2D Array. Calculating the average of the student - c++

Question

I can't figure out why I can't calculate the average. The output of the average is same as the other look the details in the picture below. My teacher told me to return the AVG to 0, but I don't know where to put it.
#include <iostream>
using namespace std;
const int ROW = 3;
const int COL = 3;
const int D = 3;
void display(int nums[ROW][COL]);
void show(int ID[D]);
int main()
{
int nums[ROW][COL];
int ID[D];
double AVG = 0;
for (int i = 1; i <= ROW; i++) {
cout << "Enter student ID[" << i << "]: ";
cin >> ID[i];
for (int j = 0; j < COL; j++) {
cout << "Enter student[" << i << "] Grades [" << j + 1 << "]:";
cin >> nums[i][j];
AVG = AVG + nums[i][j];
}
AVG = AVG / 3;
}
for (int i = 1; i <= ROW; i++) {
cout << "Student ID: " << ID[i] << endl;
for (int j = 0; j < COL; j++) {
cout << "Score: " << nums[i][j] << endl;
}
cout << "Average: " << AVG << endl;
}
return 0;
}

Answer 1

You Have some problem regarding this issues:
you take AVG as double but other variable as integer.so there is a datatype error.
2.you have problem doing array indexing.
Here Is the Solution. Here I declare AVG as an integer type array so that it remains same as your code.
#include
using namespace std;
const int ROW = 3;
const int COL = 3;
const int D = 3;
void display(int nums[ROW][COL]);
void show(int ID[D]);
int main ()
{
int nums[ROW][COL];
int ID[D];
int AVG[ROW];
for(int i=1; i<=ROW; i++)
{
cout << "Enter student ID[" << i <<"]: ";
cin>>ID[i];
AVG[i-1]=0;
for(int j=0; j<COL; j++)
{
cout<<"Enter student[" << i << "] Grades [" <<j+1<<"]:" ;
cin>>nums[i-1][j];
AVG[i-1] = AVG[i-1] + nums[i-1][j];
}
AVG[i-1] = AVG[i-1] / 3;
}
for(int i=1; i<=ROW; i++)
{
cout <<"Student ID: " << ID[i] << endl;
for(int j=0; j<COL; j++)
{
cout<<"Score: "<<nums[i-1][j]<<endl;
}
cout << "Average: " << AVG[i-1]<< endl;
}
return 0;
}

Answer 2

Currently you are calculating the average as follows;
First iteration:
(90 + 90 + 90) / 3 = 90
Second iteration:
(90 + 80 + 80 + 80)/3 = 110
Third iteration:
(110 + 70 + 70 + 70)/3 = 106
If you want to get the average of all grades, do this;
double AVG = 0;
double avg_tmp;
for (int i = 1; i <= ROW; i++) {
avg_tmp = 0;
cout << "Enter student ID[" << i << "]: ";
cin >> ID[i];
for (int j = 0; j < COL; j++) {
cout << "Enter student[" << i << "] Grades [" << j + 1 << "]:";
cin >> nums[i][j];
avg_tmp += nums[i][j];
}
AVG += avg_tmp / 3;
}

Answer 3

Your biggest issue is you invoke Undefined Behavior with your loop limits for for (int i = 1; i <= ROW; i++). In C/C++ all array indexes are Zero-Based with valid indexes of 0 <= i < n. So your i loop must be for (int i = 0; i < ROW; i++).
You do NOT need to compute the sum or average in your first set of loops. The first set of nested loops are you Input Loops where you are simply filling the elements of ID and nums. You only need AVG in your second set of nested loops (e.g. your Output Loops) Avoid mixing input and output logic.
Only declare and initialize your variables in the scope where they are needed. This will fix what your professor is talking above. If you only declare and initialize AVG within the first output-loop where the average will be computed, it will automatically be reset to zero for each student.
Putting it altogether, you could do:
#include <iostream>
const int ROW = 3; /* good job on declaring constants */
const int COL = 3;
const int D = 3;
int main()
{
int nums[ROW][COL] = {{0}}; /* initialize all plain-old arrays */
int ID[D] = {0};
for (int i = 0; i < ROW; i++) {
std::cout << "\nEnter student ID[" << i + 1 << "]: ";
/* you must validate EVERY user-input */
if (!(std::cin >> ID[i])) {
std::cerr << "error: invalid integer input.\n";
return 1;
}
for (int j = 0; j < COL; j++) {
if (!j)
std::cout.put('\n');
std::cout << "Enter student[" << i+1 << "] Grades [" << j+1 << "]: ";
if (!(std::cin >> nums[i][j])) { /* ditto */
std::cout << "error: invalid integer input.\n";
return 1;
}
}
}
for (int i = 0; i < ROW; i++) {
double AVG = 0.; /* declare / initialize in scope needed */
std::cout << "\nStudent ID: " << ID[i] << '\n';
for (int j = 0; j < COL; j++) {
std::cout << " Score: " << nums[i][j] << '\n';
AVG += nums[i][j]; /* sum each score */
}
std::cout << "Average: " << AVG / COL << '\n';
}
}
Example Use/Output
$ ./bin/studentavg
Enter student ID[1]: 1
Enter student[1] Grades [1]: 90
Enter student[1] Grades [2]: 89
Enter student[1] Grades [3]: 91
Enter student ID[2]: 2
Enter student[2] Grades [1]: 79
Enter student[2] Grades [2]: 81
Enter student[2] Grades [3]: 80
Enter student ID[3]: 3
Enter student[3] Grades [1]: 71
Enter student[3] Grades [2]: 70
Enter student[3] Grades [3]: 69
Student ID: 1
Score: 90
Score: 89
Score: 91
Average: 90
Student ID: 2
Score: 79
Score: 81
Score: 80
Average: 80
Student ID: 3
Score: 71
Score: 70
Score: 69
Average: 70
Look things over and let me know if you have further questions.

Answer 4

package test;
import java.util.*;
public class DoubleArray {
public static void main(String[] args) {
String[][] arr = {{"Ram", "100"}, {"Mohan", "80"}, {"Ram", "70"}, {"Mohan", "60"}, {"Chitra", "80"}, {"Ram", "88"}};
HashMap<String, Integer> pf = new HashMap<String, Integer>();
List<String> as = new ArrayList<>();
for (int row = 0; row < arr.length; row++) {
if (pf.containsKey(arr[row][0])) {
as.add(arr[row][0]);
pf.put(arr[row][0], ((pf.get(arr[row][0]) + Integer.parseInt(arr[row][1]))));
} else {
as.add(arr[row][0]);
pf.put(arr[row][0], Integer.parseInt(arr[row][1]));
}
}
int value = 0;
for (Map.Entry<String, Integer> entry : pf.entrySet()) {
pf.put(entry.getKey(), entry.getValue() / Collections.frequency(as, entry.getKey()));
value = Collections.max(pf.values());
if (entry.getValue() == value) {
System.out.println(entry.getKey() + "," + entry.getValue());
}
}
}
}