mapcar Functionmapcar 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.)