c++ and double arrays

[LasherHN]

I'm trying to use some double arrays, one approach that works is this:

#include <iostream>
#include <cstdlib>

using namespace std;

int main() {
int **a;
//get memory
a=new int*[5];
for(int i=0; i<5; i++)
a[i]=new int[5];
//assign
int k=0;
for(int i=0; i<x; i++)
for(int j=0; j<y; j++){
a[i][j]=k;
k++;
}
//print
for(int i=0; i<5; i++){
for(int j=0; j<5; j++){
cout<<a[i][j]<<" ";
}
cout<<endl;
}
return 0;
}

That prints a matrix-like array of 5x5 without any problems.

In another program im doing i need to do that in a separete function like this:

#include <iostream>
#include <cstdlib>

using namespace std;

void arraytest(int ***);

int main() {
int **a;
arraytest(&a);
//print
for(int i=0; i<5; i++){
for(int j=0; j<5; j++){
cout<<a[i][j]<<" ";
}
cout<<endl;
}
return 0;
}

void arraytest(int ***a){
//get memory
*a=new int*[5];
for(int i=0; i<5; i++)
*a[i]=new int[5];
//assign
int k=0;
for(int i=0; i<5; i++)
for(int j=0; j<5; j++){
*a[i][j]=k;
k++;
}
}

But the second code gives me a segmentation error. Could you tell me please what's wrong with it?

[smani]

Pass the pointer by reference:

#include <iostream>
#include <cstdlib>

using namespace std;

void arraytest(int**&);

int main() {
int **a;
arraytest(a);
//print
for(int i=0; i<5; i++)
for(int j=0; j<5; j++){
cout<<a[i][j]<<" ";
}
cout<<endl;

return 0;
}

void arraytest(int**&a){
//get memory
a=new int*[5];
for(int i=0; i<5; i++)
a[i]=new int[5];
//assign
int k=0;
for(int i=0; i<5; i++)
for(int j=0; j<5; j++){
a[i][j]=k;
k++;
}
}

[LasherHN]

Great that worked perfectly!

I get too confused with pointers/references. Java made me too lazy too care about when I learned :p

Thanks a lot smani.

[dwhitney67]

There's another way... and you should consider adopting it versus tinkering with the C-style "nuts 'n' bolts".

Here's a simple example using the std::tr1::array, which is similar (if not identical) to Boost's array:

#include <tr1/array>
#include <iostream>
#include <iomanip>

static const unsigned int ROWS = 5;
static const unsigned int COLS = 5;

typedef std::tr1::array< std::tr1::array<int, COLS>, ROWS > MyArray;


int main()
{
MyArray a;

// initialize array
for (unsigned int i = 0; i < ROWS; ++i)
{
for (unsigned int j = 0; j < COLS; ++j)
{
a[i][j] = (j + i + 1) * 2;
}
}

// print array contents
for (unsigned int i = 0; i < ROWS; ++i)
{
for (unsigned int j = 0; j < COLS; ++j)
{
std::cout << std::setw(3) << std::setfill(' ') << a[i][j] << " ";
}
std::cout << std::endl;
}
}

[LasherHN]

That looks good too, I'll save it aside for the future, for the class I'm taking I think it's better the 'c-style' way.

Thanks for the replies.

[Martin Witte]

If you don't want to use non stl stuff you can go with
#include <iostream>
#include <vector>
#include <iomanip>

using namespace std;

typedef vector<int> intvector;
typedef vector< vector<int> > int2dmatrix;

void arraytest(int2dmatrix& matrix, const int& dimension)
{
int k(0);

for(int i=0; i < dimension; ++i)
{
for(int j=0; j < dimension; ++j)
{
matrix[i][j]=k++;
}
}
}
int main() {
const int dimension(5);
int2dmatrix matrix(dimension, intvector(dimension,0));

arraytest(matrix, dimension);
//print
for(int i = 0; i < dimension; ++i)
{
for(int j = 0; j < dimension; ++j)
{
cout<< setw(3) << setfill(' ') << matrix[i][j];
}
cout<<endl;
}
return 0;
}