C++ Students score average calculator array problem - c++

Question

I am trying to write a program that calculates students last scores as a midterm, quiz1, quiz2 and final then finds their class average depends on a number of students. Program will show the class average. User should enter the scores. I am new to c++, and my problem is right now i can't find a way to connect this for loop to an array for class average. Is my code wrong for this? I don't know what to do.
#include <iostream>
using namespace std;
int main()
{
int mt, q1, q2, fnl, stdn, num;
double cls[5], std, avg;
cout << "Enter a number of students: ";
cin >> num;
for (stdn=0; stdn<num; stdn++) {
cout<<"Enter mt, q1, q2, fnl of a "<<stdn+1<<". student in order:"<<endl;
cin>>mt>>q1>>q2>>fnl;
std = mt * 30/100 + q1 * 10/100 + q2 * 10/100 + fnl * 50/100;
cout<<stdn+1<<". students total score is "<<std<<endl;
}
}

Answer 1

Type int is always rounding down the value after a decimal point.
So, (int)3.84 == 3, and therefore your std variable will probably have a wrong value.
Define all variables as double for start. To calculate avg simply add marks then devide by the number of students in the end.
double mt, q1, q2, fnl, stdn, num, grades_sum = 0, avg;
...
for(stdn=0; stdn<num; stdn++){
...
grades_sum += mt * 30/100 + q1 * 10/100 + q2 * 10/100 + fnl * 50/100;
...
}
avg = grades_sum/num;

Answer 2

You don't need an array for class average. Just add up all the students scores and divide by the number of students. That just needs one variable (I've called it std_sum) not an array. Like this
double std_sum = 0.0;
for(stdn=0; stdn<num; stdn++){
...
std = mt * 30/100 + q1 * 10/100 + q2 * 10/100 + fnl * 50/100;
std_sum = std_sum + std; // add up all student scores
}
avg = std_total/num;

Answer 3

You can add up all of the students scores, then at the end divide by the total number of students. This will give you a total average of the class. Also, I avoided integer division by replacing your fractions with decimals that add up to 1. I also edited your for loop to start at 1 and go to the number, to avoid adding 1 to everything.
#include <iostream>
using namespace std;
int main()
{
int mt, q1, q2, fnl, stdn, num;
double cls[5], std;
double classAvg = 0; // add a new variable
cout << "Enter a number of students: ";
cin >> num;
for (stdn=1; stdn <= num; stdn++) {
cout << "Enter mt, q1, q2, fnl of a " << stdn << ". student in order:" << endl;
cin >> mt >> q1 >> q2 >> fnl;
std = (mt * 0.3) + (q1 * 0.1) + (q2 * 0.1) + (fnl * 0.5);
cout << stdn << ". students total score is " << std << endl;
classAvg = classAvg + std; // start adding the totals of all the students
}
avg = classAvg/num; // find the total average by dividing by total students
cout << "Class Average is " << classAvg << endl; // display average.
}