Practical Programming, 3rd Edition

Storing Data Using Dictionaries

Here is the same bird-watching observation file that we saw in ​Set Example: Arctic Birds​:

canada goose
canada goose
long-tailed jaeger
canada goose
snow goose
canada goose
long-tailed jaeger
canada goose
northern fulmar

Suppose we want to know how often each species was seen. Our first attempt uses a list of lists, in which each inner list has two items. The item at index 0 of the inner list contains the species, and the item at index 1 contains the number of times it has been seen so far. To build this list, we iterate over the lines of the observations file. For each line, we search the outer list, looking for the species on that line. If we find that the species occurs in the list, we add one to ...