Finding matching keys in two large dictionaries and doing it fast

Question

I am trying to find corresponding keys in two different dictionaries. Each has about 600k entries.

Say for example:

    myRDP = { \'Actinobacter\':          


        

Answer 1

You could do this:

for key in myRDP:
    if key in myNames:
        print key, myNames[key]

Your first attempt was slow because you were comparing every key in myRDP with every key in myNames. In algorithmic jargon, if myRDP has n elements and myNames has m elements, then that algorithm would take O(n×m) operations. For 600k elements each, this is 360,000,000,000 comparisons!

But testing whether a particular element is a key of a dictionary is fast -- in fact, this is one of the defining characteristics of dictionaries. In algorithmic terms, the key in dict test is O(1), or constant-time. So my algorithm will take O(n) time, which is one 600,000th of the time.