c - C中使用链表时的指针问题

Question

我正在编写 C 编程代码，除了多项式乘法部分之外，大多数代码都工作正常。它有一个运行时错误。请帮助我从多项式乘法中删除此运行时错误，我找不到错误，我认为它位于第三个 for 循环中。 谢谢... 如果你能解决这个错误那就太好了

#include<math.h>
#include<stdio.h>
#include<stdlib.h>
#define MAX 17

typedef struct node
{
    int coeff;
    struct node *next;
}node;

node * init();
void read(node *h1);
void print(node *h1);
node * add(node *h1,node *h2);
node * multiply(node *h1, node *h2);

void main()
{
    node *h1=NULL,*h2=NULL,*h3=NULL;
    int option;
    do
    {
        printf("\n1 : create 1’st polynomial");
        printf("\n2 : create 2’nd polynomial");
        printf("\n3 : Add polynomials");
        printf("\n4 : Multiply polynomials");
        printf("\n5 : Quit");
        printf("\nEnter your choice :");
        scanf("%d",&option);
        switch(option)
        {
            case 1:
                h1=init();
                read(h1);
                break;

            case 2:
                h2=init();
                read(h2);
                break;

            case 3:
                h3=add(h1,h2);
                printf("\n1’st polynomial -> ");
                print(h1);
                printf("\n2’nd polynomial -> ");
                print(h2);
                printf("\n Sum = ");
                print(h3);
                break;

            case 4:
                h3=multiply(h1,h2);
                printf("\n1’st polynomial -> ");
                print(h1);
                printf("\n2’nd polynomial -> ");
                print(h2);
                printf("\n Product = ");
                print(h3);
                break;
        }
    }while(option!=5);
}

void read(node *h)
{
    int n,i,j,power,coeff;
    node *p;
    p=init();
    printf("\n Enter number of terms :");
    scanf("%d",&n);
    /* read n terms */
    for (i=0;i<n;i++)
    {
        printf("\nenter a term(power coeff.)");
        scanf("%d%d",&power,&coeff);
        for(p=h,j=0;j<power;j++)
            p=p->next;
        p->coeff=coeff;
    }
}

void print(node *p)
{
    int i;
    for(i=0;p!=NULL;i++,p=p->next)
        if(p->coeff!=0)
            printf("%dX^%d ",p->coeff,i);
}

node * add(node *h1, node *h2)
{
    node *h3,*p;
    h3=init();
    p=h3;
    while(h1!=NULL)
    {
        h3->coeff=h1->coeff+h2->coeff;
        h1=h1->next;
        h2=h2->next;
        h3=h3->next;
    }
    return(p);
}

node * multiply(node *h1, node *h2)
{
    node *h3,*p,*q,*r;
    int i,j,k,coeff,power;
    h3=init();
    for(p=h1,i=0;p!=NULL;p=p->next,i++)
        for(q=h2,j=0;q!=NULL;q=q->next,j++)
        {
            coeff=p->coeff * q->coeff;
            power=i+j;
            for(r=h3,k=0;k<power;k++)
                r=r->next;
            r->coeff=r->coeff+coeff;
        }
    return(h3);
}

node * init()
{
    int i;
    node *h=NULL,*p;
    for(i=0;i<MAX;i++)
    {
        p=(node*)malloc(sizeof(node));
        p->next=h;
        p->coeff=0;
        h=p;
    }
    return(h);
}

Answer 1

multiply 函数中至少存在一个问题:

...
for (r = h3, k = 0; k < power; k++)
  r = r->next;

r->coeff = r->coeff + coeff;
...

在某些时候，r 会变为 NULL，并且在下一步中，当您使用 r->coeff 取消引用 r 时>(r 现在是NULL)您的程序将导致未定义的行为(大多数平台上的段错误)。