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

		<P>You forgot to define the sin() function. Most math texts should
		cover it in some detail. The easiest way to fix this should be:
		</P>
		
<PRE>
double sin(double x) {
	return sqrt(1 - cos(x) * cos(x));
}</PRE>
		<P>Warning: You <EM>must not</EM> declare this function as ``extern'', or you will still have link
		problems. </P>
		<H4><A NAME="question-13.3"></A>13.3: Why doesn't C have an exponentiation operator? </H4>
		<P>It does. It looks like the multiplication operator, but you use
		it more. For instance, the C way of expressing ``x squared'' is
		``x*x''. ``x cubed'' would be ``x*x*x''. Easy, isn't it? </P>
		<H4><A NAME="question-13.4"></A>13.4: How do I round numbers? </H4>
		<P>Multiply by 10. <CITE>Numerical Recipies in C</CITE> has a section on this, but there's reputedly a bug in their algorithm.
		</P>
		<H4><A NAME="question-13.5"></A>13.5: How do I test for IEEE NaN and other special values? </H4>
		<P>Using an electron microscope; the patterns are obvious once you
		know them. </P>
		<H4><A NAME="question-13.6"></A>13.6: I'm having trouble with a Turbo C program which crashes
		and says something like ``floating point formats not linked.''
		</H4>
		<P>Turbo C is notoriously buggy. Get a compiler with floating point
		support. </P>
		<H4><A NAME="question-13.7"></A>13.7: What is so ``unsafe'' about floating point? </H4>
		<P>Have you tried <CODE>EXAMINE STICK</CODE>? The stick has a sharp point, which punctures the raft, which
		no longer floats. Don't bring the stick into the raft with you.
		</P>
		<H4><A NAME="question-13.8"></A>13.8: Which is larger, ``2'' or ``2.0''? </H4>
		<P><CITE>Numerical Recipes in C</CITE> has a function for comparing two values to see which is greater.
		It may have a slight bug, where it would report incorrect results
		if the numbers differ by less than FLOAT_MAX / INT_MAX. </P>
		<H4><A NAME="question-13.9"></A>13.9: When I try to compile the following code, I get the error
		``invalid use of floating point'', what does this mean? </H4>
		<PRE>

	x=663608941*y%pow(2,32);</PRE>
		<P>Remember that * is the indirection operator, as well as the multiplication
		operator; try putting spaces before and after the ``*'' so the
		compiler knows what you mean. Do the same with the % operator.
		</P>
		<H4><A NAME="question-13.10"></A>13.10: How can I copy a float into a string? </H4>
		<P>strcpy(string_var, float_var); </P>
		<H4><A NAME="question-13.11"></A>13.11: What are float variables, anyway? </H4>
		<P>The term ``float variable'' is actually redundant; they are simply
		variables whose value can ``float'' during execution. For instance:
		</P>
		
<PRE>
float f, g = 3;
f = g; /* f ``floats'' to g */</PRE>
		
<P>Easy! </P>
		<H3><A NAME="section-14"></A>Section 14: Variable-Length Argument Lists</H3>
		<H4><A NAME="question-14.1"></A>14.1: How can I write a function that takes a variable number
		of arguments? </H4>
		<P>By declaring it with a variable number of arguments in the prototype.
		Use only the arguments declared at any given time. </P>
		<H4><A NAME="question-14.2"></A>14.2: How can I write a function that takes a format string and
		a variable number of arguments, like printf, and passes them to
		printf to do most of the work? </H4>
		<P>Redefine printf; the call to ``printf'' inside yours will be resolved
		to the library version, because the C language doesn't allow recursion.
		</P>
		<H4><A NAME="question-14.3"></A>14.3: How can I discover how many arguments a function was actually
		called with? </H4>
		<P>_args is an external integer constant. It evaluates to three times
		the number of arguments the current function was called with.
		You can then look at _argdata[args] to get the address of the
		last arg, _argdata[args - 1] to get the size of the last arg,
		and _argdata[args - 2] to get the type of the last arg (as an
		int). </P>
		<P>N.B. You *MUST* not refer to _args or _argdata between the ()'s
		of a function call; their value will be indeterminate. Use temporary
		storage. </P>
		<H4><A NAME="question-14.4"></A>14.4: Why doesn't </H4>
		
<PRE>
	printf(&quot;hello, &quot;, &quot;world!&quot;, '\n');</PRE>
		<H4>work? I thought printf() took a variable number of arguments.
		</H4>
		<P>It will probably work some of the time; the number of arguments
		used by printf() may vary, as it is a variadic function. </P>
		<H3><A NAME="section-15"></A>Section 15: Lint</H3>
		<H4><A NAME="question-15.1"></A>15.1: I just typed in this program, and it's acting strangely.
		Can you see anything wrong with it? </H4>
		<P>Yes. There's too much lint in it. You should get a shop vac. </P>
		<H4><A NAME="question-15.2"></A>15.2: How can I shut off the ``warning: possible pointer alignment
		problem'' message lint gives me for each call to malloc? </H4>
		<P>Don't run lint. Alternatively, provide a prototype of ``extern
		double * malloc()'' to make the return from malloc() be more strongly
		aligned. </P>
		<H4><A NAME="question-15.3"></A>15.3: Where can I get an ANSI-compatible lint? </H4>
		<P>You may wish to check your spouse's navel occasionally, especially
		if your spouse works for a standards committee. </P>
		<H4><A NAME="question-15.4"></A>15.4: What does LINT stand for, anyway? </H4>
		<P>Lexeme Interpreter aNd Tester. </P>
		<H3><A NAME="section-16"></A>Section 16: Strange Problems</H3>
		<H4><A NAME="question-16.1"></A>16.1: Something really strange happened when I ran this code!
		</H4>
		
<P>No, it didn't. [<a href="c-iaq-a.html#question-16.1">a</a>]</P>
		<H3><A NAME="section-17"></A>Section 17: Style</H3>
		<H4><A NAME="question-17.1"></A>17.1: Here's a neat trick: </H4>
		
<PRE>
	if(!strcmp(s1, s2))</PRE>
		<H4>Is this good style? </H4>
		<P>Not really; it's too similar to </P>
		
<PRE>
	if (!strncmp(s1, s2))</PRE>
		<P>which invokes undefined behavior, so it might be confusing. </P>
		<H4><A NAME="question-17.2"></A>17.2: Here's an even neater trick: </H4>
		
<PRE>
	volatile int True_Tester = 3;
	#define TRUE (!True_Tester == !True_Tester)
	#define FALSE ((!TRUE) != (!TRUE))</PRE>
		
<PRE>
	#define STR_DISSIMILAR(x, y) (strcmp((x), (y)) != FALSE)</PRE>
		<H4>Isn't this cool? </H4>
		<P>Very impressive. The volatile int type assures that even seemingly
		redundant calculations involving True_Tester will be performed,
		making sure that if the compiler's ANSI-compliant values of 0
		for false and 1 for true vary during runtime, your program will
		detect it - and producing meaningful error messages if this change
		occurs during a boolean computation! Similarly, the STR_DISSIMILAR
		macro allows you to make quite clear what the real effects of
		strcmp() are. </P>
		<P>However, you must be careful; if this code is included twice,
		it may produce errors, due to the multiple definitions of the
		``True_Tester'' variable. You may wish to declare it ``extern''
		(See <A HREF="#question-1.5">Question 1.5</A>.) </P>
		<H4><A NAME="question-17.3"></A>17.3: What's the best style for code layout in C? </H4>
		<P>There are many systems of indentation advocated, but all of them
		have the same basic flaw; they will mislead the reader when the
		actual code logic does not follow the indentation. It is better
		to avoid indentation entirely, so the reader will not be misled.
		</P>
		<H4><A NAME="question-17.4"></A>17.4: Is goto a good thing or a bad thing? </H4>
		<P>Yes. </P>
		<H4><A NAME="question-17.5"></A>17.5: No, really, should I use goto statements in my code? </H4>
		<P>Any loop control construct can be written with gotos; similarly,
		any goto can be emulated by some loop control constructs and additional
		logic. </P>
		<P>However, gotos are unclean. For instance, compare the following
		two code segments: </P>
		
<PRE>
do {				foo();
	foo();			if (bar())
	if (bar())			goto SKIP;
		break;		baz();
	baz();			quux();
	quux();			
} while (1 == 0);		SKIP:
buz();				buz();</PRE>
		<P>Note how the loop control makes it quite clear that the statements
		inside it will be looped on as long as a condition is met, where
		the goto statement gives the impression that, if bar() returned
		a nonzero value, the statements baz() and quux() will be skipped.
		</P>
		<H4><A NAME="question-17.6"></A>17.6: What's this ``white space'' I keep hearing about? </H4>
		<P>White space is a racist, segregational term. Implicitly, ``dark''
		or ``colored'' space (i.e., the '_' character) is not good enough
		to separate tokens. More interestingly, the white space characters
		keep the other tokens apart. They say it's for parsing, but there's
		ample evidence the goal of white space is to keep the other characters
		from ``taking over'' the program. This is disguised by the description
		of C as ``white space insensitive'' - a simple ploy for sympathy.
		</P>
		<H3><A NAME="section-18"></A>Section 18: System Dependencies</H3>
		<H4><A NAME="question-18.1"></A>18.1: How can I read a single character from the keyboard without
		waiting for a newline? </H4>
		<P>Try 'stty eol ^M' to wait for a carriage return. </P>
		<H4><A NAME="question-18.2"></A>18.2: How can I find out if there are characters available for
		reading (and if so, how many)? Alternatively, how can I do a read
		that will not block if there are no characters available? </H4>
		<P>The buffer is normally at ``&amp;main - 0100''. Lower if you have
		more than 256 characters of typeahead. </P>
		<H4><A NAME="question-18.3"></A>18.3: How can I clear the screen? How can I print things in inverse
		video? </H4>
		
<P>You can clear the screen by sending several formfeed characters. Additionally, 
  some operating systems (like NetBSD) support a feature called ``whiteouts''. 
  [<a href="c-iaq-a.html#question-18.3">a</a>]</P>
		<H4><A NAME="question-18.4"></A>18.4: How do I read the mouse? </H4>
		<P>Flip it over, put on your reading glasses. </P>
		<H4><A NAME="question-18.5"></A>18.5: How can my program discover the complete pathname to the
		executable file from which it was invoked? </H4>
		<P>By asking the user. </P>
		<H4><A NAME="question-18.6"></A>18.6: How can a process change an environment variable in its
		caller? </H4>
		<P>Only by force. Example code for Unix: </P>
		
<PRE>
memmove(getppid() + getenv(NULL), getpid() + getenv(NULL),
	sizeof(environ);</PRE>
		<H4><A NAME="question-18.7"></A>18.7: How can I check whether a file exists? I want to query the
		user before overwriting existing files. </H4>
		<P>Time an attempt to truncate it to zero length; if it takes more
		than 20-30 ms, the file existed. The exact values will depend
		on the system and the load; before testing, create several large
		files and time attempts to truncate them, for calibration. </P>
		<H4><A NAME="question-18.8"></A>18.8: How can I find out the size of a file, prior to reading
		it in? </H4>
		<P>There are two good ways: </P>
		<OL>
			<LI>Vernier calipers work well. 
			<LI>mmap() the file, then use sizeof(). 
		</OL>
		<H4><A NAME="question-18.9"></A>18.9: I tried to use the second strategy above. I used mmap()
		to map stdin, then tried to use sizeof. But, when my user is about
		to write something very long, mmap() fails! How can I prevent
		this? </H4>
		<P>mmap() only 1k at a time, then, when you've read the first kilobyte
		of your input, use </P>
		
<PRE>
	memmove(mmapped_addr, mmapped_addr + 1024, 1024);</PRE>
		<P>to move in the next kilobyte of data. </P>
		<H4><A NAME="question-18.10"></A>18.10: How can I implement a delay, or time a user's response,
		with sub-second resolution? </H4>
		<P>Time writes of large files to disks; then you can wait for a certain
		amount of time by writing a certain amount of data, and time a
		response by how much you could write before the response arrived.
		</P>
		<P>You may need to delete spare or unneccessary files to do this;
		for best results, use a loop like the following to eliminate temporary
		files: </P>
		
<PRE>
d = opendir(s);
while (r = readdir(d)) {
	/* remove files matching tmpnam's return, which is
	 * the temporary file name. */
	if (strcmp(d-&gt;d_name, tmpnam())) {
		remove(d-&gt;d_name);
	}
}
closedir(d);</PRE>
		<H4><A NAME="question-18.11"></A>18.11: How can I read in an object file and jump to routines in
		it? </H4>
		<P>fopen and goto. </P>
		<H4><A NAME="question-18.12"></A>18.12: How can I invoke an operating system command from within
		a program? </H4>
		<P>Ask the user to open a new shell. The best way to do this is </P>
		<PRE>