chapter4.html

c语言编程-c语言作者的书。英文原版。非常好。

   int atoi(char s[])
   {
       double atof(char s[]);

       return (int) atof(s);
   }
</pre>
Notice the structure of the declarations and the return statement. The value
of the expression in
<pre>
   return <em>expression</em>;
</pre>
is converted to the type of the function before the return is taken.
Therefore, the value of <tt>atof</tt>, a <tt>double</tt>, is converted
automatically to <tt>int</tt> when it appears in this <tt>return</tt>, since
the function <tt>atoi</tt> returns an <tt>int</tt>. This operation does
potentionally discard information, however, so some compilers warn of it. The
cast states explicitly that the operation is intended, and suppresses any
warning.
<p>
<strong>Exercise 4-2.</strong> Extend <tt>atof</tt> to handle scientific
notation of the form
<pre>
   123.45e-6
</pre>
where a floating-point number may be followed by <tt>e</tt> or <tt>E</tt> and
an optionally signed exponent.

<h2><a name="s4.3">4.3 External Variables</a></h2>
A C program consists of a set of external objects, which are either variables
or functions. The adjective ``external'' is used in contrast to ``internal'',
which describes the arguments and variables defined inside functions. External
variables are defined outside of any function, and are thus potentionally
available to many functions. Functions themselves are always external, because
C does not allow functions to be defined inside other functions. By default,
external variables and functions have the property that all references to them
by the same name, even from functions compiled separately, are references to
the same thing. (The standard calls this property <em>external linkage</em>.)
In this sense, external variables are analogous to Fortran COMMON blocks or
variables in the outermost block in Pascal. We will see later how to define
external variables and functions that are visible only within a single source
file.
Because external variables are globally accessible, they provide an alternative
to function arguments and return values for communicating data between
functions. Any function may access an external variable by referring to it by
name, if the name has been declared somehow.
<p>
If a large number of variables must be shared among functions, external
variables are more convenient and efficient than long argument lists. As
pointed out in <a href="chapter1.html">Chapter 1</a>, however, this reasoning should be
applied with some caution, for it can have a bad effect on program structure,
and lead to programs with too many data connections between functions.
<p>
External variables are also useful because of their greater scope and lifetime.
Automatic variables are internal  to a function; they come into existence when
the function is entered, and disappear when it is left. External variables, on
the other hand, are permanent, so they can retain values from one function
invocation to the next. Thus if two functions must share some data, yet
neither calls the other, it is often most convenient if the shared data is
kept in external variables rather than being passed in and out via arguments.
<p>
Let us examine this issue with a larger example. The problem is to write a
calculator program that provides the operators <tt>+</tt>, <tt>-</tt>, <tt>*</tt>
and <tt>/</tt>. Because it is easier to implement, the calculator will use reverse
Polish notation instead of infix. (Reverse Polish notation is used by some
pocket calculators, and in languages like Forth and Postscript.)
<p>
In reverse Polish notation, each operator follows its operands; an infix
expression like
<pre>
   (1 - 2) * (4 + 5)
</pre>
is entered as
<pre>
   1 2 - 4 5 + *
</pre>
Parentheses are not needed; the notation is unambiguous as long as we know
how many operands each operator expects.
<p>
The implementation is simple. Each operand is pushed onto a stack; when an
operator arrives, the proper number of operands (two for binary operators)
is popped, the operator is applied to them, and the result is pushed back
onto the stack. In the example above, for instance, 1 and 2 are pushed, then
replaced by their difference, -1. Next, 4 and 5 are pushed and then replaced
by their sum, 9. The product of -1 and 9, which is -9, replaces them on
the stack. The value on the top of the stack is popped and printed when the
end of the input line is encountered.
<p>
The structure of the program is thus a loop that performs the proper operation
on each operator and operand as it appears:
<pre>
   while (<em>next operator or operand is not end-of-file indicator</em>)
       if (<em>number</em>)
           <em>push it</em>
       else if (<em>operator</em>)
           <em>pop operands</em>
           <em>do operation</em>
           <em>push result</em>
       else if (<em>newline</em>)
           <em>pop and print top of stack</em>
       else
           <em>error</em>
</pre>
The operation of pushing and popping a stack are trivial, but by the time error
detection and recovery are added, they are long enough that it is better to
put each in a separate function than to repeat the code throughout the whole
program. And there should be a separate function for fetching the next input
operator or operand.
<p>
The main design decision that has not yet been discussed is where the stack is,
that is, which routines access it directly. On possibility is to  keep it in
<tt>main</tt>, and pass the stack and the current stack position to the routines
that push and pop it. But <tt>main</tt> doesn't need to know about the variables
that control the stack; it only does push and pop operations. So we have
decided to store the stack and its associated information in external variables
accessible to the <tt>push</tt> and <tt>pop</tt> functions but not to <tt>main</tt>.
<p>
Translating this outline into code is easy enough. If for now we think of the
program as existing in one source file, it will look like this:
<p>
&nbsp;&nbsp;&nbsp;&nbsp;<tt>#include</tt><em>s</em><br>
&nbsp;&nbsp;&nbsp;&nbsp;<tt>#define</tt><em>s</em>
<p>
&nbsp;&nbsp;&nbsp;&nbsp;<em>function declarations for</em> <tt>main</tt>
<p>
&nbsp;&nbsp;&nbsp;&nbsp;<tt>main() { ... }</tt>
<p>
&nbsp;&nbsp;&nbsp;&nbsp;<em>external variables for</em> <tt>push</tt> <em>and</em> <tt>pop</tt>
<pre>
   void push( double f) { ... }
   double pop(void) { ... }

   int getop(char s[]) { ... }
</pre>
&nbsp;&nbsp;&nbsp;&nbsp;<em>routines called by</em> <tt>getop</tt>
<p>
Later we will discuss how this might be split into two or more source files.
<p>
The function <tt>main</tt> is a loop containing a big <tt>switch</tt> on the type of
operator or operand; this is a more typical use of <tt>switch</tt> than the one
shown in <a href="chapter3.html#s3.4">Section 3.4</a>.
<pre>
   #include &lt;stdio.h&gt;
   #include &lt;stdlib.h&gt;  /* for  atof() */

   #define MAXOP   100  /* max size of operand or operator */
   #define NUMBER  '0'  /* signal that a number was found */

   int getop(char []);
   void push(double);
   double pop(void);

   /* reverse Polish calculator */
   main()
   {
       int type;
       double op2;
       char s[MAXOP];

       while ((type = getop(s)) != EOF) {
           switch (type) {
           case NUMBER:
               push(atof(s));
               break;
           case '+':
               push(pop() + pop());
               break;
           case '*':
               push(pop() * pop());
               break;
           case '-':
               op2 = pop();
               push(pop() - op2);
               break;
           case '/':
               op2 = pop();
               if (op2 != 0.0)
                   push(pop() / op2);
               else
                   printf("error: zero divisor\n");
               break;
           case '\n':
               printf("\t%.8g\n", pop());
               break;
           default:
               printf("error: unknown command %s\n", s);
               break;
           }
       }
       return 0;
   }
</pre>
Because <tt>+</tt> and <tt>*</tt> are commutative operators, the order in which
the popped operands are combined is irrelevant, but for <tt>-</tt> and <tt>/</tt>
the left and right operand must be distinguished. In
<pre>
   push(pop() - pop());   /* WRONG */
</pre>
the order in which the two calls of <tt>pop</tt> are evaluated is not defined.
To guarantee the right order, it is necessary to pop the first value into a
temporary variable as we did in <tt>main</tt>.
<pre>
   #define MAXVAL  100  /* maximum depth of val stack */

   int sp = 0;          /* next free stack position */
   double val[MAXVAL];  /* value stack */

   /* push:  push f onto value stack */
   void push(double f)
   {
       if (sp &lt; MAXVAL)
           val[sp++] = f;
       else
           printf("error: stack full, can't push %g\n", f);
   }

   /* pop:  pop and return top value from stack */
   double pop(void)
   {
       if (sp &gt; 0)
           return val[--sp];
       else {
           printf("error: stack empty\n");
           return 0.0;
       }
   }
</pre>
A variable is external if it is defined outside of any function. Thus the
stack and stack index that must be shared by <tt>push</tt> and <tt>pop</tt>
are defined outside these functions. But <tt>main</tt> itself does not refer
to the stack or stack position - the representation can be hidden.
<p>
Let us now turn to the implementation of <tt>getop</tt>, the function that
fetches the next operator or operand. The task is easy. Skip blanks and tabs.
If the next character is not a digit or a hexadecimal point, return it.
Otherwise, collect a string of digits (which might include a decimal point),
and return <tt>NUMBER</tt>, the signal that a number has been collected.
<pre>
   #include &lt;ctype.h&gt;

   int getch(void);
   void ungetch(int);

   /* getop:  get next character or numeric operand */
   int getop(char s[])
   {
       int i, c;

       while ((s[0] = c = getch()) == ' ' || c == '\t')
           ;
       s[1] = '\0';
       if (!isdigit(c) && c != '.')
           return c;      /* not a number */
       i = 0;
       if (isdigit(c))    /* collect integer part */
           while (isdigit(s[++i] = c = getch()))
              ;
       if (c == '.')      /* collect fraction part */
           while (isdigit(s[++i] = c = getch()))
              ;
       s[i] = '\0';
       if (c != EOF)
           ungetch(c);
       return NUMBER;
   }
</pre>
What are <tt>getch</tt> and <tt>ungetch</tt>? It is often the case that a program
cannot determine that it has read enough input until it has read too much. One
instance is collecting characters that make up a number: until the first
non-digit is seen, the number is not complete. But then the program has read
one character too far, a character that it is not prepared for.
<p>
The problem would be solved if it were possible to ``un-read'' the unwanted
character. Then, every time the program reads one character too many, it could
push it back on the input, so the rest of the code could behave as if it had
never been read. Fortunately, it's easy to simulate un-getting a character, by
writing a pair of cooperating functions. <tt>getch</tt> delivers the next input
character to be considered; <tt>ungetch</tt> will return them before reading new
input.
<p>
How they work together is simple. <tt>ungetch</tt> puts the pushed-back
characters into a shared buffer -- a character array. <tt>getch</tt> reads
from the buffer if there is anything else, and calls <tt>getchar</tt> if the
buffer is empty. There must also be an index variable that records the
position of the current character in the buffer.
<p>
Since the buffer and the index are shared by <tt>getch</tt> and <tt>ungetch</tt>
and must retain their values between calls, they must be external to both
routines. Thus we can write <tt>getch</tt>, <tt>ungetch</tt>, and their shared
variables as:
<pre>
   #define BUFSIZE 100

   char buf[BUFSIZE];    /* buffer for ungetch */
   int bufp = 0;         /* next free position in buf */

   int getch(void)  /* get a (possibly pushed-back) character */
   {
       return (bufp &gt; 0) ? buf[--bufp] : getchar();
   }

   void ungetch(int c)   /* push character back on input */
   {
       if (bufp &gt;= BUFSIZE)
           printf("ungetch: too many characters\n");
       else
           buf[bufp++] = c;
   }
</pre>
The standard library includes a function <tt>ungetch</tt> that provides one
character of pushback; we will discuss it in <a href="chapter7.html">Chapter 7</a>.
We have used an array for the pushback, rather than a single character, to
illustrate a more general approach.
<p>
<strong>Exercise 4-3.</strong> Given the basic framework, it's
straightforward to extend the calculator. Add the modulus (%) operator and
provisions for negative numbers.
<p>
<strong>Exercise 4-4.</strong> Add the commands to print the top elements of
the stack without popping, to duplicate it, and to swap the top two elements.
Add a command to clear the stack.
<p>