www.delorie.com/gnu/docs/emacs-lisp-intro/emacs-lisp-intro_287.html search
Programming in Emacs Lisp

 [ < ] [ > ] [ << ] [ Up ] [ >> ] [Top] [Contents] [Index] [ ? ]

### 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 section 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)) ```

See section `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.)

 [ < ] [ > ] [ << ] [ Up ] [ >> ] [Top] [Contents] [Index] [ ? ]

 webmaster delorie software   privacy Copyright © 2003   by The Free Software Foundation Updated Jun 2003