Unable to assign the value to structure member using double pointer - c

Question

I am trying to create a program that implements a basic character sheet with an inventory. User
will be able to create a character, view their info, add items, and view items.
There are functions for each functionality.
I created a header file charactersheet.h to define structure and function inside it.
#include <stdio.h>
struct Item {
char *name;
int price;
double weight;
};
struct Inventory {
struct Item **item;
int numitems;
};
struct Character {
char *name;
int Level;
long XP;
struct Inventory inventory;
};
void
createCharacter(struct Character *c)
{
printf("Enter Name:");
c->name = (char *) malloc(40 * sizeof(char));
scanf("%s", c->name);
printf("Enter Level:");
scanf("%d", &c->Level);
printf("Enter XP:");
scanf("%ld", &c->XP);
}
void
viewCharacter(struct Character *b)
{
printf("%s level %d with %ld XP", b->name, b->Level, b->XP);
}
void
addItem(struct Inventory *i)
{
i->numitems = i->numitems + 1;
printf("Enter name:");
scanf("%s", i->item[i->numitems]->name);
printf("Enter price:");
scanf("%d", &(i->item[i->numitems]->price));
printf("Enter Weight:");
scanf("%d", &(i->item[i->numitems]->weight));
}
void
viewItem(struct Inventory i)
{
for (int j = 0; j < i.numitems; j++)
printf("%s,%d golds,%.2lf pounds", i.item[j]->name, i.item[j]->price, i.item[j]->weight);
}
The addItem function takes Inventory * function as input reallocate the memory to accommodate the additional item.
The viewItem function takes Inventory as parameter and list all items.
Now in the file charactersheet.c I have following implementation
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "charsheet.h"
int
main(void)
{
struct Item items;
struct Inventory inventory;
struct Character chars;
inventory.numitems = 0;
while (1) {
char choice;
printf("\nC Create character\n");
printf("V view Character\n");
printf("A Add Item\n");
printf("I view Items\n");
printf("Q Quit\n");
scanf(" %c", &choice);
switch (choice) {
case 'C':
createCharacter(&chars);
break;
case 'V':
viewCharacter(&chars);
break;
case 'A':
addItem(&inventory);
break;
case 'I':
viewItem(inventory);
break;
case 'Q':
exit(0);
default:
printf("Invalid input\n");
}
}
return 0;
}
However I am not able to implement addItem and viewItem functions. There are no errors in the code in my editor as well.

Answer 1

There were a number of issues.
In the Inventory struct, you [proably] want *item instead of **item. The double pointer made things needlessly complicated.
In addItem, you need to do a realloc on item to increase the length of the array before trying to fill in the data.
If we still had the **item, this would have required an additional malloc for each new struct.
In addItem, you were doing a scanf for name, but were not allocating it, so that was UB. Better to do the scanf on a fixed buffer and then use strdup.
In createCharacter, you're doing a fixed malloc of length 40. Better to use a [large] fixed size buffer and then do strdup
In viewItem, you were passing the struct by value. While that's legal, most of the time, you want to pass a pointer to the struct instead.
Here's some refactored code. I used preprocessor conditionals to show your/old vs. my/new code:
#if 0
// old code
#else
// new code
#endif
Anyway, here is the .h file:
#include <stdio.h>
struct Item {
char *name;
int price;
double weight;
};
struct Inventory {
#if 0
struct Item **item;
#else
struct Item *item;
#endif
int numitems;
};
struct Character {
char *name;
int Level;
long XP;
struct Inventory inventory;
};
Here is the .c file. As per my top comments, I moved the function bodies here:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "charsheet.h"
void
createCharacter(struct Character *c)
{
printf("Enter Name:");
#if 0
c->name = (char *) malloc(40 * sizeof(char));
scanf("%s", c->name);
#else
char buf[1000];
scanf("%s",buf);
c->name = strdup(buf);
#endif
printf("Enter Level:");
scanf("%d", &c->Level);
printf("Enter XP:");
scanf("%ld", &c->XP);
}
void
viewCharacter(struct Character *b)
{
printf("%s level %d with %ld XP\n", b->name, b->Level, b->XP);
}
void
addItem(struct Inventory *i)
{
i->numitems = i->numitems + 1;
#if 0
printf("Enter name:");
scanf("%s", i->item[i->numitems]->name);
printf("Enter price:");
scanf("%d", &(i->item[i->numitems]->price));
printf("Enter Weight:");
scanf("%d", &(i->item[i->numitems]->weight));
#else
struct Item *t;
t = realloc(i->item,sizeof(*t) * i->numitems);
if (t == NULL) {
perror("item");
exit(1);
}
i->item = t;
t += i->numitems - 1;
char buf[1000];
printf("Enter name:");
scanf("%s", buf);
t->name = strdup(buf);
printf("Enter price:");
scanf("%d", &t->price);
printf("Enter Weight:");
scanf("%d", &t->weight);
#endif
}
#if 0
void
viewItem(struct Inventory i)
#else
void
viewItem(struct Inventory *i)
#endif
{
for (int j = 0; j < i->numitems; j++) {
struct Item *t = &i->item[j];
printf("%s,%d golds,%.2lf pounds\n",
t->name, t->price, t->weight);
}
}
int
main(void)
{
struct Item items;
struct Inventory inventory;
struct Character chars;
inventory.numitems = 0;
#if 1
inventory.item = NULL;
#endif
#if 1
setbuf(stdout,NULL);
#endif
while (1) {
char choice;
printf("\nC Create character\n");
printf("V view Character\n");
printf("A Add Item\n");
printf("I view Items\n");
printf("Q Quit\n");
scanf(" %c", &choice);
switch (choice) {
case 'C':
createCharacter(&chars);
break;
case 'V':
viewCharacter(&chars);
break;
case 'A':
addItem(&inventory);
break;
case 'I':
#if 0
viewItem(inventory);
#else
viewItem(&inventory);
#endif
break;
case 'Q':
exit(0);
break;
default:
printf("Invalid input\n");
break;
}
}
return 0;
}
UPDATE:
what if i need **item?
Well, I think you should have some justification for the extra complexity. But, if it's a requirement, it is possible.
Also, the scanf for weight was using the wrong format. If you had enabled warnings by compiling with -Wall [which you should always do], the compiler would have flagged this for you.
Here's the code with code for double pointers. Note that I've merged the .h into the .c here for brevity:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#ifndef DOUBLE_POINTER
#define DOUBLE_POINTER 0
#endif
struct Item {
char *name;
int price;
double weight;
};
struct Inventory {
#if DOUBLE_POINTER
struct Item **item;
#else
struct Item *item;
#endif
int numitems;
};
struct Character {
char *name;
int Level;
long XP;
struct Inventory inventory;
};
void
createCharacter(struct Character *c)
{
printf("Enter Name:");
#if 0
c->name = (char *) malloc(40 * sizeof(char));
scanf("%s", c->name);
#else
char buf[1000];
scanf("%s",buf);
c->name = strdup(buf);
#endif
printf("Enter Level:");
scanf("%d", &c->Level);
printf("Enter XP:");
scanf("%ld", &c->XP);
}
void
viewCharacter(struct Character *b)
{
printf("%s level %d with %ld XP\n", b->name, b->Level, b->XP);
}
void
addItem(struct Inventory *i)
{
i->numitems = i->numitems + 1;
#if 0
printf("Enter name:");
scanf("%s", i->item[i->numitems]->name);
printf("Enter price:");
scanf("%d", &(i->item[i->numitems]->price));
printf("Enter Weight:");
scanf("%d", &(i->item[i->numitems]->weight));
#else
struct Item *t;
i->item = realloc(i->item,sizeof(*i->item) * i->numitems);
if (i->item == NULL) {
perror("item");
exit(1);
}
#if DOUBLE_POINTER
t = malloc(sizeof(*t));
i->item[i->numitems - 1] = t;
#else
t = &i->item[i->numitems - 1];
#endif
char buf[1000];
printf("Enter name:");
scanf("%s", buf);
t->name = strdup(buf);
printf("Enter price:");
scanf("%d", &t->price);
printf("Enter Weight:");
#if 0
scanf("%d", &t->weight);
#else
scanf("%lf", &t->weight);
#endif
#endif
}
#if 0
void
viewItem(struct Inventory i)
#else
void
viewItem(struct Inventory *i)
#endif
{
for (int j = 0; j < i->numitems; j++) {
#if DOUBLE_POINTER
struct Item *t = i->item[j];
#else
struct Item *t = &i->item[j];
#endif
printf("%s,%d golds,%.2lf pounds\n",
t->name, t->price, t->weight);
}
}
int
main(void)
{
#if 0
struct Item items;
#endif
struct Inventory inventory;
struct Character chars;
inventory.numitems = 0;
#if 1
inventory.item = NULL;
#endif
#if 1
setbuf(stdout,NULL);
#endif
while (1) {
char choice;
printf("\nC Create character\n");
printf("V view Character\n");
printf("A Add Item\n");
printf("I view Items\n");
printf("Q Quit\n");
scanf(" %c", &choice);
switch (choice) {
case 'C':
createCharacter(&chars);
break;
case 'V':
viewCharacter(&chars);
break;
case 'A':
addItem(&inventory);
break;
case 'I':
#if 0
viewItem(inventory);
#else
viewItem(&inventory);
#endif
break;
case 'Q':
exit(0);
break;
default:
printf("Invalid input\n");
break;
}
}
return 0;
}

Answer 2

The addItem function takes Inventory * function as input reallocate the memory to accommodate the additional item
Well, it could but it does not
void addItem(struct Inventory* i){
i->numitems =i->numitems + 1;
printf("Enter name:");
scanf("%s",i->item[i->numitems]->name);
printf("Enter price:");
scanf("%d",&(i->item[i->numitems]->price));
printf("Enter Weight:");
scanf("%d",&(i->item[i->numitems]->weight));
}
Inventory is a struct, an array of numitems pointers to item
You need to build it carefully. They would not come to life just by adding 1 to a counter.
Some things you may consider
As pointed already in the comments, INDENT your code
Use typedef to name your structs
Use a convention to struct naming, like first letter in uppercase only for structs
When you declare Item **item; you are declaring item, so write Item** item instead. This way makes clear what is what and no one needs to search for asterisks around names in declarations. item is Item**, *item is Item* and **item points to the first Item.
do not return void. In general is a waste of something, maybe an error
use menu() as a function to return the user option
do not use scanf(): it has another purpose: scan formatted input. But when you use always test the return. scanf() return an int. Read the manual.
do not put code or declare variables in a header: use another file with the same name as your header and put your code there
leave main() in another file so you can have many test programs to test your set of functions...
do not mix logic and data entry in the same function: it creates a bad dependency and limits your testing capabilities.
also see things like this
void addItem(Inventory*);
addItem() adds an item to an inventory. Write
int addItem( item Item*, Inventory* inv);
And maybe return 0 for ok or an error code, but pass pointers to Item and Inventory. It is clearer and more manageable...
maybe you need another Inventory, a dynamic array of Character
A possible header
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
typedef struct
{
char* name;
int price;
double weight;
} Item;
typedef struct
{
Item** item;
int size;
} Inventory;
typedef struct
{
Inventory inventory;
int level;
char* name;
long xp;
} Character;
typedef struct
{
Character** item;
int limit;
int size;
} Cast;
int addItem(Item*, Inventory*);
Cast* buildCast(unsigned);
Character* createCharacter(Character*);
int viewCharacter(Character*);
int viewItem(Inventory*);
Using typedef to the structs, a convention that first letter in uppercase means a struct name, and a Cast struct to hold the Character set.
I am adding a buildCast() that shows the usual way to build this sort of thing, the same thing the system does for every program building argc and argv as
int main( int argc, char** argv)
typedef struct
{
Character** item;
int limit;
int size;
} Cast;
You can write
Cast* buildCast(unsigned count)
{
// builds an array of 'count' pointers
// to 'Character'
if( count == 0 ) return NULL;
Cast* one = (Cast*) malloc(sizeof(Cast));
one->limit = count;
one->size= 0; // empty
one->item = (Character**) malloc( count * sizeof(Character*));
for( int i =0; i<count; i+= 1)
one->item[i] = NULL;
return one;
}
And call as in
Cast* cast = buildCast(300);
To have an array of 300-max pointers to Character initially empty
This is just an example using your code and has errors as I did not correct the issues on memory allocation, for example.
You can look at buildCast() and see how to build your inventory struct.
I will left a set of files here that you can compile and run and move on if you decided to go this way. It is just your code refactored.
1 of 3 soch.h header:
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
typedef struct
{
char* name;
int price;
double weight;
} Item;
typedef struct
{
Item** item;
int size;
} Inventory;
typedef struct
{
Inventory inventory;
int level;
char* name;
long xp;
} Character;
typedef struct
{
Character** item;
int limit;
int size;
} Cast;
int addItem(Item*, Inventory*);
Cast* buildCast(unsigned);
Character* createCharacter(Character*);
char menu();
int viewCharacter(Character*);
int viewItem(Inventory*);
2 of 3 sohc.c functions
#include"soch.h"
int addItem(Item* it, Inventory* inv)
{
printf("Enter name:");
scanf("%s",it->name);
printf("Enter price:");
scanf("%d",&(it->price));
printf("Enter Weight:");
scanf("%lf",&(it->weight));
inv->item[inv->size] = it;
inv->size =inv->size + 1;
return 0;
}
Cast* buildCast(unsigned count)
{
// builds an array of 'count' pointers
// to 'Character'
if( count == 0 ) return NULL;
Cast* one = (Cast*) malloc(sizeof(Cast));
one->limit = count;
one->size= 0; // empty
one->item = (Character**) malloc( count * sizeof(Character*));
for( int i =0; i<count; i+= 1)
one->item[i] = NULL;
return one;
}
Character* createCharacter(Character* c)
{
Character* one = NULL;
printf("Enter Name:");
c->name = (char*) malloc(40*sizeof(char));
scanf("%s",c->name);
printf("Enter Level:");
scanf("%d",&c->level);
printf("Enter XP:");
scanf("%ld",&c->xp);
return one;
}
char menu()
{
char choice[30];
printf("\nC Create character\n");
printf("V view Character\n");
printf("A Add Item\n");
printf("I view Items\n");
printf("Q Quit\n");
fgets( choice, sizeof(choice), stdin);
return choice[0];
};
int viewCharacter(Character* b)
{
printf("%s level %d with %ld XP",
b->name,
b->level,
b->xp);
return 0;
}
int viewItem(Inventory* i)
{
for(int j=0; j<i->size; j++)
{
printf(
"%s,%d golds,%.2lf pounds\n",
i->item[j]->name,
i->item[j]->price,
i->item[j]->weight);
};
return 0;
}
main function to test buildCast() and menu()
#include "soch.h"
int main(void)
{
Inventory inventory;
Item item;
Character chars;
Cast* cast = buildCast(300);
while(1)
{
switch(menu())
{
case 'C':
createCharacter(&chars);
break;
case 'V':
viewCharacter(&chars);
break;
case 'A':
addItem(&item, &inventory);
break;
case'I':
viewItem(&inventory);
break;
case 'Q':
exit(0);
default:
printf("Invalid input\n");
break;
}; // switch()
}; // while()
return 0;
};
In practice you would build structs like Inventory in blocks of N pointers, for efficiency, and if needed call realloc() when more pointers are needed. But for a simple program you can write a fixed array of pointers or malloc() one by one and see how it goes.
Write back if you choose to go this way. Take a look at the provided code