C.4.4 The mapcar Function

mapcar is a function that calls its first argument with each element of its second argument, in turn. The second argument must be a sequence.

The ‘map’ part of the name comes from the mathematical phrase, “mapping over a domain”, meaning to apply a function to each of the elements in a domain. The mathematical phrase is based on the metaphor of a surveyor walking, one step at a time, over an area he is mapping. And ‘car’, of course, comes from the Lisp notion of the first of a list.

For example,

(mapcar '1+ '(2 4 6))
     ⇒ (3 5 7)

The function 1+ which adds one to its argument, is executed on each element of the list, and a new list is returned.

Contrast this with apply, which applies its first argument to all the remaining. (See Readying a Graph, for an explanation of apply.)

In the definition of one-fiftieth, the first argument is the anonymous function:

(lambda (arg) (/ arg 50))

and the second argument is full-range, which will be bound to list-for-graph.

The whole expression looks like this:

(mapcar (lambda (arg) (/ arg 50)) full-range))

See Mapping Functions in The GNU Emacs Lisp Reference Manual, for more about mapcar.

Using the one-fiftieth function, we can generate a list in which each element is one-fiftieth the size of the corresponding element in list-for-graph.

(setq fiftieth-list-for-graph
      (one-fiftieth list-for-graph))

The resulting list looks like this:

(10 20 19 15 11 9 6 5 4 3 3 2 2
1 1 1 1 0 1 0 0 0 0 0 0 0 0 0 0 0 4)

This, we are almost ready to print! (We also notice the loss of information: many of the higher ranges are 0, meaning that fewer than 50 defuns had that many words or symbols—but not necessarily meaning that none had that many words or symbols.)