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.

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.

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