Computing an average on a project

closed account (9hv4jE8b)
I earlier made a 2-d array called grades[][] and now I'm trying to find out the average on a specific column of the array (i.e. project 1 that has Num_of_Students grades in it). 

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float compute_ave(int grades[][8], int Num_of_Students)
{ 
    int sum=0;
    int j; //project number
    float average;
    
    cout << "Give the number of the project: ";
    cin >> j;
       for(int i=0;i<Num_of_Students;i++){
           sum+=grades[i][j];                                             
            }
       average=sum/Num_of_Students; 
       return average;
}


But this outputs some crazy 1.39e+008 and I dont know why..
TechnoCat (39)
That's just the format. If you want the whole thing, you have to write that into your code. I'm not sure, but I think adding cout.setf (ios :: fixed, ios :: floatfield) ; cout.precision (9) ; before you output average should work.
hanst99 (2869)
In case you didn't know,
1.39e+008

means: 1.39*10^(008)

If you got such a large number, you probably forgot to initialize one or more values inside of the array.
Last edited on
closed account (9hv4jE8b)
hanst, I build the array from a nested for loop. heres my whole (tentative) code. Does it look like part of my array is messed up which is causing this? (collect_data is the array builder func)

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#include<iostream>
#include<cmath>

using namespace std;

void collect_data (int grades[][8], int Num_of_Students); //Gets data to store in array
float compute_ave(int grades[][8], int Num_of_Students); //Prototype for computing the average of a specific exam
float class_average(int grades[][8], int Num_of_Students);


int main()
{
     
     cout << "Student Exam Database\nThis program stores the exam scores\n";
     cout << "of a number of students and produces\nthe average and the letter grades." << endl << endl;

     int choice; //for picking option
     int flag=0; //because you must input data first
     int Num_of_Students; //Number of students in class
     int grade;
     int grades[][8]={}; //the empty array
              
     do{ 
         cout << "Please choose an option." << endl;
         cout << "1. To store students exams." << endl;
         cout << "2. To compute class average on a specific exam." << endl;
         cout << "3. To compute overall class average in the course." << endl;
         cout << "4. To see the letter grade of a specific student." << endl;
         cout << "Please choose: ";
         cin >> choice;
             if (choice==1 && flag==0){
                cout << "Please give number of students: " <<endl;
                cin >> Num_of_Students;
                collect_data(grades, Num_of_Students);
                     
                flag=1;}
                
             else if(choice==2 && flag==0){
                  cout << "No scores stored yet." <<endl;
                  }
             else if(choice==2 && flag==1){
                  float average;
                  average=compute_ave(grades, Num_of_Students);
                  cout << average;
                  }
             else if(choice==3 && flag==0){
                  cout << "No scores stored yet." <<endl;
                  }
             else if(choice==3 && flag==1){
                  float totalaverage;
                  
                  totalaverage=class_average(grades, Num_of_Students);
                  cout << totalaverage << endl;
                  }
         
                              
          }while(choice!=0);


system("pause");

}


void collect_data (int grades[][8], int Num_of_Students) //a nested for loop that builds a 2-D array
{   int j; //counter for which project it is
    int i; //counter for which student

    
    for(j=0;j<8;j++){
        for(i=0;i<Num_of_Students;i++){
            cout << "Please give grade of student " << i+1 <<  " in project " <<j+1 << ": ";
            cin >> grades[i][j];
            
            }
    } 

}  

float compute_ave(int grades[][8], int Num_of_Students)
{ 
    int sum=0;
    int project_number;
    int j; //project number adjusted for array
    float average;
    
    cout << "Give the number of the project: ";
    cin >> project_number;
        j=project_number-1; //i.e. project "1" is grades[i][0]
       for(int i=0;i<Num_of_Students;i++){
           sum+=grades[i][j];                                             
            }
       average=sum/Num_of_Students; 
       return average;
}
buffbill (467)
I`m not absolutely sure but:
L21 I don`t think the array is initialized to 0 for each element when [ ] is empty. If this was a single dimensional array like grades[8]={} all elements would be initialized to 0.
...and why use a 2D array in any case?
L72 why are you incrementing i and j in the body of the i for loop?
closed account (9hv4jE8b)
Ok I'll see what I can do about initializing a 2-d array to zero...
I'm using a 2-d array because my professor is making us.

As for L72, that's just a cout syntax bc i=0 is student 1 and j=0 is project one. So to get it to read out correctly i had to put i+1, j+1.
My for loop acts in such a way that grades[0][0], grades[1][0], grades[2][0].....etc til it reaches num of students, then goes to the second project.
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