Extract rows/columns from ** matrices in C/C++ -

i have square matrix double **a

i know how iterate through matrix:

for (int = 0; < matrix_size; i++) {  (int j = 0; j < matrix_size; j ++) {    int val = a[i][j];    printf("val: %d\n", val);  } } 

however, i'm wondering how can assign entire row or column variable given have ** matrix (the ** pointer pointer nonsense still little confusing. believe saying list of list of doubles).

to add little more background, i'm trying extract rows , columns can perform cuda matrix multiplication. see lot of documentation online uses one-dimensional vectors represent matrices (i.e. double* a) however, getting confused **

a two-dimensional array of doubles (double **) can looked @ one-dimensional array of one-dimensional arrays of doubles.

double **arr; // initialized for(int rownumber = 0; rownumber < matrix_size; ++rownumber) {     double *row = arr[rownumber];     // row     for(int colnumber = 0; colnumber < matrix_size; ++colnumber)     {         double value = row[colnumber];         // value     } } 

in above example, row pointer contiguous row of values initial array. works because 2 dimensional array allocated this:

double **arr = new double*[row_count]; for(int rownumber = 0; rownumber < row_count; ++rownumber) {     arr[rownumber] = new double[col_count]; } 

getting pointer column in matrix (like did row above) not possible because values in column not contiguous, values in each row contiguous.