00000004.htm

一份很好的linux入门资料

&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;a&nbsp;3)&nbsp;</I><BR>
3&nbsp;<BR>
＞<I>&nbsp;(cond&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;((evenp&nbsp;a)&nbsp;a)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;if&nbsp;a&nbsp;is&nbsp;even&nbsp;return&nbsp;a&nbsp;<BR>
&nbsp;&nbsp;&nbsp;((&gt;&nbsp;a&nbsp;7)&nbsp;(/&nbsp;a&nbsp;2))&nbsp;&nbsp;&nbsp;&nbsp;;else&nbsp;if&nbsp;a&nbsp;is&nbsp;bigger&nbsp;than&nbsp;7&nbsp;return&nbsp;a/2&nbsp;<BR>
&nbsp;&nbsp;&nbsp;((&lt;&nbsp;a&nbsp;5)&nbsp;(-&nbsp;a&nbsp;1))&nbsp;&nbsp;&nbsp;&nbsp;;else&nbsp;if&nbsp;a&nbsp;is&nbsp;smaller&nbsp;than&nbsp;5&nbsp;return&nbsp;a-1&nbsp;<BR>
&nbsp;&nbsp;&nbsp;(t&nbsp;17)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;else&nbsp;return&nbsp;17&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
2&nbsp;<BR>
&nbsp;<BR>
If&nbsp;the&nbsp;action&nbsp;in&nbsp;the&nbsp;selected&nbsp;cond&nbsp;clause&nbsp;is&nbsp;missing,&nbsp;cond&nbsp;returns&nbsp;what&nbsp;<BR>
the&nbsp;condition&nbsp;evaluated&nbsp;to:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(cond&nbsp;((+&nbsp;3&nbsp;4)))&nbsp;</I><BR>
7&nbsp;<BR>
&nbsp;<BR>
Here's&nbsp;a&nbsp;clever&nbsp;little&nbsp;recursive&nbsp;function&nbsp;which&nbsp;uses&nbsp;cond.&nbsp;You&nbsp;might&nbsp;be&nbsp;<BR>
interested&nbsp;in&nbsp;trying&nbsp;to&nbsp;prove&nbsp;that&nbsp;it&nbsp;terminates&nbsp;for&nbsp;all&nbsp;integers&nbsp;x&nbsp;at&nbsp;<BR>
least&nbsp;1.&nbsp;(If&nbsp;you&nbsp;succeed,&nbsp;please&nbsp;publish&nbsp;the&nbsp;result.)&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(defun&nbsp;hotpo&nbsp;(x&nbsp;steps)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;;hotpo&nbsp;stands&nbsp;for&nbsp;Half&nbsp;Or&nbsp;Triple&nbsp;Plus&nbsp;One&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;(cond&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;((=&nbsp;x&nbsp;1)&nbsp;steps)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;((oddp&nbsp;x)&nbsp;(hotpo&nbsp;(+&nbsp;1&nbsp;(*&nbsp;x&nbsp;3))&nbsp;(+&nbsp;1&nbsp;steps)))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(t&nbsp;(hotpo&nbsp;(/&nbsp;x&nbsp;2)&nbsp;(+&nbsp;1&nbsp;steps)))&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;)&nbsp;<BR>
A&nbsp;<BR>
＞<I>&nbsp;(hotpo&nbsp;7&nbsp;0)&nbsp;</I><BR>
16&nbsp;<BR>
&nbsp;<BR>
The&nbsp;LISP&nbsp;case&nbsp;statement&nbsp;is&nbsp;like&nbsp;a&nbsp;C&nbsp;switch&nbsp;statement:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;x&nbsp;'b)&nbsp;</I><BR>
B&nbsp;<BR>
＞<I>&nbsp;(case&nbsp;x&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;(a&nbsp;5)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;((d&nbsp;e)&nbsp;7)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;((b&nbsp;f)&nbsp;3)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;(otherwise&nbsp;9)&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
3&nbsp;<BR>
&nbsp;<BR>
The&nbsp;otherwise&nbsp;clause&nbsp;at&nbsp;the&nbsp;end&nbsp;means&nbsp;that&nbsp;if&nbsp;x&nbsp;is&nbsp;not&nbsp;a,&nbsp;b,&nbsp;d,&nbsp;e,&nbsp;or&nbsp;<BR>
f,&nbsp;the&nbsp;case&nbsp;statement&nbsp;will&nbsp;return&nbsp;9.&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Iteration&nbsp;<BR>
&nbsp;<BR>
The&nbsp;simplest&nbsp;iteration&nbsp;construct&nbsp;in&nbsp;LISP&nbsp;is&nbsp;loop:&nbsp;a&nbsp;loop&nbsp;construct&nbsp;<BR>
repeatedly&nbsp;executes&nbsp;its&nbsp;body&nbsp;until&nbsp;it&nbsp;hits&nbsp;a&nbsp;return&nbsp;special&nbsp;form.&nbsp;For&nbsp;<BR>
example,&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(setq&nbsp;a&nbsp;4)&nbsp;</I><BR>
4&nbsp;<BR>
＞<I>&nbsp;(loop&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;(setq&nbsp;a&nbsp;(+&nbsp;a&nbsp;1))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;(when&nbsp;(&gt;&nbsp;a&nbsp;7)&nbsp;(return&nbsp;a))&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
8&nbsp;<BR>
＞<I>&nbsp;(loop&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;(setq&nbsp;a&nbsp;(-&nbsp;a&nbsp;1))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;(when&nbsp;(&lt;&nbsp;a&nbsp;3)&nbsp;(return))&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
NIL&nbsp;<BR>
&nbsp;<BR>
The&nbsp;next&nbsp;simplest&nbsp;is&nbsp;dolist:&nbsp;dolist&nbsp;binds&nbsp;a&nbsp;variable&nbsp;to&nbsp;the&nbsp;elements&nbsp;of&nbsp;<BR>
a&nbsp;list&nbsp;in&nbsp;order&nbsp;and&nbsp;stops&nbsp;when&nbsp;it&nbsp;hits&nbsp;the&nbsp;end&nbsp;of&nbsp;the&nbsp;list.&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(dolist&nbsp;(x&nbsp;'(a&nbsp;b&nbsp;c))&nbsp;(print&nbsp;x))&nbsp;</I><BR>
A&nbsp;<BR>
B&nbsp;<BR>
C&nbsp;<BR>
NIL&nbsp;<BR>
&nbsp;<BR>
Dolist&nbsp;always&nbsp;returns&nbsp;nil.&nbsp;Note&nbsp;that&nbsp;the&nbsp;value&nbsp;of&nbsp;x&nbsp;in&nbsp;the&nbsp;above&nbsp;<BR>
example&nbsp;was&nbsp;never&nbsp;nil:&nbsp;the&nbsp;NIL&nbsp;below&nbsp;the&nbsp;C&nbsp;was&nbsp;the&nbsp;value&nbsp;that&nbsp;dolist&nbsp;<BR>
returned,&nbsp;printed&nbsp;by&nbsp;the&nbsp;read-eval-print&nbsp;loop.&nbsp;<BR>
&nbsp;<BR>
The&nbsp;most&nbsp;complicated&nbsp;iteration&nbsp;primitive&nbsp;is&nbsp;called&nbsp;do.&nbsp;A&nbsp;do&nbsp;statement&nbsp;<BR>
looks&nbsp;like&nbsp;this:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(do&nbsp;((x&nbsp;1&nbsp;(+&nbsp;x&nbsp;1))&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(y&nbsp;1&nbsp;(*&nbsp;y&nbsp;2)))&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;((&gt;&nbsp;x&nbsp;5)&nbsp;y)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;(print&nbsp;y)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;(print&nbsp;'working)&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
1&nbsp;<BR>
WORKING&nbsp;<BR>
2&nbsp;<BR>
WORKING&nbsp;<BR>
4&nbsp;<BR>
WORKING&nbsp;<BR>
8&nbsp;<BR>
WORKING&nbsp;<BR>
16&nbsp;<BR>
WORKING&nbsp;<BR>
32&nbsp;<BR>
&nbsp;<BR>
The&nbsp;first&nbsp;part&nbsp;of&nbsp;a&nbsp;do&nbsp;specifies&nbsp;what&nbsp;variables&nbsp;to&nbsp;bind,&nbsp;what&nbsp;their&nbsp;<BR>
initial&nbsp;values&nbsp;are,&nbsp;and&nbsp;how&nbsp;to&nbsp;update&nbsp;them.&nbsp;The&nbsp;second&nbsp;part&nbsp;specifies&nbsp;a&nbsp;<BR>
termination&nbsp;condition&nbsp;and&nbsp;a&nbsp;return&nbsp;value.&nbsp;The&nbsp;last&nbsp;part&nbsp;is&nbsp;the&nbsp;body.&nbsp;A&nbsp;<BR>
do&nbsp;form&nbsp;binds&nbsp;its&nbsp;variables&nbsp;to&nbsp;their&nbsp;initial&nbsp;values&nbsp;like&nbsp;a&nbsp;let,&nbsp;then&nbsp;<BR>
checks&nbsp;the&nbsp;termination&nbsp;condition.&nbsp;As&nbsp;long&nbsp;as&nbsp;the&nbsp;condition&nbsp;is&nbsp;false,&nbsp;it&nbsp;<BR>
executes&nbsp;the&nbsp;body&nbsp;repeatedly;&nbsp;when&nbsp;the&nbsp;condition&nbsp;becomes&nbsp;true,&nbsp;it&nbsp;<BR>
returns&nbsp;the&nbsp;value&nbsp;of&nbsp;the&nbsp;return-value&nbsp;form.&nbsp;<BR>
&nbsp;<BR>
The&nbsp;do*&nbsp;form&nbsp;is&nbsp;to&nbsp;do&nbsp;as&nbsp;let*&nbsp;is&nbsp;to&nbsp;let.&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Non-local&nbsp;Exits&nbsp;<BR>
&nbsp;<BR>
The&nbsp;return&nbsp;special&nbsp;form&nbsp;mentioned&nbsp;in&nbsp;the&nbsp;section&nbsp;on&nbsp;iteration&nbsp;is&nbsp;an&nbsp;<BR>
example&nbsp;of&nbsp;a&nbsp;nonlocal&nbsp;return.&nbsp;Another&nbsp;example&nbsp;is&nbsp;the&nbsp;return-from&nbsp;form,&nbsp;<BR>
which&nbsp;returns&nbsp;a&nbsp;value&nbsp;from&nbsp;the&nbsp;surrounding&nbsp;function:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(defun&nbsp;foo&nbsp;(x)&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;(return-from&nbsp;foo&nbsp;3)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;x&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
FOO&nbsp;<BR>
＞<I>&nbsp;(foo&nbsp;17)&nbsp;</I><BR>
3&nbsp;<BR>
&nbsp;<BR>
Actually,&nbsp;the&nbsp;return-from&nbsp;form&nbsp;can&nbsp;return&nbsp;from&nbsp;any&nbsp;named&nbsp;block&nbsp;--&nbsp;it's&nbsp;<BR>
just&nbsp;that&nbsp;functions&nbsp;are&nbsp;the&nbsp;only&nbsp;blocks&nbsp;which&nbsp;are&nbsp;named&nbsp;by&nbsp;default.&nbsp;You&nbsp;<BR>
can&nbsp;create&nbsp;a&nbsp;named&nbsp;block&nbsp;with&nbsp;the&nbsp;block&nbsp;special&nbsp;form:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(block&nbsp;foo&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;(return-from&nbsp;foo&nbsp;7)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;3&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
7&nbsp;<BR>
&nbsp;<BR>
The&nbsp;return&nbsp;special&nbsp;form&nbsp;can&nbsp;return&nbsp;from&nbsp;any&nbsp;block&nbsp;named&nbsp;nil.&nbsp;Loops&nbsp;are&nbsp;<BR>
by&nbsp;default&nbsp;labelled&nbsp;nil,&nbsp;but&nbsp;you&nbsp;can&nbsp;make&nbsp;your&nbsp;own&nbsp;nil-labelled&nbsp;blocks:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(block&nbsp;nil&nbsp;</I><BR>
&nbsp;&nbsp;&nbsp;&nbsp;(return&nbsp;7)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;3&nbsp;<BR>
&nbsp;&nbsp;)&nbsp;<BR>
7&nbsp;<BR>
&nbsp;<BR>
Another&nbsp;form&nbsp;which&nbsp;causes&nbsp;a&nbsp;nonlocal&nbsp;exit&nbsp;is&nbsp;the&nbsp;error&nbsp;form:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(error&nbsp;&quot;This&nbsp;is&nbsp;an&nbsp;error&quot;)&nbsp;</I><BR>
Error:&nbsp;This&nbsp;is&nbsp;an&nbsp;error&nbsp;<BR>
&nbsp;<BR>
The&nbsp;error&nbsp;form&nbsp;applies&nbsp;format&nbsp;to&nbsp;its&nbsp;arguments,&nbsp;then&nbsp;places&nbsp;you&nbsp;in&nbsp;the&nbsp;<BR>
debugger.&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Funcall,&nbsp;Apply,&nbsp;and&nbsp;Mapcar&nbsp;<BR>
&nbsp;<BR>
Earlier&nbsp;I&nbsp;promised&nbsp;to&nbsp;give&nbsp;some&nbsp;functions&nbsp;which&nbsp;take&nbsp;functions&nbsp;as&nbsp;<BR>
arguments.&nbsp;Here&nbsp;they&nbsp;are:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(funcall&nbsp;#'+&nbsp;3&nbsp;4)&nbsp;</I><BR>
7&nbsp;<BR>
＞<I>&nbsp;(apply&nbsp;#'+&nbsp;3&nbsp;4&nbsp;'(3&nbsp;4))&nbsp;</I><BR>
14&nbsp;<BR>
＞<I>&nbsp;(mapcar&nbsp;#'not&nbsp;'(t&nbsp;nil&nbsp;t&nbsp;nil&nbsp;t&nbsp;nil))&nbsp;</I><BR>
(NIL&nbsp;T&nbsp;NIL&nbsp;T&nbsp;NIL&nbsp;T)&nbsp;<BR>
&nbsp;<BR>
Funcall&nbsp;calls&nbsp;its&nbsp;first&nbsp;argument&nbsp;on&nbsp;its&nbsp;remaining&nbsp;arguments.&nbsp;<BR>
&nbsp;<BR>
Apply&nbsp;is&nbsp;just&nbsp;like&nbsp;funcall,&nbsp;except&nbsp;that&nbsp;its&nbsp;final&nbsp;argument&nbsp;should&nbsp;be&nbsp;a&nbsp;<BR>
list;&nbsp;the&nbsp;elements&nbsp;of&nbsp;that&nbsp;list&nbsp;are&nbsp;treated&nbsp;as&nbsp;if&nbsp;they&nbsp;were&nbsp;additional&nbsp;<BR>
arguments&nbsp;to&nbsp;a&nbsp;funcall.&nbsp;<BR>
&nbsp;<BR>
The&nbsp;first&nbsp;argument&nbsp;to&nbsp;mapcar&nbsp;must&nbsp;be&nbsp;a&nbsp;function&nbsp;of&nbsp;one&nbsp;argument;&nbsp;mapcar&nbsp;<BR>
applies&nbsp;this&nbsp;function&nbsp;to&nbsp;each&nbsp;element&nbsp;of&nbsp;a&nbsp;list&nbsp;and&nbsp;collects&nbsp;the&nbsp;<BR>
results&nbsp;in&nbsp;another&nbsp;list.&nbsp;<BR>
&nbsp;<BR>
Funcall&nbsp;and&nbsp;apply&nbsp;are&nbsp;chiefly&nbsp;useful&nbsp;when&nbsp;their&nbsp;first&nbsp;argument&nbsp;is&nbsp;a&nbsp;<BR>
variable.&nbsp;For&nbsp;instance,&nbsp;a&nbsp;search&nbsp;engine&nbsp;could&nbsp;take&nbsp;a&nbsp;heuristic&nbsp;function&nbsp;<BR>
as&nbsp;a&nbsp;parameter&nbsp;and&nbsp;use&nbsp;funcall&nbsp;or&nbsp;apply&nbsp;to&nbsp;call&nbsp;that&nbsp;function&nbsp;on&nbsp;a&nbsp;<BR>
state&nbsp;description.&nbsp;The&nbsp;sorting&nbsp;functions&nbsp;described&nbsp;later&nbsp;use&nbsp;funcall&nbsp;<BR>
to&nbsp;call&nbsp;their&nbsp;comparison&nbsp;functions.&nbsp;<BR>
&nbsp;<BR>
Mapcar,&nbsp;along&nbsp;with&nbsp;nameless&nbsp;functions&nbsp;(see&nbsp;below),&nbsp;can&nbsp;replace&nbsp;many&nbsp;<BR>
loops.&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Lambda&nbsp;<BR>
&nbsp;<BR>
If&nbsp;you&nbsp;just&nbsp;want&nbsp;to&nbsp;create&nbsp;a&nbsp;temporary&nbsp;function&nbsp;and&nbsp;don't&nbsp;want&nbsp;to&nbsp;<BR>
bother&nbsp;giving&nbsp;it&nbsp;a&nbsp;name,&nbsp;lambda&nbsp;is&nbsp;what&nbsp;you&nbsp;need.&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;#'(lambda&nbsp;(x)&nbsp;(+&nbsp;x&nbsp;3))&nbsp;</I><BR>
(LAMBDA&nbsp;(X)&nbsp;(+&nbsp;X&nbsp;3))&nbsp;<BR>
＞<I>&nbsp;(funcall&nbsp;*&nbsp;5)&nbsp;</I><BR>
8&nbsp;<BR>
&nbsp;<BR>
The&nbsp;combination&nbsp;of&nbsp;lambda&nbsp;and&nbsp;mapcar&nbsp;can&nbsp;replace&nbsp;many&nbsp;loops.&nbsp;For&nbsp;<BR>
example,&nbsp;the&nbsp;following&nbsp;two&nbsp;forms&nbsp;are&nbsp;equivalent:&nbsp;<BR>
&nbsp;<BR>
＞<I>&nbsp;(do&nbsp;((x&nbsp;'(1&nbsp;2&nbsp;3&nbsp;4&nbsp