How to multiply numbers if they meet a certain condition?

I have a project where I need to multiply certain numbers in a vector if they meet certain conditions, like being positive or divisilble by three.

I tried something like this:

V = [5, 17, -3, 8, 0, -7, 12, 15, 20, -6, 6, 4, -7, 16];

if rem(find(V>0),3)==0

% multiply values

end

It didn't seem to work. How would I go about finding these values?

[Voss]

rem(find(V>0),3)==0

That tells you whether the index of each positive element of V is divisible by 3.

V = [5, 17, -3, 8, 0, -7, 12, 15, 20, -6, 6, 4, -7, 16];

temp = find(V>0) % elements #1,2,4,7,8,9,11,12,14 are positive

temp = 1x9

1 2 4 7 8 9 11 12 14

idx = rem(temp,3)==0

idx = 1x9 logical array
   0   0   0   0   0   1   0   1   0

temp(idx) % this tells you that 9 and 12 (not elements #9 and 12 in V, which are 20 and 4) are divisible by 3

ans = 1x2

9 12

Answer 1

[Voss]

V = [5, 17, -3, 8, 0, -7, 12, 15, 20, -6, 6, 4, -7, 16]

V = 1x14

5 17 -3 8 0 -7 12 15 20 -6 6 4 -7 16

idx = V>0 | rem(V,3)==0

idx = 1x14 logical array
   1   1   1   1   1   0   1   1   1   1   1   1   0   1

Now idx is a logical vector saying whether each element of V is positive or divisible by 3.

You can do stuff with that, e.g., get just those elements of V:

V_pos_or_3x = V(idx)

V_pos_or_3x = 1x12

5 17 -3 8 0 12 15 20 -6 6 4 16

Another example, get the elements of V that are positive and divisible by 3:

idx = V>0 & rem(V,3)==0

idx = 1x14 logical array
   0   0   0   0   0   0   1   1   0   0   1   0   0   0

V_pos_and_3x = V(idx)

V_pos_and_3x = 1x3

12 15 6