MATLAB逻辑索引：a（find（b <1））= 0是否等于a（b（b <1））= 0？

Question

As my question suggests, my goal is to find all the indices of the values in b less than 1, and set these same indices in a to zero.

The first expression ie

a(find(b<1)) = 0

does what I want, but matlab is suggesting that I use logical indices to improve performance. Does the second expression do the same thing?

a(b(b<1)) = 0

Answer 1

No.

a(b<1) = 0

does the same thing.

b(b<1)

returns the values of b where b is smaller than 1. This is not a logical value (which it should be for logical indexing) and it is probably not of the same size as b (unless all values are less than 1).

Answer 2

find returns the actual indices of elements values are less than a 1. on the other hand, b<1 returns vector with length equal to b and it has 0's for elements fulfill the condition and 1's for those does not fill fill condition. Let suppose, you have b vector:

b = [2 3 4 5 6 -1 9 -2]
find(b<2)
ans =

     6     8
>> b<2

ans =

     0     0     0     0     0     1     0     1
 b(b<2)

ans =

    -1    -2
a(b(b<1)) =0
Subscript indices must either be real positive integers or logicals.

So both operations are not same. b<1 returns the logical array and find(b) returns the indices of elements fulfill condition.