How to remove values from an array which do not create output?

Suppose, I have two arrays like below and some conditions:

a=1:10;
b=1:10;
c=[];

for i=1:length(a)
    for j=1:length(b)
        if(a(i)>5)
            if(b(j)>7)
                c=[c i*j];
            end
        end
    end
end

plot3(a,b,c);

The plot3 command will show an error as a,b,c are not of equal length. Now I want to remove the values of a and b which do not produce c. What can I do? How can I plot successfully here? Thanks in advance.

Answer 1

[Walter Roberson]

a=1:10;
b=1:10;
[A,B] = ndgrid(a,b);
C = A.*B;
mask = A > 5 & B > 7;
C = C(mask);
A = A(mask);
B = B(mask);
scatter3(A,B,C)

The above is the general case. But since your tests are independent, you could instead

a=1:10;
b=1:10;
A = a(a>5);
B = b(b>7);
C = A.*B(:);
size(A), size(B), size(C)

ans = 1×2

1 5

ans = 1×2

1 3

ans = 1×2

3 5

surf(A,B,C)