Make a new struct variable when users needs it - c

Question

I'm just getting started with C, and having issues with struct. For instance I have:
struct student {
int id;
char name[25]
};
I want the user to add as many students as he needs:
int count = 0;
while (stop == 0){
struct student count
scanf("%d", count.id);
scanf("%s", count.name);
scanf("%d", stop);
}
It looks like I've to create struct student count (where count is a number) and keep creating these. So, I would like to create something like struct student 0, then struct student 1 and so on, so I can reference each student by it's count or number.
How would I get something like this to work?

Answer 1

This will automatically allocate memory when user requests it. It starts from a dimension of 1, up to virtually infinite (actually, up to the space available in RAM).
Of course if you want, you can change the initial size of size, as well as the growth rate of the array.
// Example program
#include <stdio.h> /* printf, scanf */
#include <stdlib.h> /* for realloc, malloc */
// Supposing student is defined like this:
struct student
{
int id;
char name[25];
};
int main()
{
int stop = 0;
int count = 0;
int size = 0;
// Using an array of students
size = 1;
struct student* students = malloc(size * sizeof(struct student));
while (stop == 0)
{
if(count >= size)
{
size ++;
students = realloc (students, size * sizeof(struct student));
if (students == NULL)
{
printf ("Failed to allocate more memory.\n");
return 0;
}
}
scanf("%d", &(students[count].id));
scanf(" %24[0-9a-zA-Z ]", &(students[count].name));
scanf("%d", &stop);
count = count + 1;
}
int i = 0;
for (i = 0; i < count; ++i)
printf("%d => %d %s\n", i, students[i].id, students[i].name);
}

Answer 2

I think you would like to create multiple instances of the struct in your first code sample for each user that is entered in the console, handled by the while-loop.
The easiest way to achieve this, is to use an array. I suggest you to first use an array with a fixed size, that means that you specify the array size in your code. This array will allow you to add as many student instances into it as the array size you've specified.
A simple example would be something like this:
// Define the student struct
struct student {
int id;
char name[25];
};
// ...
// Create an array with a fixed size to put the students in, and define a counter
struct student students[128];
int count = 0;
while(stop == 0){
// Create the struct to fill
struct student newStudent;
// Fill the struct with the user supplied data
scanf("%d", newStudent.id);
scanf("%s", newStudent.name);
scanf("%d", stop);
// Add the struct to the array, and increase the count afterwards
students[count++] = newStudent;
}
In the above example, I've added an array with a fixed size of 128, which you can change to whatever size you'd like. In the while-loop, an instance of a new struct is made, which is similar to before. This struct is being filled afterwards with data fed from the console. At the end of the while-loop the struct instance is added to the array of students. This will give you an array of all the students you've entered.
There is a downside to this solution however, and that's that in most cases, much more memory is consumed than is really used. This is because for the computer, it feels like 128 whole instances (or any other array size if specified) are stored in RAM, this can be quite expensive if only two instances will be really used. Also, like I said earlier, make sure to keep in mind that fixing the array size limits the amount of entries, this can also have a negative effect on your code. If you don't want to have these consequences, you may want to take a look at the solution described bellow.
You can make the size of the array dynamic, this is a little more advanced. If you'd like to achieve something like this, make sure to take a look at memory allocation functions, like Sourav Ghosh pointed out in a comment. You may also want to take a look at the code-example Michael made.
I hope this helps to solve the problem you're having. Happy coding!