How to extract indices with a condition?

For example I have Y = X(X==42), but I need the indices also!

[Stephen23]

X==42 gives the logical indices, which will be faster than using find.

[rbih rbih]

hi Mr M. i just wondering about the value that you're searching index for (42), isn't that related to the quiz of rigid body kinematics course (Concept Check 9 - Euler Angle Differential Kinematic Equations) on coursera!

just for fun, have a good day.

Answer 1

[Birdman]

To find the index where X is equal to 42, type

 idx=find(X==42)
 Y=X(idx)

Answer 2

[Walter Roberson]

find(X==42) and store in a variable. Then X(that_variable)

[Mr M.]

Thanks! and what if I have zero-indices ind0 = find(X==0), but I need non-zero indices also. Are there any faster way using ind0, or I have to use another find: find(X~=0) ?

[Birdman]

For nonzero indices, simply type

 find(X~=0)

[Mr M.]

yes, I know, as you can see above, but my question was: is it possible to do faster? by using ind0

[Mr M.]

with logical indexing its trivial, but ind0 is a vector of integer indices

[Birdman]

It is possible to do faster, yes, by using logical indexing.

[Walter Roberson]

X(setdiff(1:numels(X), ind0))

[Ron]

can you please this statement? thankyou.

[Rik]

@Ron Your comment seems to be missing some words. If you're asking for an explanation of Walters last comment: did you read the documentation for the functions he used? (There is also probably a typo: numels should be numel)

[Ron]

@Rik, thanks for replying. Yes my comment was somewhat arbitrary here. Yes I was talking about "X(setdiff(1:numels(X), ind0))", this statement. I couldnt understand what it means. Yes you are right about the typo, thanks for correcting me.

[Walter Roberson]

setdiff(1:numels(X), ind0)

returns all the indices of X that do not appear in ind0

It is questionable whether this will be any faster than

X(X ~= 0)

but the original poster wanted a different way.