将C移植到Go，难以理解某些指针语法

Question

I'm currently porting some C (as part of a wider R package) to Go. Because the C in question is used as part of an R package, it has to make extensive use of pointers. The R package is changepoint.np.

As somebody who isn't experienced in C, I've managed to understand most of it. However, the following code has me a bit stumped:

double *sumstat; /* matrix in R: nquantile rows, n cols */
int *n;          /* length of data */
int *minseglen;  /* minimum segment length */
int *nquantiles; /* num. quantiles in empirical distribution */

...[abridged for brevity]...

int j;
int isum;
double *sumstatout;
sumstatout = (double *)calloc(*nquantiles,sizeof(double));
for (j = *minseglen; j < (2*(*minseglen)); j++) {
    for (isum = 0; isum < *nquantiles; isum++) {
        *(sumstatout+isum) = *(sumstat+isum+(*nquantiles*(j))) - *(sumstat+isum+(*nquantiles*(0)));
    }
}

Specifically, this line (in the inner for loop):

*(sumstatout+isum) = *(sumstat+isum+(*nquantiles*(j))) - *(sumstat+isum+(*nquantiles*(0)));

I've read various pages and Stackoverflow questions/answers about C pointers and arrays, and if I understood them correctly, this line would be translated into Go as:

n := len(data)
nquantiles := int(4 * math.Log(float64(len(data))))

sumstatout[isum] = sumstat[isum*n + nquantiles*j] - sumstat[isum*n + nquantiles*0]

Where n is the number of columns (*n in the C code), and nquantiles is the number of rows (*nquantiles in the C code).

However this produces an error (index out of range, obviously) where the original code does not.

Where am I going wrong?

Answer 1

In the line:

sumstatout[isum] = sumstat[isum*n + nquantiles*j] - sumstat[isum*n + nquantiles*0]

I see two strange things:

1) Where did the n in isum*n come from? The n is not part of the orginal expression.

2) nquantiles is a pointer in the original code so it can't be used that way.

In C it should rather be:

sumstatout[isum] = sumstat[isum + *nquantiles*j] - sumstat[isum]

The original C code treats a (contiguous) memory area as a 2D matrix. Like this:

int i, j;
int cols = ..some number..;
int rows = ..some number..;
double* matrix = malloc(cols * rows * sizeof *matrix);
for (i = 0; i < rows; ++i)
    for (j = 0; j < rows; ++j)
        *(matrix + i*cols       +       j) = ... some thing ...;
                   ^^^^^^              ^^^
                 Move to row i        Move to column j

That is equivalent to:

int i, j;
int cols = ..some number..;
int rows = ..some number..;
double matrix[rows][cols];
for (i = 0; i < rows; ++i)
    for (j = 0; j < cols; ++j)
        matrix[i][j] = ... some thing ...;