How to copy a "double pointer"? - c

Question

I'm a novice in C. I have a function with such a structure (this code is not reproducible, this is "pseudo-code", but I hope it's enough to explain my problem):
unsigned** myfunction(double*** A, unsigned nx, unsigned ny, unsigned nz){
unsigned* cells[nz-1];
unsigned totallength = 0;
unsigned lengths[nz-1];
unsigned** bottomTypes = function1(A, nx, ny, 0);
for(unsigned k=0; k<nz-1; k++){
unsigned** topTypes = function1(A, nx, ny, k+1);
unsigned** cellTypes = function2(bottomTypes, topTypes);
unsigned length
unsigned** goodcells = function3(cellTypes, &length);
cells[k] = malloc(length * sizeof(unsigned));
for(unsigned l=0; l<length; l++){
cells[k][l] = goodcells[0][l] + k;
}
lengths[k] = length;
totallength += length;
bottomTypes = function1(A, nx, ny, k+1); // the same as topTypes
}
unsigned** out = malloc(4 * sizeof(unsigned*));
......
......
return out;
}
As you can see, at the end of the loop, I do bottomTypes = function1(A, nx, ny, k+1); which is the same as topTypes previousy introduced. Thus function1(A,nx,ny,k+1) is called twice. That's because I've not been able to copy topTypes in bottomTypes. The code produced the expected output when I do like this.
Instead of doing like this, to avoid the doube call, I've tried
**bottomTypes = **topTypes;
or
bottomTypes = malloc((nx-1) * sizeof(unsigned*));
for ( int i = 0; i < nx-1; ++i ){
bottomTypes[i] = malloc((ny-1)*sizeof(unsigned));
memcpy(bottomTypes[i], topTypes[i], ny-1);
}
The code compiles but I don't get the expected resut when I do that.
What is the right way to copy topTypes into bottomTypes ?
I hope this is clear. Otherwise do not hesitate to say me it's not clear and I'll try to make a reproducibe exampe, but it's not easy. I've found similar questions on SO but none of them allows me to solve my issue.
Edit
Below is a complete reproducibe example. Not very minimal, I admit.
#include <stdlib.h> // for malloc
#include <stdio.h> // for printf
// function to create a "three-dimensional array" (I know this is not the correct wording)
// from a given function
double*** fun2array(double f(unsigned,unsigned,unsigned), unsigned nx, unsigned ny, unsigned nz){
double*** out = (double***) malloc(nx * sizeof(double**));
for(unsigned i=0; i<nx; i++){
out[i] = (double**) malloc(ny * sizeof(double*));
for(unsigned j=0; j<ny; j++){
out[i][j] = (double*) malloc(nz * sizeof(double));
for(unsigned k=0; k<nz; k++){
out[i][j][k] = f(i,j,k);
}
}
}
return out;
}
// a three-dimensional array
double fun(unsigned i, unsigned j, unsigned k){
return i+j+k;
}
double*** A = fun2array(fun, 2, 3, 4);
// function to "slice" a 3d-array to a 2D-array (a "matrix")
double** toMatrix(double*** A, unsigned nx, unsigned ny, unsigned k){
double** out = (double**) malloc(nx * sizeof(double*));
for(unsigned i=0; i<nx; i++){
out[i] = (double*) malloc(ny * sizeof(double));
for(unsigned j=0; j<ny; j++){
out[i][j] = A[i][j][k];
}
}
return out;
}
// function to convert double matrix to integer matrix
unsigned** floorMatrix(double** M , unsigned nx, unsigned ny){
unsigned** out = (unsigned**) malloc(nx * sizeof(unsigned*));
for(unsigned i=0; i<nx; i++){
out[i] = (unsigned*) malloc(ny * sizeof(unsigned));
for(unsigned j=0; j<ny; j++){
out[i][j] = (unsigned) M[i][j];
}
}
return out;
}
// function to sum 2 "matrices"
unsigned** matricialSum(unsigned** M1, unsigned** M2, unsigned nx, unsigned ny){
unsigned** out = (unsigned**) malloc(nx * sizeof(unsigned*));
for(unsigned i=0; i<nx; i++){
out[i] = (unsigned*) malloc(ny * sizeof(unsigned));
for(unsigned j=0; j<ny; j++){
out[i][j] = M1[i][j] + M2[i][j];
}
}
return out;
}
unsigned myfunction(double*** A, unsigned nx, unsigned ny, unsigned nz){
unsigned** bottomTypes = floorMatrix(toMatrix(A, nx, ny, 0), nx, ny);
unsigned** cellTypes;
for(unsigned k=0; k<nz-1; k++){
unsigned** topTypes = floorMatrix(toMatrix(A, nx, ny, k+1), nx, ny);
cellTypes = matricialSum(bottomTypes, topTypes, nx, ny);
bottomTypes = floorMatrix(toMatrix(A, nx, ny, k+1), nx, ny); // the same as topTypes
}
return cellTypes[0][0];
}
int main(){
unsigned result = myfunction(A, 2, 3, 4);
printf("result:%u\n", result);
return 0;
}
2nd edit
I have a solution, but surey not optimal:
unsigned** copyMatrix(unsigned** M, unsigned nx, unsigned ny){
unsigned** MM = malloc(nx * sizeof(unsigned*));
for(unsigned i=0; i<nx; i++){
MM[i] = malloc(ny * sizeof(unsigned));
for(unsigned j=0; j<ny; j++){
MM[i][j] = M[i][j];
}
}
return MM;
}
Then in the example code I free bottomTypes and then I do
unsigned** bottomTypes = copyMatrix(topTypes, nx-1, ny-1);

Answer 1

I finally understand your question, you write **bottomTypes = **topTypes; in place of bottomTypes = topTypes;. This is simply replace pointer by another.
However there are a lot of potential issue with your code, you have memory leak, you use ThreeStar, you cast return of malloc, etc. If your purpose to use C is to be fast, I advice you to change your data structure. malloc() is expensive and unless you want to be able to swap two lines of your matrix with O(1), there are no need to have one malloc for every lines of yours matrix.
So:
Do I cast the result of malloc?
You could use FAM if you don't want too many malloc()
You have memory leak each time you drop topTypes and bottomTypes or cellTypes, I suppose it only in your [mcve].
In your orignal code you use VLA, this should be use carefully.
You use unsigned instead of size_t for index, not a big problem if you don't use very big matrix.
Your [mcve] with little fix, memory leak everywhere:
#include <stdlib.h> // for malloc
#include <stdio.h> // for printf
// function to create a "three-dimensional array" (I know this is not the
// correct wording)
// from a given function
double ***fun2array(double (*f)(size_t, size_t, size_t), size_t nx, size_t ny,
size_t nz) {
double ***out = malloc(nx * sizeof *out);
for (size_t i = 0; i < nx; i++) {
out[i] = malloc(ny * sizeof *out[i]);
for (size_t j = 0; j < ny; j++) {
out[i][j] = malloc(nz * sizeof *out[i][j]);
for (size_t k = 0; k < nz; k++) {
out[i][j][k] = f(i, j, k);
}
}
}
return out;
}
// a three-dimensional array
double fun(size_t i, size_t j, size_t k) { return i + j + k; }
// function to "slice" a 3d-array to a 2D-array (a "matrix")
double **toMatrix(double ***A, size_t nx, size_t ny, size_t k) {
double **out = malloc(nx * sizeof *out);
for (size_t i = 0; i < nx; i++) {
out[i] = malloc(ny * sizeof *out[i]);
for (size_t j = 0; j < ny; j++) {
out[i][j] = A[i][j][k];
}
}
return out;
}
// function to convert double matrix to integer matrix
unsigned **floorMatrix(double **M, size_t nx, size_t ny) {
unsigned **out = malloc(nx * sizeof *out);
for (size_t i = 0; i < nx; i++) {
out[i] = malloc(ny * sizeof *out[i]);
for (size_t j = 0; j < ny; j++) {
out[i][j] = M[i][j];
}
}
return out;
}
// function to sum 2 "matrices"
unsigned **matricialSum(unsigned **M1, unsigned **M2, size_t nx, size_t ny) {
unsigned **out = malloc(nx * sizeof *out);
for (size_t i = 0; i < nx; i++) {
out[i] = malloc(ny * sizeof *out[i]);
for (size_t j = 0; j < ny; j++) {
out[i][j] = M1[i][j] + M2[i][j];
}
}
return out;
}
unsigned myfunction(double ***A, size_t nx, size_t ny, size_t nz) {
unsigned **bottomTypes = floorMatrix(toMatrix(A, nx, ny, 0), nx, ny);
unsigned **cellTypes = bottomTypes;
for (size_t k = 1; k < nz; k++) {
unsigned **topTypes = floorMatrix(toMatrix(A, nx, ny, k), nx, ny);
cellTypes = matricialSum(bottomTypes, topTypes, nx, ny);
bottomTypes = topTypes;
}
return cellTypes[0][0];
}
int main(void) {
double ***A = fun2array(fun, 2, 3, 4);
unsigned result = myfunction(A, 2, 3, 4);
printf("result:%u\n", result);
}