How to find multiples of x in a row vector?

I searched the forms and didn't find any pertinent information on finding multiples of a specific number or set of numbers in a row vector. Ex given x=[1:100] how would I find a= multiples of 3 or combining two functions and find b = multiples of 3 but not multiples of 2. c = multiples of 3 or multiples of 2. etc.

I tried using the find function as a=find mult(x,3). However that doesnt work. Any suggestions would be much appreciated.

Answer 1

[Guillaume]

First of all, it's entirely possible you don't need the find function. Unless you actually need the indices of the element that match your condition, you're better off simply using logical indexing.

Secondly, to find something that matches multiple conditions, you simply use logical operators to combine theses conditions.

Finally, to find if a number is a multiple of another, you simply divide the former by the latter and see if the remainder is 0. The function to get the remainder is rem ( mod also works).

So, to get the elements of x that are multiple of 3 AND NOT multiple of 2:

x(rem(x, 3) == 0 && rem(x, 2) ~= 0)

[Walter Roberson]

I would probably use mod() for this myself, but rem() is also quite reasonable here.

http://www.mathworks.com/help/matlab/ref/mod.html#bue4xl3-6

[Kyle Gray]

The only problem with this solution is that the && key doesn't work with the simple row vector. I tried modifying the solution to x(rem(x, 3) == 0), x(rem(x, 2) ~= 0), but it simply displays two answers instead of combining the solutions. Is there a way around this?

[Walter Roberson]

x(rem(x, 3) == 0 & rem(x, 2) ~= 0)

[Guillaume]

Oh yes, sorry a silly typo on my part. It should have read & not &&.