Replace selected elements in a vector if meeting some criteria

Assume A=rand(10,1) is a column vector. B is a vector of position assume B=[2 4 7]. I want to check if any of the 2,4,7th rows elements in A is <0.5. If yes, change those elements in A to zero. I want finally modified A.

Within a loop, I can assess each elements of B (i.e. 2,4,7 one after another) and check the condition and make changes in A - that's straightforward. Is there any smarter way?