空链表条件为真——C语言

如何解决空链表条件为真——C语言

我使用结构和指针在 C 中创建程序。我必须将两个多项式加到一个简化的多项式中。我遇到的问题是，如果 L 为空，则用于测试链表是否为空的 IsEmpty 函数返回 true，然后打印“空列表”。在将 addPolynomial 函数放在一起后，我不确定为什么我的链表是空的。该函数位于 list.c 文件中。任何帮助将不胜感激，谢谢！

头文件：

#ifndef _List_H
#define _List_H

 struct Node;  //OK
 typedef struct Node *PtrToNode; //OK
 typedef PtrToNode List; //OK
 typedef PtrToNode Position; //OK

 List MakeEmpty( List L ); //OK
 int IsEmpty( List L );
 int IsLast( Position P,List L );
 List addPolynomial(List L1,List L2);
 Position FindPrevious( int e,List L );
 void Insert( int c,int e,List L );
 void DeleteList( List L );
 Position Header( List L );
 Position First( List L );
 Position Advance( Position P ); 
 int RetrieveCoefficient( Position P ); //OK
 int RetrieveExponent( Position P ); //OK
#endif    /* _List_H */

打印/测试文件：

#include <stdio.h>
#include <stdlib.h>
#include "list.h"

void PrintList( const List L ){
Position P = Header( L );
if( IsEmpty( L ) ) printf( "Empty list\n" );
else{
    do{
        P = Advance( P );
        if(RetrieveCoefficient( P )>0){
            printf( "%+dx^%d ",RetrieveCoefficient( P ),RetrieveExponent( P ));
        }
        else{
            printf( "%dx^%d ",RetrieveExponent( P ));
        }
    } while( !IsLast( P,L ) );
    printf( "\n" );
}
}

main(){
List L1,L2,L3;
Position P1,P2;

L1 = MakeEmpty( NULL );
P1 = Header( L1 );
L2 = MakeEmpty( NULL );
P2 = Header( L2 );
L3 = MakeEmpty( NULL );

Insert(3,5,L1);
Insert(2,3,L1);
Insert(-7,8,L1);
Insert(4,9,L1);
Insert(-9,1,L2);
printf("Polynomial 1:\n");
PrintList( L1 );

Insert(4,7,L2);
Insert(5,6,L2);
Insert(-4,L2);
Insert(-3,L2);
printf("Polynomial 2:\n");
PrintList( L2 );

L3 = addPolynomial(L1,L2);
printf("Polynomial 3:\n");
PrintList( L3 );

DeleteList( L1 );
DeleteList( L2 );
DeleteList( L3 );
return 0;
}

list.c 文件：

#include "list.h"
#include <stdlib.h>
#include "fatal.h"

/* Figure 3.23 Type declaration for linked list implementation of Polynomial ADT */
struct Node{
int Coefficient;
int Exponent;
PtrToNode Next;
};

List MakeEmpty( List L ){
if( L != NULL ) DeleteList( L );
L = (List) malloc( sizeof( struct Node ) ); 
if( L == NULL ) FatalError( "Out of memory!" );
L->Next = NULL;
return L;
}

/* Figure 3.8 Function to test whether a linked list is empty */
/* Return true if L is empty */
int IsEmpty( List L ){return L->Next == NULL;}

/* Figure 3.9 Function to test whether current position is the last in a linked list */
/* Return true if P is the last position in list L */
/* Parameter L is unused in this implementation */
int IsLast( Position P,List L ){
return P->Next == NULL;
}
/* END */

List addPolynomial(List L1,List L2){
List L; //Result of L1+L2
Position P,P1,P2;
L = MakeEmpty( NULL );
P = Header( L );
P1 = First(L1);
P2 = First(L2);

// While loops to be used

while (P1->Next && P2->Next)
{
    //move pointer and leave 1st as is if exponent is greater than 2nd poly
    if (P1->Exponent > P2->Exponent){

        P->Exponent = P1->Exponent;
        P->Coefficient = P1->Coefficient;
        P1 = P1->Next;
    }
    //if exponent in 2nd is larger then store L2 and move pointer
    else if (P1->Exponent < P2->Exponent){

        P->Exponent = P2->Exponent;
        P->Coefficient = P2->Coefficient;
        P2 = P2->Next;
    }
    //if both are the same then they're added together
    else{

        P->Exponent = P1->Exponent;
        P->Coefficient = P1->Coefficient + P2->Coefficient;         
        P1 = P1->Next;
        P2 = P2->Next;
    }
    
    P->Next = /*(struct Node *)*/malloc(sizeof(struct Node));
    P = P->Next;
    P->Next = NULL;
    
}
while (P1->Next || P2->Next)
{
    if (P1->Next) {

        P->Exponent = P1->Exponent;
        P->Coefficient = P1->Coefficient;
        P1 = P1->Next;
    }

    if (P2->Next) {

        P->Exponent = P2->Exponent;
        P->Coefficient = P2->Coefficient;
        P2 = P2->Next;
    }

    P->Next = /*(struct Node *)*/malloc(sizeof(struct Node));
    P = P->Next;
    P->Next = NULL;

}
L = P;
return L;
}

/* Figure 3.13 Insertion routine for linked lists */
/* Insert (after legal position P) */
/* Header implementation assumed */
/* Parameter L is unused in this implementation */
void Insert( int c,List L){
    Position TmpCell;
    Position P = FindPrevious( e,L );

/* 1*/  TmpCell = malloc( sizeof( struct Node ) ); //(Position)
/* 2*/  if( TmpCell == NULL ) FatalError( "Out of space!!!" );
/* 3*/  TmpCell->Coefficient = c;
/* 4*/  TmpCell->Exponent = e;
/* 5*/  TmpCell->Next = P->Next;
/* 6*/  P->Next = TmpCell;
}

/* Figure 3.12 FindPrevious - the Find routine for use with Delete */
/* If X is not found,then Next field of returned value is NULL */
/* Assumes a header */
Position FindPrevious( int e,List L ){
    Position P;
/* 1*/  P = L;
/* 2*/  while (P->Next != NULL && P->Next->Exponent != e)
/* 3*/  P = P->Next;
/* 4*/  return P;
}

/* Figure 3.15 Correct way to delete a list */
/* Correct DeleteList algorithm */
void DeleteList( List L ){
    Position P,Tmp;
/* 1*/  P = L->Next;  /* Header assumed */
/* 2*/  L->Next = NULL;
/* 3*/  while( P != NULL ){
/* 4*/      Tmp = P->Next;
/* 5*/      free( P );
/* 6*/      P = Tmp;
    }
}

Position Header( List L ){  return L;}
Position First( List L ){   return L->Next;}
Position Advance( Position P ){ return P->Next;}
int RetrieveCoefficient( Position P ){  return P->Coefficient;}
int RetrieveExponent( Position P ){ return P->Exponent;}

fatal.h 文件：

#include <stdio.h>
#include <stdlib.h>

#define Error( Str )        FatalError( Str )
#define FatalError( Str )   fprintf( stderr,"%s\n",Str ),exit( 1 )

输出到终端：

多项式 1： +3x^5 +2x^3 -7x^8 +4x^9

多项式 2： -9x^1 +4x^7 +5x^6 -4x^3 -3x^5

多项式 3： 空列表

解决方法

你只需要删除函数L = P;末尾的行addPolynomial，然后它就可以工作了。