谭浩强版的C语言课本上的例题，运行显示 'NULL'的宣告是不相等的 这是怎么回事
#include <stdio.h>#include <malloc.h>
#define NULL 0
#define LEN sizeof(struct student)
struct student
{
long num;
float score;
struct student *next;
};
int n;
struct student *creat(void)
{
struct student *p1,*p2,*head;
p1=p2=(struct student *)malloc(LEN);
printf("please input a number and score:\n");
scanf("%ld%f",&p1->num,&p1->score);
head=NULL;n=0;
while(p1->num!=0)
{
n=n+1;
if(n==1)
head=p1;
else
p2->next=p1;
p2=p1;
p1=(struct student *)malloc(LEN);
printf("please input a number and score:\n");
scanf("%ld%f",&p1->num,&p1->score);
}
p2->next=NULL;
return head;
}
void print(struct student *head)
{
struct student *p;
p=head;
if(head!=NULL)
do
{printf("%ld,%f\n",p->num,p->score);
p=p->next;}
while(p!=NULL);
}
struct student *del(struct student *head,long num)
{
struct student *p1,*p2;
if(head==NULL)
printf("the list is NULL\n");
else
{
p1=head;
while(num!=p1->num&&p1->next!=NULL)
{
p2=p1;
p1=p1->next;
}
if(num!=p1->num)
printf("no found!\n");
else
{
if(p1==head)
head=head->next;
else
{p2->next=p1->next;
free(p1);}
}
return head;
}
}
struct student *insert(struct student *head,struct student *stu)
{
struct student *p0,*p1,*p2;
p0=stu;
if(head==NULL)
{head=p0;
p0->next=NULL;}
else
{
p1=head;
while(p0->num>p1->num&&p1->next!=NULL)
{p2=p1;p1=p1->next;}
if(p0->num<=p1->num)
{
if(head==p1)
{head=p0;p0->next=p1;}
else
{p2->next=p0;p0->next=p1; }
}
else
{p1->next=p0;p0->next=NULL;}
}
return head;
}
main()
{
struct student *head,*stu;
long del_num;
head=creat();
print(head);
printf("please input the del number:\n");
scanf("%ld",&del_num);
while(del_num!=0)
{ head=del(head,del_num);
print(head);
printf("please input the del number:\n");
scanf("%ld",&del_num);
}
printf("please input the insert num and score:\n");
stu=(struct student *)malloc(LEN);
scanf("%ld%f",&stu->num,&stu->score);
while(stu->num!=0)
{
head=insert(head,stu);
print(head);
stu=(struct student *)malloc(LEN);
printf("please input the insert num and score:\n");
scanf("%ld%f",&stu->num,&stu->score);
}
getch();
}
