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Expression: Useful Anonymity, Up: Printing the Whole Graph [Contents][Index]

`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.)

`lambda`

Expression: Useful Anonymity, Up: Printing the Whole Graph [Contents][Index]