为什么我的weighted_grade变量不起作用？

Question

I need to write a C++ program what can be used to determine grades at the end of the semester. For each student, who is identified by an integer number between 1 and 60, four examination grades must be kept. Additionally, two final grade averages must be computed. The first grade average is simply the average of all four grades. The second grade average is computed by weighting the four grades as follows: the first grade gets a weight of 0.2, the second grade gets a weight of 0.3, the third grade a weight of 0.3, and the fourth grade a weight of 0.2; that is computed as:

0.2 * grade1 + 0.3 * grade2+ 0.3* grade3 + 0.2 * grade4

Using this information, you are to construct a 60 X 7 two dimensional array, in which the first column is used for the student number, the next four columns for the grades, and the last two columns for the computed final grades. The output of the program should be a display of the data in the completed array.

Here's what I have, but my second average is off. For example, my 1st student should have both averages of 100. However Avg2 is coming out to equal 1000 and I have no idea why. Help?

#include <iostream>
#include <math.h>
using namespace std;

int grade_Calc(int sg[][5]);

int main()
{
     int student_grades[5][5] =
{
    {1, 100, 100, 100, 100}, //inputs an array of students 1-5's grades
    {2, 100, 0, 100, 0},
    {3, 82, 94, 73, 86},
    {4, 64, 74, 84, 94},
    {5, 94, 84, 74, 64},
};

grade_Calc(student_grades);
return 0;
}


int grade_Calc(int sg[][5])
{
int sum_for_avg = 0;
double weighted_grade = 0.0;
double simple_avg = 0.0;
double sum2 = 0.0;

cout << "Stdnt" << "\t" << "Grd1" << "\t" << "Grd2" << "\t" << "Grd3" << "\t" << "Grd4" << "\t" << "Avg1" << "\t" << "Avg2" << endl;
for (int r = 0; r < 5; r++)
{
    for (int c = 0; c < 5; c++)
    {
        cout << sg[r][c] << "\t\t";

        if (c != 0)
        {
            sum_for_avg += sg[r][c];
            if ((c == 1) || (c == 4))
            {
                weighted_grade += (0.3 * sg[r][c]);
            }
            else
            {
                weighted_grade += (0.2 * sg[r][c]);
            }
        }
    }

    simple_avg = (sum_for_avg / 4.0);
    cout << "\t" << simple_avg << "\t" << weighted_grade;
    for (int k = 0; k < 5; k++)
    {
        if (k != 0)
        {
            sum2 += pow((sg[r][k] - simple_avg), 2);
        }
    }

    cout << sqrt(sum2 / 4.0);
    sum_for_avg = 0;
    weighted_grade = 0.0;
    cout << endl;
}
return 0;
}

Answer 1

You have to reset the value of sum_for_avg for every student, at every line of your table, that is, at the beginning of your for loop over r . As it is, it starts correctly at 0 for the first student, but then for each following student it starts from the value you already have.

EDIT: turns out I was too hasty. sum_for_avg is reset after all - just not where I would do it, that is, it's reset at the end of the loop. So the average is right. What is happening is that you are also printing another value, which is the standard deviation, and since there are no spaces it looks like it's 1000, whereas it is the concatenation of 100 and 0. Then again, the standard deviation is also wrong, for the reason that I gave at the beginning: you are not resetting it. So add this line:

sum2 = 0;

just before your loop that calculates the standard deviation. And print at least a space between your weighted average and your standard deviation.

Answer 2

Actually Avg2 is coming out 100. But then you run this bit of code for reasons not outlined in your question:

for (int k = 0; k < 5; k++)
{
    if (k != 0)
    {
        sum2 += pow((sg[r][k] - simple_avg), 2);
    }
}

cout << sqrt(sum2 / 4.0);

The cout at the end is writing a 0 right after Avg2's 100 so it looks like it's printing 1000.

So, you print out:

Stdnt   Grd1    Grd2    Grd3    Grd4    Avg1    Avg2
1       100     100     100     100     100     100

and then along comes cout << sqrt(sum2 / 4.0); to print out another 0 and you get

Stdnt   Grd1    Grd2    Grd3    Grd4    Avg1    Avg2
1       100     100     100     100     100     1000

Clean that bit of code up and you'll find the bug spotted by @Fabio Turati.