condition on two arrays

Hi,

Given three arrays x,z and y:

z = [5,5,7,7,7,11,15,15,29,29,29]

y = [1,2,3,2,1,1,1,1,2,3,1]

x = [5,7,29]

x contains some elements of z uniquely. For these elements, I wanted to count how often each of their appearance in z is accompanied with a "1" for the same index in y. For instance z contains two "5", at index 1 and at index 2, but only index 1 in y is equal to 1, so the result should be "1".

I coded the following, but this gives me completely wrong results:

for i = 1:length(x)
sumElems = sum(z==x(i)&&y==1)
end

[John D'Errico]

Why would you extend the length of the loop to be length(y)? In fact, that loop should generate an error as you wrote it, since length(y) is 11, and you index x(i).

There are only 3 distinct values in x, and you claim to be only interested in the members of x.

Yes, there will surely be a solution. But you need to be clear about the problem you are trying to solve.

[MiauMiau]

That was an error but how am I not "clear about the problem I am trying to solve"...?

[John D'Errico]

Are you interested ONLY in the elements that lie in x? If you are, then why are you looping over the length of y?

Note that there are some elements of z that are not members of x, yet they too have a corresponding 1 in y.

As I said, the code that you wrote originally will fail to run at all, so we cannot use that to infer what you are looking to get. (I do see that you have now changed the code to loop only over the length of x.)

Answer 1

[John D'Errico]

Since it now appears that you care only about those elements of z that also lie in y, the solution is simple. The code you wrote will ALMOST work, IF you save the results properly.

For example, this should work:

sumelems = zeros(size(x));
for i = 1:length(x)
  sumElems(i) = sum(z==x(i)&y==1);
end

Note that I used a &, NOT the && operator. && is used only in tests like an if statement, or in a while statement.

Could I have written the above more simply, without using a loop? Yes. But why bother writing code that will be far less readable, for a tiny problem? Don't pre-optimize code just because a solution may seem more "elegant".

[MiauMiau]

Yes I had the exact same but with two &&! So that was the problem..well for elegance: sometimes I try to learn more effectively begining with simple problems. But thank you!

[John D'Errico]

I guess that knowing when to use & and && is not always obvious.

&& was introduced (as well as ||) to allow tests to short-circuit in an if statement. So, if you had a test like

if A & B

then if A is false, there is no reason to even evaluate the test B. So && short circuits the test, failing if A is false. You use

if A && B

instead, as a more efficient test. Similarly,

if A | B

is not as efficient as

if A || B

because the || operator allows the if statement to not evaluate the test B when A is true. Again, no reason to evaluate a test when you don't care about the answer.

So use "&&" and || essentially only in if and while statements. (I may have missed some other minor cases that I can't think of at the moment.)