Building vector with for- and if statements

My goal is to find all divisors of 30 and put them in a vector. My code right now looks like:

k=[];
c=[1:1:30];
  for  i=1:30
    if mod(30,c(i))==0
        v=[c(i)]
    end
  end

I am successful in finding the divisors of 30, but the output is:

v =

     1

v =

     2

v =

     3

v =

     5
and so on...

I want to put all of the divisors in one single vector v, what am I missing? I have tried searching for answers but I could not find any, maybe because I don't understand MATLAB code well enough.

Thank you for your help!

Answer 1

[Roger Stafford]

Make two changes:

k = 0;
c = 1:1:30;
for  i=1:30
  if mod(30,c(i))==0
    k = k+1;
    v(k) = c(i);
  end
end

However, it is better practice to first allocate memory to v:

k = 0;
c = 1:1:30;
v = zeros(size(c));
for  i=1:30
  if mod(30,c(i))==0
    k = k+1;
    v(k) = c(i);
  end
end
v = v(1:k);

Also you can probably make it faster:

c = 1:30;
v = c(mod(30,c)==0);

[Henry Eriksson]

Thank you for your quick answer! I'm curious about one thing...

What exactly does k=k+1 do? It seems to appear out of nowhere. My guess is that its counting how many entries of c that are divisors of 30 for example, but how does it do that? Is there anywhere I can read about it? Assuming I am right of course.

[Stephen Cobeldick]

@Henry Erikson: the k value is defined to have to value one; each time there is a valid output its value increments by one, giving 1, 2, 3, etc. This value is used as an index to allocate the output value into the array v.

"but how does it do that? Is there anywhere I can read about it?"

This is not some MATLAB magic, but a very basic usage of loops which could be used in almost any language which has loops. Some introductory courses to programming might explain this: search for "loop variable" or the like.

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

[Henry Eriksson]

Very helpful, thank you! :)