Solution for Exercise 22-17

ex2217

/* An interactive linked list program */
/* Dan Gookin, Beginning C Programming 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;
char *filename="linked.list";

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

	load();				/* read in linked list from memory */

	while(choice!='Q')
	{
		choice=menu();
		switch (choice)
		{
			case 'S':
				show();
				break;
			case 'A':
				add();
				break;
			case 'R':
				delete();
				break;
			case 'Q':
				break;
			default:
				break;
		}
	}
	save();
	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)
	{
		if(current==NULL)	/* ensure that 'r' is in range */
		{
			puts("Record not found");
			return;
		}
		previous=current;
		current=current->next;
		c++;
	}
	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);
}

void load(void)
{
	FILE *f;
	struct typical loaded;

	f = fopen(filename,"r");
	if(!f)					/* assume the file just doesn't exist */
		return;
							/* read in all the records */
	while(fread(&loaded,sizeof(struct typical),1,f))
	{
		if(first==NULL)		/* This code copied from create() */
		{
			first=create();
			current=first;
		}
		else
		{
			current=first;
			while(current->next)
				current=current->next;
			new=create();
			current->next=new;
			current=new;
		}
		current->value=loaded.value;	/* Save value from file */
						/* don't save pointers! */
		current->next=NULL;
	}
	fclose(f);
}

void save(void)
{
	int count=1;
	FILE *f;

	if(first==NULL)			/* this list is empty */
	{
		puts("Nothing to save");
		return;
	}
	current=first;
	f = fopen(filename,"w");
	if(!f)
	{
		puts("File open error");
		exit(1);
	}
	while(current)			/* last record == NULL */
	{
		fwrite(current,sizeof(struct typical),1,f);
		current=current->next;
		count++;
	}
	count--;
	fclose(f);
	printf("%d Records written\n",count);
}

Notes

* This code does contain a bug: If you remove all the records, then the first record equals NULL, which is tested for in the save() function. The function doesn't save anything at that point, which means any records already saved to disk are retained. Here is the suggested fix:

	if(first==NULL)			/* this list is empty */
	{
		puts("Nothing to save");
       f = fopen(filename,"w");
		if(!f)
		{
			puts("File open error");
			exit(1);
		}
		fclose(f);
		return;
	}

I would like to thank reader Tingfeng for pointing out this bug and suggesting the above fix.

Exercise ex2301 →