Non-NULL User input - c

Question

I'm trying to understand the whole concept of pointers, structures, etc so I've created a program that gets user input for two different books and then swaps the info of the two books. I had no trouble doing that, however, a problem arose- when I pressed enter the name of the book would be plain blank and at the output I would, of course, see a blank space. My problem is, how am I able to limit the user to input any letter (A-Z, a-z) and not blank space?
A string of characters when input to an array, they get saved in consecutive memory addresses. We also know that 'NULL' is represented as '\0' in arrays.
With the above things in mind, I performed multiple tests in which, ALL of them failed to yield the desired results.
Below are some attempts that I made.
1st Attempt
while (pBook1->name[0] == '\0')
{
printf("\n Please enter a valid book name: ");
fgets(pBook1->name, MAX, stdin);
}
2nd Attempt
while (strcmp(pBook1->name, ""))
{
printf("\n Please enter a valid book name: ");
fgets(pBook1->name, MAX, stdin);
}
Also, consider the following code as the source code of my program:
#include <stdio.h>
#include <string.h>
#define MAX 50
struct Books
{
char name[MAX];
int ID;
float price;
};
void swap(struct Books *, struct Books *);
void main()
{
struct Books Book1, Book2, *pBook1, *pBook2;
pBook1 = &Book1;
pBook2 = &Book2;
// Input for the 1st book
printf("\n 1st Book \n ------------------------------");
printf("\n Enter the name: ");
fgets(pBook1->name, MAX, stdin);
while (pBook1->name[0] == '\0')
{
printf("\n Please enter a valid book name: ");
fgets(pBook1->name, MAX, stdin);
}
printf("\n Enter the ID: ");
scanf("%d", &pBook1->ID);
printf("\n Enter the price: ");
scanf("%f", &pBook1->price);
// Input for the 2nd book
printf("\n 2nd Book \n ------------------------------");
printf("\n Enter the name: ");
fgets(pBook2->name, MAX, stdin);
while (pBook2->name[0] == '\0')
{
printf("\n Please enter a valid book name: ");
fgets(pBook2->name, MAX, stdin);
}
printf("\n Enter the ID: ");
scanf("%d", &pBook2->ID);
printf("\n Enter the price: ");
scanf("%f", &pBook2->price);
printf("\n Let's swap the info of the two books...");
swap(pBook1, pBook2);
printf("\n The info of the two books is now:");
printf("\n------------------------------ \n 1st Book \n ------------------------------------");
printf("\n Name \t\t ID \t Price \n %s \t\t %d \t %f", pBook1->name, pBook1->ID, pBook1->price);
printf("\n------------------------------ \n 2nd Book \n ------------------------------------");
printf("Name \t\t ID \t Price \n %s \t\t %d \t %f", pBook2->name, pBook2->ID, pBook2->price);
}
void swap(struct Books *pB1, struct Books *pB2)
{
char temp[MAX];
strcpy(temp, pB1->name);
strcpy(pB1->name, pB2->name);
strcpy(pB2->name, temp);
int tempID = pB1->ID, tempPrice = pB1->price;
pB1->ID = pB2->ID;
pB2->ID = tempID;
pB1->price = pB2->price;
pB2->price = tempPrice;
}

Answer 1

fgets reads until it encounters EOF, \n or N-1 bytes have been read. So if a user of your program presses enter, it will read \n and stop. Which means that pBook1->name[0] == '\n'. That is why your check for equality with "" fails and why pBook1->name[0] == '\0' fails.
See this example.
That means that you need to check for \n and \0 in case the user entered Ctrl-D which is how you enter EOF on *nix systems.

Answer 2

When you press enter pBook1->name[0] will become \n. You can use some functions as strlen to be sure there something in name.