I probably miss something very basic, but how given two arrays a and b, can I find positions in a where elements of b are located?  If a were sorted, I could use searchsorted, but I don't want to get valid positions for elements that are not in a.  In my case, a has unique elements, but in the general case I would accept the first match.  In other words, I am looking for an array analog of list.index() method.