Logical indexing without for loop

Hi all,

This is probably a simple question, but I would love some input from those who can lend it. I have a vector [A] I would like to index according to a condition comparing the nth entry of A with the one following it (n+1).

For instance, if A is [1,2,3,4,5,6,7,6,5,4,3,2,1], and I would like to know which entries of A are larger than the entry preceding it, I would try to do the following:

 for i = 1:size(A)
  if i > 1
     j = A(i) > A(i-1);
  end
end

Where j = [0,1,1,1,1,1,1,0,0,0,0,0,0]

Is there a way to do this without using a loop? Any help would be appreciated, thank you.

Answer 1

[Renato Agurto]

 j = [0 A(2:end) > A(1:end-1)]

[Tyler]

Awesome, thank you so much! If I wanted to do something more complicated, such as wanting the absolute value of the difference of the two entries to be greater than a certain value, could I do that?

Aa = [0 abs(A(1:end)-A(1:end-1)) > .001]

This doesn't quite work, but I might be forgetting something obvious.

[Walter Roberson]

You used A(1:end)-A(1:end-1) which subtracts two vectors of different sizes. Renato used A(2:end) for the first operation.

Answer 2

[Walter Roberson]

[abs(diff(A)) > 0.001, false]

The false compensates for the fact that diff(A) is one shorter than A itself. You will find that in practice it is not a problem to use a logical index shorter than the array dimension, so for most purposes

abs(diff(A)) > 0.001

would be just as good.

[Tyler]

Thank you for your help!