c-iaq.html

稀疏矩阵、链表、图、队列、二叉树、多叉树、排序、遗传算法等的实现

system("echo Please open a new shell now.");
sprintf(cmdstring, "echo Enter the command '%s' in it.", cmd);
system(cmdstring);</PRE>
		<P>This will not work if you haven't declared cmdstring properly.
		</P>
		<H4><A NAME="question-18.13"></A>18.13: How can I ensure objects of my class are always created
		via ``new'' rather than as locals or global/static objects? </H4>
		<P>Read the C++ FAQ. </P>
		<H3><A NAME="section-19"></A>Section 19: Miscellaneous</H3>
		<H4><A NAME="question-19.1"></A>19.1: What can I safely assume about the initial values of variables
		which are not explicitly initialized? If global variables start
		out as ``zero,'' is that good enough for null pointers and floating-point
		zeroes? </H4>
		
<P>They're always zero. [<a href="c-iaq-a.html#question-19.1">a</a>]</P>
		<H4><A NAME="question-19.2"></A>19.2: How can I write data files which can be read on other machines
		with different word size, byte order, or floating point formats?
		</H4>
		<P>The traditional solution, pioneered by Microsoft, is to sell enough
		copies of your proprietary, slow, and limited software that everyone
		else supports your formats. </P>
		<H4><A NAME="question-19.3"></A>19.3: How can I insert or delete a line (or record) in the middle
		of a file? </H4>
		<P>Using fcntl(), lock the line or record in the file exclusively.
		Now, using another thread, read the file, at each byte, trying
		to write that byte back. Whenever you succeed, write that byte
		into another file. Then copy the new file over the old file, releasing
		the lock first. </P>
		<H4><A NAME="question-19.4"></A>19.4: How can I return several values from a function? </H4>
		<P>Code like this ought to work. </P>
		
<PRE>
long int foo() {
	return 2L +3; /* returns both values */
}</PRE>
		<H4><A NAME="question-19.5"></A>19.5: If I have a char * variable pointing to the name of a function
		as a string, how can I call that function? </H4>
		<P>Try the following: </P>
		
<PRE>
eval(s);</PRE>
		<P>Now all you need to do is write eval(). </P>
		<H4><A NAME="question-19.6"></A>19.6: I seem to be missing the system header file &lt;math.h&gt;. Can
		someone send me a copy? </H4>
		<P>A lot of people claim that it is useless to send people headers
		from other machines. Not so! It can be informative, and can show
		you a lot about how blatantly stupid your request was, although
		it can't show you anything you wouldn't have known in an instant
		had you thought before posting. </P>
		<P>Of course, we'd be happy to send you the header files... </P>
		<P>----cut here---- </P>
		
<PRE>
/* math.h rev 7.0b (3/7/95) */
/* RCS log: #log% - can anyone tell me why this doesn't work?
 * - joe, 2/12/93
 */

/*
 * Copyright 1995 Berserkley Software Systems &amp;&amp; Analytic Overdrive
 */

/* Parts of this header, including in particular the second and
 * third clauses of the first sentance of the fourth comment, were
 * based on copyright agreements from other sources, including
 * Xerox corporation.
 */

/*
 * math.h - math related macros and headers
 */

#ifndef _MATH_H
#define _MATH_H

/*
 * global data and definitions
 */

#ifdef __LITERAL_BIBLICAL_FUNDEMENTALISM
#define PI 3.0				/* 1 Kings 7:23 */
#endif

/*
 * common (portable) structures and functions
 */

/*
 * machine specific data
 */
#include &lt;machine/math.h&gt;

#endif /* _MATH_H // prevent multiple inclusion by using C++ comments*/
----cut here----</PRE>
		
<P>(Morons.) [<a href="c-iaq-a.html#question-19.6">a</a>]</P>
		<H4><A NAME="question-19.7"></A>19.7: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP, perl)
		functions from C? (And vice versa?) </H4>
		<P>You can do things like this: </P>
		
<PRE>
DO CALL FORTRAN;		fortran();
__LINE__ BASIC;			basic();
sub pascal;			pascal();
(((((lisp)))))			lithp(); [*]
&amp;perl_c;			perl():</PRE>
		<P>(You can't call Ada from C; it's unsafe.) </P>
		<P>[*] C is pass by value, of course. </P>
		<H4><A NAME="question-19.8"></A>19.8: Does anyone know of a program for converting Pascal or FORTRAN
		(or LISP, Ada, awk, ``Old'' C, ...) to C? </H4>
		<P>Nope. However, the psychic friends network may have a lead. And
		they're not just a psychic, they're also a friend. </P>
		<H4><A NAME="question-19.9"></A>19.9: Is C++ a superset of C? Can I use a C++ compiler to compile
		C code? </H4>
		<P>C++ is a superset of something, we're not sure what. You can use
		a C++ compiler to compile C code, but the results may surprise
		you. </P>
		<H4><A NAME="question-19.10"></A>19.10: Where can I get copies of all these public-domain programs?
		</H4>
		<P>From <A HREF="ftp://ftp.microsoft.com/">ftp://ftp.microsoft.com/</A> . Some of the code may look copyrighted; don't worry! The small
		companies that wrote it in the first place are not available for
		comment. </P>
		<H4><A NAME="question-19.11"></A>19.11: When will the next International Obfuscated C Code Contest
		(IOCCC) be held? How can I get a copy of the current and previous
		winning entries? </H4>
		<P>Next week. You missed the deadline. Tough, sucker. </P>
		<H4><A NAME="question-19.12"></A>19.12: Why don't C comments nest? How am I supposed to comment
		out code containing comments? Are comments legal inside quoted
		strings? </H4>
		<P>We believe it has something to do with captivity; C comments in
		the wild mate and nest normally. The San Diego Zoo believes it
		has managed to convince some C comments to nest, but it's hard
		to tell how much of that is really in the preprocessor, and how
		much of it is just bovine fecal matter. </P>
		<H4><A NAME="question-19.13"></A>19.13: How can I get the ASCII value corresponding to a character,
		or vice versa? </H4>
		<P>chr$(foo); You would have known this if you had an integer basic
		in ROM. </P>
		<H4><A NAME="question-19.14"></A>19.14: How can I implement sets and/or arrays of bits? </H4>
		<P>With linked lists of bitfields. You may also wish to simply use
		a large set of constants and some clever use of the switch statement,
		i.e.: </P>
		
<PRE>
enum { zero, one, two, three };</PRE>
		
<PRE>
int bitwise_or(int n, int m) {
	switch (n) {
	case three:
		return three;
		break;
	case two:
		switch (m) {
		case one: case three: return three; break;
		default: return two; break;
		}
		break;
	case one:
		switch (m) {
		case two: case three: return three; break;
		default: return one; break;
		}
		break;
	default: case zero:
		switch (m) {
		case one: return one; break;
		case two: return two; break;
		case three: return three; break;
		case zero: default: return zero; break;
		}
		break;
	}
}</PRE>
		<P>Obviously, you'll need to increase this slightly to deal with
		more than two bits. This is much more readable than the alleged
		``C'' solution: </P>
		
<PRE>
int bitwise_or(int n,int m){return n|m;}</PRE>
		<P>Note how the lack of whitespace around operators obscures the
		functionality of the code. A clear argument for explicit statement
		of program logic over arcane operators, if I ever saw one. </P>
		<P>The enum at the top isn't declared ``const int'', because the
		resulting ``const poisoning'' would require casts during all of
		the switch statements. </P>
		<H4><A NAME="question-19.15"></A>19.15: What is the most efficient way to count the number of bits
		which are set in a value? </H4>
		<P>Start a counter at zero and add one to it for each bit set. Some
		operating systems may provide a call to do this. For values over
		INT_MAX/2, start the counter at CHAR_BIT * sizeof(int) and subtract
		one for each bit not set. </P>
		<H4><A NAME="question-19.16"></A>19.16: How can I make this code more efficient? </H4>
		<P>Remove the comments; the no-op instructions generated by comments
		can slow your code down signifigantly. Similarly, shorten variable
		names. Most compilers, to implement pass by value, actually pass
		the names of variables in the stack; shorter variable names will
		reduce stack usage, and consequently execution time. If your compiler
		has good loop optimization, replace </P>
		
<PRE>
foo();</PRE>
		<P>with </P>
		
<PRE>
do {
	foo();
} while (1 != 1);</PRE>
		<P>which will likely receive more optimization. </P>
		<H4><A NAME="question-19.17"></A>19.17: Are pointers really faster than arrays? How much do function
		calls slow things down? Is <CODE>++i</CODE> faster than <CODE>i = i + 1</CODE>? </H4>
		<P>Yes. About 10 ms per call. Only on machines which feature preincrement
		addressing. </P>
		<H4><A NAME="question-19.18"></A>19.18: This program crashes before it even runs! (When single-stepping
		with a debugger, it dies before the first statement in main.)
		</H4>
		<P>You probably declared main as ``void main(void)''. It's also possible
		that the first statement in main is abort(); - by the as if rule,
		the compiler can abort at any time before then, too. Some compilers
		have bugs, and will produce buggy code for any module which includes
		the letters ``a'', ``b'', ``o'', ``r'', and ``t'' in that order
		before the first function declaration. </P>
		<H4><A NAME="question-19.19"></A>19.19: What do ``Segmentation violation'' and ``Bus error'' mean?
		</H4>
		<P>C programs are very territorial, and divide their code into segments.
		Violating these segments can trigger riots; similarly, pointers
		and integral constants are at the front of the bus, wheras arrays,
		strings, and other second-class data types are required to be
		at the rear of the bus. When they start forgetting their places,
		you can get a bus error. This is what the whole ``integral'' type
		thing is about - integrated bussing. </P>
		<H4><A NAME="question-19.20"></A>19.20: My program is crashing, apparently somewhere down inside
		malloc, but I can't see anything wrong with it. </H4>
		<P>Your vendor's library is buggy; complain loudly. Don't send them
		any example code; they just ask for that so they can steal your
		trade secrets. </P>
		<H4><A NAME="question-19.21"></A>19.21: Does anyone have a C compiler test suite I can use? </H4>
		<P>Yes. Unfortunately, it's probably broken. It's hard to tell. </P>
		<H4><A NAME="question-19.22"></A>19.22: Where can I get a YACC grammar for C? </H4>
		<P>You can't; YACC is written in C. </P>
		<H4><A NAME="question-19.23"></A>19.23: I need code to parse and evaluate expressions. </H4>
		<P>Ask any first year CS student. You may also wish to use your C
		compiler. </P>
		<H4><A NAME="question-19.24"></A>19.24: I need a sort of an ``approximate'' strcmp routine, for
		comparing two strings for close, but not necessarily exact, equality.