Question about Indexing a function

A = [-1 -2 0 1 2];
B = A(A<0);

What does A(A<0) mean exactly? and is there a way to do its function with an if statement? because I tried this and it did not work.

if A<0
B = A;
end

[Stephen23]

"What does A(A<0) mean exactly?"

A(A<0)
  ^^^  logical index
^^   ^ indexing into A

Basic MATLAB concepts, like how to use indexing, are explained here:

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

"and is there a way to do its function with an if statement?"

It could be done with a bit of effort, but that would be a very poor use of MATLAB.

Answer 1

[Dyuman Joshi]

B = A(A<0) means to allocate all the elements of A which are less than zero to B.

In case you want to use if statement to obtain the same, use a for loop as follows -

B=[];
for i=1:numel(A)
if A(i)<0
B = [B A(i)];
end
end

However, using B = A(A<0) should be opted.

[Mashari Alharbi]

Thank you for your answer, but what does x mean in the alternative way?

[Dyuman Joshi]

My bad, that is supposed to be A and not x. I have edited it.

Answer 2

[Steven Lord]

That operation is logical indexing. Also see the "Indexing with Logical Values" section on this documentation page for more information.