eintr: mapcar
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. (
Readying a Graph Readying a Graph, for a
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))
Mapping Functions (elisp)Mapping Functions, 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.)
Info Catalog
eintr: lambda
eintr: Print Whole Graph
eintr: Another Bug