// Source: "Software Design ...", John A Robinson, Newnes, 2004, page 106.
#include	<iostream>
#include 	<fstream>
#include 	<cstdlib>
using namespace std;
#define 	MAXSTUDENTS	80
#define		MAXNAMELENGTH	30

struct student
	{
	char name[MAXNAMELENGTH];
	long id;
	float mark;
	};

float average_mark(int num_students, student *p);
int	load_data(char *fname, student **pp);

int main(int argc, char **argv)
	{
	student *syde321;	// Will point to the base of the array of structures
	int num_students;
	if (argc != 2)
		{
		cerr << "Usage: average filename\n";
		return(-1);
		}
	if ((num_students =	load_data(*(argv+1),&syde321)) < 0)
		return(-1);

	cout << "Average was " << average_mark(num_students, syde321) << "\n";
	return(0);
	}

float average_mark(int num_students, student *p)
	{
	int i;
	float sum_marks;
	sum_marks = 0.0;
	for (i = 0; i < num_students; i++)
		sum_marks += p[i].mark;	// Remember, a pointer passed to a function can be treated
					// like the start address of an array.
	return(sum_marks/num_students);
	}

int load_data(char *fname, student **pp)
	{
	char c;
	student *memory;
	unsigned int numstudents;
	ifstream in(fname);
	if (!in)
		{
		cerr << "Can't open " << fname << " for reading\n";
		return(-1);
		}
	in >> numstudents;	
	if (numstudents > MAXSTUDENTS)
		{
		cerr << "Too many database entries\n";
		return(-1);
		}
	memory = new student[numstudents];	// Memory is allocated here
	// Don't catch bad_alloc exception from new: Just exit if memory exhausted.
	// Write into student pointer
	*pp = memory;
	for(int i = 0; i < numstudents; i++)
		{
		in >> memory[i].name;
		in >> memory[i].id;
		in >> memory[i].mark;
		}
	in.close();
	memory[numstudents].id = 0;
	// Sentinel
	return(numstudents);
	}