Your homework tonight is to beautify this code (which we collectively authored in class today) and to make it accept the array size (so long as it's <= MAX_ROWS) and the array values (g_coeffs) as input from the user.  You do not have to worry about the case stated in the code when the row's lead coefficient is 0 (big extra credit, though, if you eliminate this nasty bug -- hint: you'll have to "pivot" your rows somehow; hint2:  it may not be possible -- not all arrays produce unique solutions!).

Good luck, and don't forget the quiz Friday on C++ arrays.

// gauss.cpp -- Gaussian Elimination
// by David Kosbie and AP Computer Science P8

#include <stdio.h>
#include <iostream.h>

const int MAX_ROWS = 8;

int g_rows = 3; // change to input from user

// change this to be loaded by user
double g_coeffs[MAX_ROWS][MAX_ROWS+1] = {
 {-2, 1.5, 4, 2.5},
 {1, -1.5, 3, 7.5},
 {3, 4.5, -2, 8.5}
};
 

void reduceCoeffToOne(int row, int col)
{
 // ignore case when it is 0 (bug -- fix later)
 double divisor = g_coeffs[row][col];
 int col2;
 for (col2=col; col2<=g_rows; col2++)
  g_coeffs[row][col2] = g_coeffs[row][col2] / divisor;
}

void eliminateColumn(int col)
{
 // 1. set top left to 1
 int row=col;
 reduceCoeffToOne(row,col);
 // 2. For each row below, eliminate that row using this row
 int row2;
 for (row2 = col+1; row2 < g_rows; row2++)
 {
  reduceCoeffToOne(row2,col);
  // now subtract row from row2
  int col2;
  for (col2 = col; col2 <= g_rows; col2++)
   g_coeffs[row2][col2] -= g_coeffs[row][col2];
 }

}

// if you solve for variable in <row>, then all
// variables in <row2> > <row> must be solved already.
void solveVariable(int row)
{
 double answer = g_coeffs[row][g_rows];
 int col;
 for (col=row+1; col<g_rows; col++)
 {
  // You must think about this one
  answer -= g_coeffs[row][col] * g_coeffs[col][g_rows];
  g_coeffs[row][col] = 0;
 }
 g_coeffs[row][g_rows] = answer;
}
 

void solveMatrix()
{
 int col;
 int row;
 // eliminate all the columns
 for (col=0; col<g_rows; col++)
  eliminateColumn(col);
 // solve for each variable, BOTTOM UP
 for (row=g_rows-1; row>=0; row--)
  solveVariable(row);

}

void printAnswers()
{
 int row;
 cout << "Here are the answers:" << endl;
 for (row = 0; row < g_rows; row++)
  cout << "x[" << row << "] = "
       << g_coeffs[row][g_rows] << endl;
}
 

void main()
{
 cout << "Gaussian Elimination blah blah...";
 solveMatrix();
 printAnswers();
 getchar();
}