Solution for Exercise 20-12

ex2012

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

struct typical {
    int value;
    struct typical *next;
};
struct typical *first;
struct typical *current;
struct typical *new;

int menu(void);
void add(void);
void show(void);
void delete(void);
struct typical *create(void);

/* The main function works with input only
   Everything else is handled by a function */
int main()
{
    int choice='\0';    /* get the while loop to spin */
    first=NULL;

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

    return(0);
}

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

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

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

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

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

/* Remove a record from the list */
void delete(void)
{
    struct typical *previous;   /* must save previous record */
    int r,c;

    if(first==NULL)         /* check for empty list */
    {
        puts("No records to remove");
        return;
    }
    puts("Choose a record to remove:");
    show();
    printf("Record: ");
    scanf("%d",&r);
    while(getchar()!='\n')      /* remove excess input */
        ;
    c=1;
    current=first;
    previous=NULL;      /* first record has no previous */
    while(c!=r)
    {
        previous=current;
        current=current->next;
        c++;
        if(current==NULL)   /* ensure that 'r' is in range */
        {
            puts("Record not found");
            return;
        }
    }
    if(previous==NULL)      /* Special case for first record */
        first=current->next;
    else                    /* point previous record at next */
        previous->next=current->next;
    printf("Record %d removed.\n",r);
    free(current);          /* release memory */
}

/* Build an empty structure and return its address */
struct typical *create(void)
{
    struct typical *a;

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

Notes

* This code is shown inaccurately in the book, in Listing 20-8. Specifically a chunk of code in the delete() function is located at the wrong spot. Here's the code chunk:

        if(current==NULL)   /* ensure that 'r' is in range */
        {
            puts("Record not found");
            return;
        }

In the book, this chunk is shown at Line 130, which works but not for the situation where the record number entered is one greater than the number of records in the linked list. In that case, the code references a non-existent record and crashes. By moving the above chunk after the c++ statement this bug is fixed.

Exercise ex2101 →