Finding the average of individual rows and columns in random array - c++

Question

I've got to make a program that creates a 10x10 array of random numbers 0-9 and find the average of each individual row and column and also the whole array. I've got the array right but i'm stuck on how to select a single row or column then find the average of it, any tips?
#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
const int numRows = 10;
const int numCols = 10;
int val[numRows][numCols];
int i, j;
double sum = 0, avgR, avgC;
void randNum() // Creates 10x10 array of random nums 0-9
{
srand(2622); // random seed
for (i = 0; i < numRows; i++) // Creates random rows
{
cout << endl << endl;
for (j = 0; j < numCols; j++) // Creates random columns
{
val[i][j] = rand() % 10;
cout << setw(6) << val[i][j];
}
}
cout << endl << endl;
}
void randAvg() // finds average of each row and column
{
for (i = 0; i < numCols; i++)
{
sum += val[i][j];
}
avgR = sum / numRows;
cout << " " << avgR << endl;
}
int main() // calls each function
{
randNum();
randAvg();
}

Answer 1

A two dimensional array has a row and a column. When you access each elements of such an array in a loop,it is better to reflect that structure with your loops.
So this in your code:
for (i = 0; i < numCols; i++)
{
sum += val[i][j];
}
Is trying to access a single column(given by j) in each row(given by i) of your array
Since the value of j is now made 10 by the call to the randNum() function, you are accessing one element past your array (technically this happens when you are at the last row) which is not good. That is why it's not a good idea to use global variables to share information between functions.
With these:
//assuming sum and avgR are declared
//average of a row
for (int i = 0; i < numRows; i++)//iterate each row(one loop for the row)
{
sum=0;//reset sum for each row
for (int j = 0; j < numCols; j++)//iterate each column in each row(one loop for the column)
sum += val[i][j];//will sum up the contents of the current row
avgR=sum/numCols;//give you the average of each row
}
//average of a column
for (int j = 0; j < numCols; j++)//iterate each column
{
sum=0;//reset sum for each column
for (int i = 0; i < numRows; i++)//iterate each row for each column
sum += val[i][j];//will sum up the contents of the current column
avgR=sum/numRows;//give you the average of each column
}
Am i sure you will be able to figure out how to get the average of the whole array.

Answer 2

In either case, you will have to iterate over all array entries. While building the column average, you iterate over each column and in each iteration, you iterate over all rows, computing the average.
During the row average, you iterate over all rows and in each iteration compute the average over all columns:
void randAvg()
{
for (int i = 0; i < numCols; i++)
{
float sum = 0.f;
for (int j = 0; j < numRows; j++) {
sum += val[i][j];
}
float avg = sum / numRows;
cout << "Average of column " << i << ": " << avg << endl;
}
for (int i = 0; i < numRows; i++)
{
float sum = 0.f;
for (int j = 0; j < numCols; j++) {
sum += val[j][i];
}
float avg = sum / numRows;
cout << "Average of row " << i << ": " << avg << endl;
}
float sum = 0.f;
for (int i = 0; i < numRows; i++)
{
for (int j = 0; j < numCols; j++) {
sum += val[j][i];
}
}
float avg = sum / (numRows*numCols);
cout << "Overall average: " << avg << endl;
}
Disclaimer: Untested code, should work though.