c - 为什么在多项式加/多源代码中使用指向结构的指针

Question

/* Program to multiply two polynomials. */

/#include <stdio.h>
#include <conio.h>

#define MAX 10

struct term
{
 int coeff ;
 int exp ;
} ;

struct poly
{
 struct term t [10] ;
 int noofterms ;
} ;

void initpoly ( struct poly *) ;
void polyappend ( struct poly *, int, int ) ;
struct poly polyadd ( struct poly, struct poly ) ;
struct poly polymul ( struct poly, struct poly ) ;
void display ( struct poly ) ;

void main( )
{
 struct poly p1, p2, p3 ;

 clrscr( ) ;

 initpoly ( &p1 ) ;
 initpoly ( &p2 ) ;
 initpoly ( &p3 ) ;

 polyappend ( &p1, 1, 4 ) ;
 polyappend ( &p1, 2, 3 ) ;
 polyappend ( &p1, 2, 2 ) ;
 polyappend ( &p1, 2, 1 ) ;

 polyappend ( &p2, 2, 3 ) ;
 polyappend ( &p2, 3, 2 ) ;
 polyappend ( &p2, 4, 1 ) ;

 p3 = polymul ( p1, p2 ) ;

 printf ( "\nFirst polynomial:\n" ) ;
 display ( p1 ) ;

 printf ( "\n\nSecond polynomial:\n" ) ;
 display ( p2 ) ;

 printf ( "\n\nResultant polynomial:\n" ) ;
 display ( p3 ) ;

 getch( ) ;
}

/* initializes elements of struct poly */
void initpoly ( struct poly *p )
{
 int i ;
 p -> noofterms = 0 ;
 for ( i = 0 ; i < MAX ; i++ )
 {
  p -> t[i].coeff = 0 ;
  p -> t[i].exp = 0 ;
 }
}

/* adds the term of polynomial to the array t */
void polyappend ( struct poly *p, int c, int e )
{
 p -> t[p -> noofterms].coeff = c ;
 p -> t[p -> noofterms].exp =  e ;
 ( p -> noofterms ) ++ ;
}

/* displays the polynomial equation */
void display ( struct poly p )
{
 int flag = 0, i ;
 for ( i = 0 ; i < p.noofterms ; i++ )
 {
  if ( p.t[i].exp != 0 )
   printf ( "%d x^%d + ", p.t[i].coeff, p.t[i].exp ) ;
  else
  {
   printf ( "%d", p.t[i].coeff ) ;
   flag = 1 ;
  }
 }
 if ( !flag )
  printf ( "\b\b  " ) ;

}
/* adds two polynomials p1 and p2 */
struct poly polyadd ( struct poly p1, struct poly p2 )
{
 int i, j, c ;
 struct poly p3 ;
 initpoly ( &p3 ) ;

 if ( p1.noofterms > p2.noofterms )
  c = p1.noofterms ;
 else
  c = p2.noofterms ;

 for ( i = 0, j = 0 ; i <= c ; p3.noofterms++ )
 {
  if ( p1.t[i].coeff == 0 && p2.t[j].coeff == 0 )
   break ;
  if ( p1.t[i].exp >= p2.t[j].exp )
  {
   if ( p1.t[i].exp == p2.t[j].exp )
   {
    p3.t[p3.noofterms].coeff = p1.t[i].coeff + p2.t[j].coeff ;
    p3.t[p3.noofterms].exp = p1.t[i].exp ;
    i++ ;
    j++ ;
   }
   else
   {
    p3.t[p3.noofterms].coeff = p1.t[i].coeff ;
    p3.t[p3.noofterms].exp = p1.t[i].exp ;
    i++ ;
   }
  }
  else
  {
   p3.t[p3.noofterms].coeff = p2.t[j].coeff ;
   p3.t[p3.noofterms].exp = p2.t[j].exp ;
   j++ ;
  }
 }
 return p3 ;
}

/* multiplies two polynomials p1 and p2 */
struct poly polymul ( struct poly p1, struct poly p2 )
{
 int coeff, exp ;
 struct poly temp, p3 ;

 initpoly ( &temp ) ;
 initpoly ( &p3 ) ;

 if ( p1.noofterms != 0 && p2.noofterms != 0 )
 {
  int i ;
  for ( i = 0 ; i < p1.noofterms ; i++ )
  {
   int j ;

   struct poly p ;
   initpoly ( &p ) ;

   for ( j = 0 ; j < p2.noofterms ; j++ )
   {
    coeff = p1.t[i].coeff * p2.t[j].coeff ;
    exp = p1.t[i].exp + p2.t[j].exp ;
    polyappend ( &p, coeff, exp ) ;
   }

   if ( i != 0 )
   {
    p3 = polyadd ( temp, p ) ;
    temp = p3  ;
   }
   else
    temp = p ;
  }
 }
 return p3 ;
}

在这段代码中我遇到了一些问题:

1. 使用成员变量“noofterms”的目的是什么
2. 在函数 void initpoly ( struct poly *), void polyappend ( struct poly *, int, int ) 中，多项式被视为指向结构类型的指针。但是在函数 struct poly polyadd ( struct poly, struct poly ),struct poly polymul ( struct poly, struct poly ), void display ( struct poly ) 简单结构变量被使用。

谁能解释一下原因...

Answer 1

以指针作为参数的函数可以改变原始对象。如果您传递一个简单的结构变量，它会被复制，并且该函数只能更改副本。

noofterms 计算构成多项式的项数。