Nested structures and dynamic memory allocation - c

Question

I am currently learning C and trying to create an app that allows students to input their name, address, student id and any subject id they have while at university (yes this is a task part of foundations of tech course). Basically the following code tries to create an array of students (that is unlimited in size) by using dynamic allocating memory. Further a student can have unlimited amounts of subjects (that is nested within the student subject and is dynamically allocated)
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
int main()
{
typedef struct {
char subjectID[10];
char subjectName[10];
float grades;
}subject;
typedef struct {
char name[10];
int id;
struct subject *enrol;
char address[100];
int status;
int totalSubject;
}student; // This structure is to act like an unlimited list of students
int option=0, q=0, j=0, k=0, p=0, delete=0, position=0;
int count=0; // keeps the count of numbers added
student * studentAddress;
studentAddress = NULL; //Init structure
studentAddress->enrol = NULL; // Init nested structure
int flag = 0; //variable to indicate whether the number to delete is found or not
while (1) // Life cycle of the app
{
printf("1 - Add new student\n");
printf("2 - Display all students\n");
printf("3 - Delete student by id \n");
printf("4 - Delete student by name \n");
printf("5 - Quit\n");
scanf("%d",&option);
switch (option)
{
case 1: studentAddress = realloc(studentAddress, sizeof(student)*(count+1)); //using realloc memory is requested for the new number
printf("Enter your name\n");
fflush(stdin);
gets(studentAddress[count].name);
fflush(stdin);
printf("Enter your id\n");
scanf("%d", &studentAddress[count].id);
printf("Enter your address\n");
fflush(stdin);
gets(studentAddress[count].address);
fflush(stdin);
int subjectCounter = 0;
subjectCounter = 0;
char subjectContinue[10];
for (p = 0; p < subjectCounter; p++)
{
studentAddress[count].enrol = (struct subject *)realloc(studentAddress[count].enrol, (sizeof(subject)*(subjectCounter+1)));
// studentAddress[count].enrol[subjectCounter] = (struct subject *)realloc(sizeof(subject)*(subjectCounter+1));
// Above is teacher paste
printf("What is the id of your subject %d\n", (subjectCounter+1));
fflush(stdin);
gets(studentAddress[count].enrol[subjectCounter].subjectID);
fflush(stdin);
printf("Would you like to add another subject id\n");
fflush(stdin);
gets(subjectContinue);
fflush(stdin);
if (strcmp(subjectContinue, "Y") == 0)
{
subjectCounter++;
}
studentAddress[count].totalSubject = subjectCounter;
}
count++;
printf("\n");
break;
case 2:
for (j=0; j<count; j++)
{
if(studentAddress[j].status != 0)
printf("%s\n %d\n %s\n", studentAddress[j].name, studentAddress[j].id, studentAddress[j].address);
for (q = 0; q < studentAddress[count].totalSubject; q++)
{
printf("%s", studentAddress[j].enrol[q].subjectID);
}
}
printf("\n");
break;
case 3: k=count;
flag=0;
printf("Type the id of the person record you want to delete\n");
scanf("%d", &delete);
for (j=0; j < count; j++)
{
if (delete == studentAddress[j].id)
{
strcpy(studentAddress[j].name, " ");
strcpy(studentAddress[j].address, " ");
studentAddress[j].id = 0;
studentAddress[j].status = 0;
flag = 1;
}
}
if(flag==0)
printf("id not in the list ...\n");
else
printf("Deletion successful ...\n");
printf("\n");
break;
case 4:
flag=0;
char deletChar[100];
printf("Type the name of the person record you want to delete\n");
fflush(stdin);
gets(deletChar);
fflush(stdin);
for (j=0; j < count; j++)
{
if (strcmp(deletChar, studentAddress[j].name)==0)
{
flag = 1;
strcpy(studentAddress[j].name, " ");
strcpy(studentAddress[j].address, " ");
studentAddress[j].id = 0;
studentAddress[j].status = 0;
flag = 1;
}
}
if(flag==0)
printf("name not in the list ...\n");
else
printf("Deletion successful ...\n");
printf("\n");
break;
return 0;
case 5:
return 0;
}
}
return 0;
}
I try running this code and get the error on line 68 and similar on line 95
subscript of pointer to incomplete type 'struct subject'
gets(studentAddress[count].enrol[subjectCounter].subjectID);
plus I get this note (from the compiler) below the error
forward declaration of 'struct subject' struct subject *enrol;
My tutor and lecture have both tried helping me, but they have had no success. They are like most of my code looks correct, but it will not work. Sorry I am new to stack overflow so if you want me to add more information please let me know.

Answer 1

The easiest way to solve this is to create a function.
struct student* allocate_student() {
struct subject* new_subject = allocate_subject();
struct student* new_struct = malloc(sizeof(struct student));
// initialize all fields, which will eventually include
new_struct->enrol = new_subject;
return new_struct;
}
This code fails to check for any error conditions. You should add that. In addition, allocate_subject() is also not defined. Considering the offered example, odds are good you can figure out how to write allocate_subject().
If you add in parameters, allocate_student() might be a bad method name. With enough parameters to construct any student, you might want to call it construct_student(...)