Logical Indexing with Condition Involving Functions

Suppose I created a function F, and I want to find all elements a in an array A such that F(a) is some value of the function. Is there any way to do this with logical indexing (or some other method)? I tried to use idx = (F(A) == 1), but it has been giving me error messages about different numbers of elements. Thanks in advance!

[Stephen23]

"but it has been giving me error messages about different numbers of elements"

Please show the complete error message. This means all of the red text.

[Andrew Anderson]

"Unable to perform assignment because the left and right sides have a different number of elements."

Answer 1

[VBBV]

%if true
  idx = find(A(a) == 1)

Use it with find function for element indices

[Stephen23]

Note that the question specifically requests logical indices, not linear indices.

[Andrew Anderson]

This may suffice, as I do not have to use logical indexing (I will edit the question accordingly). I will try it out some time.