[yt-users] How to pass value of the field corresponds to the index to the scalar function!!!

Dear Yt-users, I need to pass the value of the available/derived field corresponds to the index selected based on some criterion (using numpy.where()) to a scalar function. It should list out the indexes wheres the condition satisfies. It happens when this has been done outside the field but instead of getting 3D indexes it gives 1D YT_Array indexes I think. But I need to pass this value of field corresponds to the selected index to scalar function inside the field, which may give some problem (please find the attachment to see what exactly I am trying to do). The attached script can run on the ENZO test data set, enzo_tiny_cosmology. Many thanks and regards -Prateek