Swapping structures in C

Question

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

struct name * create_node(char *,char *,int );
struct name *add_node(struct name *,char *,char *,int);
void output(struct name *,char *);
void free_variables(struct name *);
struct name *bubble_sort(struct name *,int);
void swap(struct name **,struct name **);

typedef struct name
{
    char firstname[100];
    char secondname[100];
    int pno;
    struct name *next;
}harsha;

harsha *create_node(char *first,char *second, int phone)
{
    harsha *pnode=(struct name *)malloc(sizeof(struct name));
    strcpy(pnode->firstname,first);
    strcpy(pnode->secondname,second);
    pnode->pno=phone;
    pnode->next=NULL;
    return pnode;
}

harsha *bubble_sort(harsha *pnode,int count)
{
    int i,j,n;

    n=count;
    for(i=n;i>1;i--)
    {
        for(j=1;j<n;j++)
        {
            if(strcmp(pnode[j].firstname,pnode[j-1].firstname)<0)
            {
                swap(&(pnode+j),&(pnode+j-1));
            }
        }
    }
    return pnode;
}

void swap(harsha **pnode1,harsha **pnode2)
{
    harsha *temp;
    temp=*pnode1;
    *pnode1=*pnode2;
    *pnode2=temp;
}


harsha *add_node(harsha *pnode,char *first,char *second,int phone)
{
    if(pnode==NULL)
    {
        pnode=create_node(first,second,phone);
        return pnode;
    }
    else
    {
        while(pnode->next!=NULL)
        {
            pnode=pnode->next;
        }
        pnode->next=create_node(first,second,phone);
    }
    return pnode;
}

void output(harsha *pnode,char *str)
{
    while(pnode->next!=NULL)
    {
        if(strcmp(pnode->secondname,str)==0)
        {
            printf("%s",pnode->firstname);
            printf("%d\n",pnode->pno);
        }
        pnode=pnode->next;
    }
    if(strcmp(pnode->secondname,str)==0)
        {
            printf("%s",pnode->firstname);
            printf("%d\n",pnode->pno);
        }
}

void free_variables(harsha *pnode)
{
    if(pnode->next!=NULL)
    free_variables(pnode->next);

    free(pnode);
}   

int main()
{
    harsha *head=NULL;

    //int i;
    char first[50];
    char second[50];
    char sname[50];
    int phone;
    char option='y';
    int count=0;

    while(tolower(option)=='y')
    {
        count++;
        printf("enter the first name:");
        scanf("%s",first);

        printf("enter the second name:");
        scanf("%s",second);

        printf("enter the phone number:");
        scanf("%d",&phone);
        fflush(stdin);
        if (head==NULL)
        head=create_node(first,second,phone);
        else 
        add_node(head,first,second,phone);

        printf("enter the option:");
        scanf("%c",&option);

    }
    fflush(stdin);
    head=bubble_sort(head,count);
    printf("enter the second name:");
    scanf("%s",sname);

    output(head,sname);

    free_variables(head);
return 0;
}

What I have done here is-I have created a linked list with the structures of personal information of an individual.After that I thought of sorting(ascending order) the linked list based on the first name of the structures.

When I tried to run this program.I have got an error

error:non-lvalue in unary `&'

while passing the parameters to the swap function in the bubble sort function.

Can someone help me out and please explain what lvalue means.Thanks..

Answer 1

The error is in this line:

 swap(&(pnode+j),&(pnode+j-1));

pnode is a pointer and when you add it with j, it produces a rvalue. Taking address-of & an rvalue is not possible, hence compiler gives error.

Answer 2

In addition to the change KingsIndian gave, you need to change the swap function:

void swap(harsha *pnode1,harsha *pnode2)
{
    harsha temp;
    temp=*pnode1;
    *pnode1=*pnode2;
    *pnode2=temp;
}

Also, it may make more sense to you to call it this way:

swap(&pnode[j], &pnode[j-1]);