];
for(i=0;i<t;i++)
a12[i]=new int [t];
for(i=0;i<t;i++)
for(j=0;j<t;j++)
a12[i][j]=a[i][j+t];
a21=new int*[t];
for(i=0;i<t;i++)
a21[i]=new int [t];
for(i=0;i<t;i++)
for(j=0;j<t;j++)
a21[i][j]=a[i+t][j];
}
Ocaml code
let matrix n x y =
let t = n/2 in
let a11 = Array.make_matrix t t 0 in
for i = 0 to t-1 do
for j = 0 to t-1 do
a11.(i).(j) <- x.(i).(j)
done
done
and
a12 = Array.make_matrix t t 0 in
for i = 0 to t-1 do
for j = 0 to t-1 do
a12.(i).(j) <- x.(i).(j+t)
done
done
and
a21 = Array.make_matrix t t 0 in
for i = 0 to t-1 do
for j = 0 to t-1 do
a21.(i).(j) <- x.(i+t).(j)
done
done
;;
The problem is that the value of t is not getting passed inside the a12 and a21 arrays and it is getting unbounded.

Answer 1

The and for a let can't appear in the body of the let. Instead, use multiple lets:
let matrix n x y =
let t = n/2 in
let a11 = Array.make_matrix t t 0 in
for i = 0 to t-1 do
for j = 0 to t-1 do
a11.(i).(j) <- x.(i).(j)
done
done;
let a12 = Array.make_matrix t t 0 in
for i = 0 to t-1 do
for j = 0 to t-1 do
a12.(i).(j) <- x.(i).(j+t)
done
done;
let a21 = Array.make_matrix t t 0 in
for i = 0 to t-1 do
for j = 0 to t-1 do
a21.(i).(j) <- x.(i+t).(j)
done
done

Answer 2

Just to clarify, the following has the same problem as your original code:
# let x = 4 in () and y = 7 in ();;
Error: Syntax error
Since let defines an expression, this amounts to writing () and (), which indeed is not legal OCaml.
The correct use of let ... and looks like this:
# let x = 4 and y = 7 in ();;
Warning 26: unused variable x.
Warning 26: unused variable y.
- : unit = ()
There are several let ... and let ... but just one in.