Solution for Exercise 22-17

ex2217

/* An interactive linked-list program */
/* Dan Gookin, C Programming For Dummies */
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

struct lili {
    int value;
    struct lili *next;
};
struct lili *first;
struct lili *current;

int menu(void);
void add(void);
void show(void);
void delrec(void);
struct lili *create(void);
void load(char *file);
void save(char *file);

/* the main function works with input only
      everything else is handled by a function */
int main()
{
    /* initialize variables */
    char filename[] = "linked.list";
    int choice='\0';
    first=NULL;

    /* load in the file if it exists */
    load(filename);

    while(choice!='Q')
    {
        choice=menu();
        switch (choice)
        {
            case 'S':
                show();
                break;
            case 'A':
                add();
                break;
            case 'R':
                delrec();
                break;
            case 'Q':
                break;
        }
    }

    /* save the file before exiting */
    save(filename);

    return(0);
}

/* output the main menu and collect input */
int menu(void)
{
    int ch;

    printf("S)how, A)dd, R)emove, Q)uit: ");
    ch=getchar();
    /* delete excess input */
    while(getchar()!='\n')
        ;
    return(toupper(ch));
}

/* add an item to the end of the linked list */
void add(void)
{
    /* special case for the first item */
    if(first==NULL)
    {
        first=create();
        current=first;
    }
    /* otherwise, find the last item */
    else
    {
        current=first;
        /* find the NULL */
        while(current->next)
            current=current->next;
        current->next=create();
        current=current->next;
    }
    printf("Type a value: ");
    scanf("%d",&current->value);
    current->next=NULL;
    /* delete excess input */
    while(getchar()!='\n') 
        ;
}

/* output all structures in the linked list */
void show(void)
{
    int count=1;

    /* is the list empty? */
    if(first==NULL)
    {
        puts("Nothing to show");
        return;
    }
    puts("Showing all records:");
    current=first;
    /* loop until current==NULL */
    while(current)
    {
        printf("Record %d: %d\n",
                count,current->value);
        current=current->next;
        count++;
    }
}

/* delete a record from the list */
void delrec(void)
{
    /* the previous record must be saved */
    struct lili *previous;
    int r,c;

    /* is the list empty? */
    if(first==NULL)
    {
        puts("No records to remove");
        return;
    }

    puts("Choose a record to remove:");
    /* output the list */
    show();
    printf("Record: ");
    scanf("%d",&r);
    /* delete excess input */
    while(getchar()!='\n') 
        ;
    c=1;
    /* reset the list */
    current=first;
    /* initialize the previous pointer */
    previous=NULL;
    while(c!=r)
    {
        previous=current;
        current=current->next;
        c++;
        /* bail on the end of the list */
        if(current==NULL)
        {
            puts("Record not found");
            return;
        }
    }
    /* 'current' holds the record to delete */
    /* is it the first record? */
    if(previous==NULL)
        first=current->next;
    /* otherwise, reset the next reference */
    else
        previous->next=current->next;
    printf("Record %d removed.\n",r);
    /* release memory of the current record */
    free(current);
}

/* build an empty structure & return its address */
struct lili *create(void)
{
    struct lili *a;

    a=(struct lili *)malloc(sizeof(struct lili)*1);
    if(a==NULL)
    {
        puts("Some kind of malloc() error");
        exit(1);
    }

    return(a);
}

void load(char *file)
{
    FILE *fp;
    struct lili loaded;
    int r,records;

    fp = fopen(file,"r");
    /* if an error, assume that the file
       doesn't exist */
    if( fp==NULL )
        return;

    /* read in the records */
    records = 0;
    while( !feof(fp) )
    {
        r = fread(&loaded,sizeof(struct lili),1,fp);
        if( r==0 )
            break;
        /* this code lifted from the create() function */
        if(first==NULL)
        {
            first=create();
            current=first;
        }
        else
        {
            current=first;
            /* find the NULL */
            while(current->next)
                current=current->next;
            current->next=create();
            current=current->next;
        }
        /* update the new structure with the
           data pulled in from the file */
        current->value = loaded.value;
        /* ignore the pointer value retrieved */
        current->next = NULL;
        records++;
    }
    fclose(fp);
    printf("%d records read from '%s'\n",records,file);
}

void save(char *file)
{
    int records;
    FILE *fp;

    /* open the file */
    fp = fopen(file,"w");
    if( fp==NULL )
    {
        printf("Unable to create '%s'\n",file);
        return;
    }

    /* write the records */
    records = 0;
    current = first;
    /* if the list is empty, first==NULL
       and nothing is written */
    while(current)
    {
        fwrite(current,sizeof(struct lili),1,fp);
        current = current->next;
        records++;
    }

    fclose(fp);
    printf("%d records written\n",records);
}

Output

4 records read from 'linked.list'
S)how, A)dd, R)emove, Q)uit: s
Showing all records:
Record 1: 10
Record 2: 20
Record 3: 30
Record 4: 40
S)how, A)dd, R)emove, Q)uit: d
S)how, A)dd, R)emove, Q)uit: r
Choose a record to remove:
Showing all records:
Record 1: 10
Record 2: 20
Record 3: 30
Record 4: 40
Record: 2
Record 2 removed.
S)how, A)dd, R)emove, Q)uit: s
Showing all records:
Record 1: 10
Record 2: 30
Record 3: 40
S)how, A)dd, R)emove, Q)uit: q
3 records written

Notes

* The reference in the book is incorrect. This Exercise solution is based on Exercise 20-13, not 20-12.

* Click here to obtain a downloadable copy of this code. (Right-click the link to save the file.)

Exercise ex2301 →