Hi there.

I wonder if anyone can help me.
I have this code but it only works for a 3 by 3 matrix. I was wondering if anyone could help me change it to a for loop so you can work out different matrixes like a 4 by 30 matrix.

#include <iostream>
#include <cmath>
using namespace std;
void gauss(int N, // number of unknowns
float A [20] [21], // coefficients and constants
float result[20],
bool& err)
// Solve system of N linear equations with N unknowns
// using Gaussian elimination with scaled partial pivoting
// First N rows and N+1 columns of A contain the system
// with right-hand sides of equations in column N+1
// err returns true if process fails; false if it is successful
// original contents of A are destroyed
// solution appears in column N
{
int indx[20];
float scale[20];
float maxRatio;
int maxIndx;
int tmpIndx;
float ratio;
float sum;

for (int i = 0; i < N; i++) indx = i; // index array initialization
// determine scale factors
for (int row = 0; row < N; row++)
{
scale[row] = abs(A[row][0]);
for (int col = 1; col < N; col++)
{
if (abs(A[row][col]) > scale[row]) scale[row] = abs(A[row][col]);
}
}
// forward elimination
for (int k = 0; k < N; k++)
{
// determine index of pivot row
maxRatio = abs(A[indx[k]] [k])/scale[indx[k]];
maxIndx = k;
for (int i = k+1; i < N; i++)
{
if (abs(A[indx] [k])/scale[indx] > maxRatio)
{
maxRatio = abs(A[indx] [k])/scale[indx];
maxIndx = i;
}
}
if (maxRatio == 0) // no pivot available
{
err = true;
return;
}
tmpIndx =indx[k]; indx[k]=indx[maxIndx]; indx[maxIndx] = tmpIndx;
// use pivot row to eliminate kth variable in "lower" rows
for (int i = k+1; i < N; i++)
{
ratio = -A[indx] [k]/A[indx[k]] [k];
for (int col = k; col <= N; col++)
{
A[indx] [col] += ratio*A[indx[k]] [col];
}
}
}
// back substitution
for (int k = N-1; k >= 0; k--)
{
sum = 0;
for (int col = k+1; col < N; col++)
{
sum += A[indx[k]] [col] * A[indx[col]] [N];
}
A[indx[k]] [N] = (A[indx[k]] [N] - sum)/A[indx[k]] [k];
}

for (int k = 0; k < N; k++) result[k] = A[indx[k]] [N];
}
int main()
{
float A[20][21];
float X[20];
bool err;
int N;
float x1,x2,x3,y1,y2,y3,z1,z2,z3,b1,b2,b3;
cout << "Enter X1: ";
cin>> x1;
A[0][0] = x1;
cout << "Enter Y1: ";
cin>> y1;
A[0][1] = y1;
cout << "Enter Z1: ";
cin>> z1;
A[0][2] = z1;
cout << "Enter B1: ";
cin>> b1;
A[0][3] = b1;
//----------
cout << "Enter X2: ";
cin>> x2;
A[1][0] = x2;
cout << "Enter Y2: ";
cin>> y2;
A[1][1] = y2;
cout << "Enter Z2: ";
cin>> z2;
A[1][2] = z2;
cout << "Enter B2: ";
cin>> b2;
A[1][3] = b2;

//-----------
cout << "Enter X3: ";
cin>> x3;
A[2][0] = x3;
cout << "Enter Y3: ";
cin>> y3;
A[2][1] = y3;
cout << "Enter Z3: ";
cin>> z3;
A[2][2] = z3;
cout << "Enter B3: ";
cin>> b3;
A[2][3] = b3;

gauss(3, A, X, err);
for (int i=0; i<3; i++) cout << X << " ";
std::cin.get();
return 0;
}

Welcome to DaniWeb! TO allow us to most effectively help you, please use code tags when you post code.

Why are 'A' and 'result' their current size? Shouldn't they be much smaller if the code was written explicitly for 3x3 matrices?

Also, about half of the code you have posted is for user input. I suggest hardcoding values that you know the result of into the program for testing. This will allow you (and us) to compile and test much more quickly. You can add the user input back when you are confident it is working correctly.

David

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