23.2. Map

You previously were introduced to accumulating a list by transforming each of the elements. Here we revisit that pattern.

The following function produces a new list with each item in the original list doubled. It is an example of a mapping, from the original list to a new list of the same length, where each element is doubled.

The doubleStuff function is an example of the accumulator pattern, in particular the mapping pattern. On line 3, new_list is initialized. On line 5, the doubled value for the current item is produced and on line 6 it is appended to the list we’re accumulating. Line 7 executes after we’ve processed all the items in the original list: it returns the new_list. Once again, codelens helps us to see the actual references and objects as they are passed and returned.


This pattern of computation is so common that python offers a more general way to do mappings, the map function, that makes it more clear what the overall structure of the computation is. map takes two arguments, a function and a sequence. The function is the mapper that transforms items. It is automatically applied to each item in the sequence. You don’t have to initialize an accumulator or iterate with a for loop at all.


Technically, in a proper Python 3 interpreter, the map function produces an “iterator”, which is like a list but produces the items as they are needed. Most places in Python where you can use a list (e.g., in a for loop) you can use an “iterator” as if it was actually a list. So you probably won’t ever notice the difference. If you ever really need a list, you can explicitly turn the output of map into a list: list(map(...)). In the runestone environment, map actually returns a real list, but to make this code compatible with a full python environment, we always convert it to a list.

As we did when passing a function as a parameter to the sorted function, we can specify a function to pass to map either by referring to a function by name, or by providing a lambda expression.

Of course, once we get used to using the map function, it’s no longer necessary to define functions like tripleStuff and quadrupleStuff.

Check Your Understanding

1. Using map, create a list assigned to the variable greeting_doubled that doubles each element in the list lst.
2. Below, we have provided a list of strings called abbrevs. Use map to produce a new list called abbrevs_upper that contains all the same strings in upper case.
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