LU Decomposition from Numerical Recipes not working; what am I doing wrong?

Question

I\'ve literally copied and pasted from the supplied source code for Numerical Recipes for C for in-place LU Matrix Decomposition, problem is its not working.

I\'m s

Answer 1

I think there's something inherently wrong with your indices. They sometimes have unusual start and end values, and the outer loop over j instead of i makes me suspicious.

Before you ask anyone to examine your code, here are a few suggestions:

• double-check your indices
• get rid of those obfuscation attempts using sum
• use a macro a(i,j) instead of a[i*MAT1+j]
• write sub-functions instead of comments
• remove unnecessary parts, isolating the erroneous code

Here's a version that follows these suggestions:

#define MAT1 3
#define a(i,j) a[(i)*MAT1+(j)]

int h_NR_LU_decomp(float *a, int *indx)
{
        int i, j, k;
        int n = MAT1;

        for (i = 0; i < n; i++) {
                // compute R
                for (j = i; j < n; j++)
                        for (k = 0; k < i-2; k++)
                                a(i,j) -= a(i,k) * a(k,j);

                // compute L
                for (j = i+1; j < n; j++)
                        for (k = 0; k < i-2; k++)
                                a(j,i) -= a(j,k) * a(k,i);
        }

        return 0;
}

Its main advantages are:

• it's readable
• it works

It lacks pivoting, though. Add sub-functions as needed.

My advice: don't copy someone else's code without understanding it.

Most programmers are bad programmers.