I have a simple question regarding pointers of the form **a. I have been trying to incorporate some external code into a program of mine, and one of the functions takes on a pointer like this as an argument. Furthermore, when the thing is used in the function, it is referred to as a two dimensional array. This is confusing to me, since I'm new to C. I'm looking for conceptual help here.

I've got the thing copied in properly, I think, but I don't know what preceisely to put in this argument slot. I've tried putting in the array, a pointer to the array, and a pointer to the pointer to the array. Everything either won't compile, or calling the function yields a segmentation fault.

I'm including the code, assuming that might help clarify things a bit. It's the first phase of a two part algorithm to get the eigenvalues and eigenvectors of a real symmetric matrix. This thing puts the matrix in a special form (tri-diagonal form) via similarity transforms.

Thanks!

Code:

void tred2(int n, double **a, double d[], double e[]){
int l,k,j,i;
float scale, hh, h, g, f;
for (i=n; i>=2; i--){
l=i-1;
h=scale=0.0;
if(l>1){
for(k=1;k<=l;k++)
scale +=fabs(a[i][k]);
if(scale == 0.0)
e[i]=a[i][l];
else{
for(k=1;k<=l;k++){
a[i][k] /= scale;
h += a[i][k]*a[i][k];
}
f=a[i][l];
g = (f >= 0.0 ? -sqrt(h) : sqrt(h));
e[i]=scale*g;
h -= f*g;
a[i][l]=f-g;
f=0.0;
for(j=1; j<=l; j++){
a[j][i]=a[i][j]/h;
g=0.0;
for(k=1;k<=j;k++)
g += a[j][k]*a[i][k];
for(k=j+1;k<=l;k++)
g += a[k][j]*a[i][k];
e[j]=g/h;
f += e[j]*a[i][j];
}
hh=f/(h+h);
for(j=1;j<=1;j++){
f=a[i][j];
e[j]=g=e[j]-hh*f;
for(k=1;k<=j;k++)
a[j][k] -= (f*e[k]+g*a[i][k]);
}
}
} else
e[i]=a[i][j];
d[i]=h;
}
d[1]=0.0;
e[1]=0.0;
for(i=1;i<=n;i++){
l=i-1;
if(d[i]){
for (j=1; j<=1; j++){
g=0.0;
for(k=1;k<=l;k++)
g += a[i][k]*a[k][j];
for(k=1;k<=l;k++)
a[k][j] -= g*a[k][i];
}
}
d[i]=a[i][i];
a[i][i]=1.0;
for(j=1;j<=l;j++)
a[j][i]=a[i][j]=0.0;
}
}