How to remove an element based on a condition

Hey guys,

So, I need to write the code for finding prime numbers and I'm trying to do so by removing the elements which can be perfectly divided. In order to remove the elements, I'm using the instructions given here:

http://stackoverflow.com/questions/8785251/matlab-remove-elements-from-array-which-are-less-than-average

Instead of a simply mean condition as above, I'm using the remainder function and a loop as follows:

r=1:20;
n=length(r)
for i=2:1:n/2
r(rem(r,i)=0)=[];
end
disp(r)

Right now, I'm just doing a test for an array from 1:20. The loop for i cycles through all the possible values and as soon as it finds an element that can be perfectly divided by any i, I want the element to be removed from the array. But, I'm getting the following error:

 subscript indices must be either positive integers less than 2^31
or logicals
error: called from
    prime5 at line 4 column 14

Can someone please explain the error and how I can circumvent it? Thanks.

Answer 1

[Image Analyst]

You need to use == instead of =. And since r shrinks as you move along deleting elements, you must start at the end and work toward the front:

r = 1:20;
n = length(r)
for i = n/2 : -1 : 2
  r(rem(r, i) == 0) = [];
end
disp(r)