C++ contiguous memory operation - c++

Question

I have c++ program in which I am calculating determinant of a matrix using normal array which is as follows:
/* rand example: guess the number */
#include <stdio.h> /* printf, scanf, puts, NULL */
#include <stdlib.h> /* srand, rand */
#include <time.h> /* time */
#include <iostream>
#include <cstdlib>
#include <vector>
using namespace std;
int** generateStandardMatrix(int dimension);
void ijMinor(int *matrix[], int *minorMatrix[], int size, int row, int column);
int determinant(int *matrix[], int size);
void ijMinor(int *matrix[], int *minorMatrix[], int size, int row, int column) {
for (int i = 0; i < size; i++) {
for (int j = 0; j < size; j++) {
if (i < row) {
if (j < column)minorMatrix[i][j] = matrix[i][j];
else if (j == column)continue;
else minorMatrix[i][j - 1] = matrix[i][j];
}
else if (i == row)continue;
else {
if (j < column)minorMatrix[i - 1][j] = matrix[i][j];
else if (j == column)continue;
else minorMatrix[i - 1][j - 1] = matrix[i][j];
}
}
}
}
int determinant(int *matrix[], int size) {
if (size == 1)return matrix[0][0];
else {
int result = 0, sign = -1;
for (int j = 0; j < size; j++) {
int **minorMatrix;
minorMatrix = new int*[size - 1];
for (int k = 0 ; k < size - 1 ; k++)
minorMatrix[k] = new int[size - 1];
ijMinor(matrix, minorMatrix, size, 0, j);
sign *= -1;
result += sign * matrix[0][j] * determinant(minorMatrix, size - 1);
for (int i = 0; i < size - 1; i++) {
delete minorMatrix[i];
}
}
return result;
}
}
int main (int argc, char* argv[])
{
/* initialize random seed: */
srand (time(NULL));
// int iSecret, iGuess;
int dimension = atoi(argv[1]);
int rowCount = dimension , colCount = dimension;
//2d array storing the integer values
int** ary = new int*[dimension];
//vector of vector storing the indices across the array for the threads to pick up from
vector<vector<int> > vec;
ary = generateStandardMatrix(dimension);
printf("Array value : %d\n", ary[0][0]);
int detVal = determinant(ary, dimension);
printf("determinant value : %d\n", detVal);
return 0;
}
int** generateStandardMatrix(int dimension) {
int** ary = new int*[dimension];
int counter = 0;
for (int i = 0; i < dimension; ++i) {
ary[i] = new int[dimension];
counter = counter + 1;
for (int j = 0; j < dimension; ++j)
{
ary[i][j] = counter;
std::cout << ary[i][j] << "\t" << std::flush;
}
std::cout << std::endl;
}
return ary;
}
I want to replace it with code in which I allocate memory for the array before the start of the algorithm and then change the determinant and the ijMonor functions so that they don't create new array's but use the same array only.
The determinant will take parameter like: determinant(int *matrix, int *startOfMyWorkspace, int size) so that it knows where to start.
I am not good at c++ and so far I was not able to do it.
Can someone please provide some sample code.
I allocated some memory for array and created and array but was unable to change the ijMinor and determinant functions for that.
This is how I am allocating memory:
int main (int argc, char* argv[])
{
/* initialize random seed: */
srand (time(NULL));
// int iSecret, iGuess;
int dimension = atoi(argv[1]);
int *a;
size_t const N_BYTES = dimension * dimension * sizeof(int);
a = (int*)malloc(N_BYTES);
createData(dimension,a);
return 0;
}
void createData(int const dimension, int* const a)
{
int row, col;
srand((unsigned)time(NULL));
int counter;
for(int row = 0; row < dimension; row++) {
counter = counter + 1;
for(int col = 0; col < dimension; col++) {
int i = col + row * dimension;
a[i] = counter;
std::cout << a[i] << "\t" << std::flush;
}
std::cout << std::endl;
}
}

Answer 1

Try this.
Note if you use new to allocate an array, you need to use delete[] to free all of it. You'll get away with delete (i.e. it won't crash) but this will only free the first element. Your other functions are the same as you posted.
You're dynamically allocating space for minorMatrix in determinant function, but it's hard to see how that could be preallocated. I've modified determinant function to use allocate_arr and deallocate_arr.
int ** allocate_arr(int dimension)
{
int** a = new int*[dimension];
for (int i = 0; i < dimension; ++i)
a[i] = new int[dimension];
return a;
}
void deallocate_arr(int dimension, int **a)
{
for (int i = 0; i < dimension; ++i)
delete[] a[i];
delete[] a;
}
int determinant(int *matrix[], int size) {
if (size == 1)return matrix[0][0];
else {
int result = 0, sign = -1;
for (int j = 0; j < size; j++) {
int **minorMatrix = allocate_arr(size - 1);
ijMinor(matrix, minorMatrix, size, 0, j);
sign *= -1;
result += sign * matrix[0][j] * determinant(minorMatrix, size - 1);
deallocate_arr(size - 1, minorMatrix);
}
return result;
}
}
void generateStandardMatrix(int dimension, int**ary) {
int counter = 0;
for (int i = 0; i < dimension; ++i) {
counter = counter + 1;
for (int j = 0; j < dimension; ++j)
{
ary[i][j] = counter;
std::cout << ary[i][j] << "\t" << std::flush;
}
std::cout << std::endl;
}
}
int main(int argc, char* argv[])
{
srand(time(NULL));
int dimension = atoi(argv[1]);
int** a = allocate_arr(dimension);
generateStandardMatrix(dimension, a);
printf("Array value : %d\n", a[0][0]);
int detVal = determinant(a, dimension);
printf("determinant value : %d\n", detVal);
// ... do more computations here, reusing `a` ...
deallocate_arr(dimension, a);
return 0;
}