If statement / conditional operations

Hi all,

I'm trying to write a code that does something based on the adjacent cells. I'm not sure the best way to do this, but have started using the 'if' statement.

arb is a 3D array, whilst arbgrad is the 1st derivative / gradient of arb.

            if ((arb(j,i-1,k)+arb(j,i+1,k)+(arb(j+1,i,k))+arb(j-1,i,k))./4)>(arb(j,i,k));
                if arbgrad(j,i,k)<0;
                    BB1(arbgrad<0)=(arbgrad<0).*1;
                else 
                    BB2(arbgrad>0)=(arbgrad>0).*-1;
                end
            else
                if arbgrad(j,i,k)<0
                    BB3(arbgrad<0)=(arbgrad<0).*1;
                else 
                    BB4(arbgrad>0)=(arbgrad>0).*-1;
                end
            end
    arbgrad(j,i,k)=BB1(j,i,k)+BB2(j,i,k)+BB3(j,i,k)+BB4(j,i,k);

The error message keeps coming up:

In an assignment A(:) = B, the number of elements in A and B must be the same.

I'm not sure why because arbgrad, arb and BB1-4 all have the same amount of elements.... Any help would be greatly appreciated.

Thanks!

Answer 1

[Adam]

(arbgrad<0)

is a logical vector of the same length as arbgrad.

BB3(arbgrad<0)

which you try to assign it to is a vector of length equal to the number of true (1) values in the above vector.

Do you maybe mean to assign:

BB3(arbgrad<0) = arbgrad(arbgrad<0)

?

[Chris Brown]

Yes, thank you. Appreciate your help.