Simple grading program returning the wrong answer - c++

Question

Trying to write a simple program that takes six test grades, input by the user, drops the lowest, and then displays average of the remaining five. The program compiles fine, but outputs the wrong answer. Any pointers to where the logic error is occurring would be helpful. I am using these values as sample grades:
85.2
92.3
78.0
51.5
91.6
87.0
and the value returned should be 86.82, but the program is instead returning 80.08.
#include <iostream>
using namespace std;
const int SIZE = 6;
double grades[SIZE];
double low(double[]);
int main()
{
double preavg = 0;
double avg = 0;
cout << "This program will take an input of six test grades\n"
<< "and determine the average of the highest five grades.\n\n"
<< "Please input the first test grade\n";
cin >> grades[0];
cout << "Please input the second test grade\n";
cin >> grades[1];
cout << "Please input the third test grade\n";
cin >> grades[2];
cout << "Please input the fourth test grade\n";
cin >> grades[3];
cout << "Please input the fifth test grade\n";
cin >> grades[4];
cout << "Please input the sixth test grade\n";
cin >> grades[5];
for (int i = 0; i < SIZE; i++)
{
preavg += grades[i];
}
avg = (preavg - low(grades)) / (SIZE -1);
cout << "Your average grade is " << avg << "\n";
system("PAUSE");
return 0;
}
double low(double grades[SIZE])
{
double lowest;
lowest = 100;
for (int i = 0; i < SIZE; i++)
{
if (grades[i] < lowest)
lowest = grades[i];
}
return lowest;
}

Answer 1

Your issue here is possibly coming from the fact you're doing division in double precision using a const int. If you cast SIZE as a double and write the rest of your double numbers with a .0 on the end, it works fine:
#include <iostream>
using namespace std;
const int SIZE = 6;
double grades[SIZE];
double low(double[]);
int main()
{
double preavg = 0.0;
double avg = 0.0;
cout << "This program will take an input of six test grades\n"
<< "and determine the average of the highest five grades.\n\n"
<< "Please input the first test grade\n";
cin >> grades[0];
cout << "Please input the second test grade\n";
cin >> grades[1];
cout << "Please input the third test grade\n";
cin >> grades[2];
cout << "Please input the fourth test grade\n";
cin >> grades[3];
cout << "Please input the fifth test grade\n";
cin >> grades[4];
cout << "Please input the sixth test grade\n";
cin >> grades[5];
for (int i = 0; i < SIZE; i++)
{
preavg += grades[i];
}
avg = (preavg - low(grades)) / ((double)SIZE - 1.0);
cout << "Your average grade is " << avg << "\n";
system("PAUSE");
return 0;
}
double low(double grades[SIZE])
{
double lowest;
lowest = 100.0;
for (int i = 0; i < SIZE; i++)
{
if (grades[i] < lowest)
lowest = grades[i];
}
return lowest;
}