How can I create a 3 dimension int with pointer in c++? - c++

Question

int b, c, d;
cin >>b>>c>>d;
int **a[b];
for(int i=0;i<b;++i){
*a[i]=new int[c];
for(int j=0;j<c;j++){
a[i][j]=new int[d];
}
}
I would like to create a 3 dimension int.I think the problem is when I create the second dimension it is not a pointer and I can not create a 3th on it.

Answer 1

As #yacsha posted, you're better off using <vector> from the STL, rather than using raw pointers and new (or even pointers at all).
But, if creating a '3 dimension int' is really what you want, then you must declare your variables properly, allocate, then delete them:
#include <iostream>
int main()
{
int b, c, d;
std::cin >> b >> c >> d;
// declaring a variable to a pointer that points to a pointer ... etc.
// really hard to maintain and read
int*** a = new int**[b];
for (int i = 0; i < b; ++i) {
a[i] = new int*[c];
for (int j = 0; j < c; j++) {
a[i][j] = new int[d];
for (int k = 0; k < d; k++)
std::cin >> a[i][j][k];
}
}
// using it like a normal 3d array would be used
for (int i = 0; i < b; i++) {
for (int j = 0; j < c; j++) {
for (int k = 0; k < d; k++)
std::cout << a[i][j][k] << ' ';
std::cout << '\n';
}
std::cout << '\n';
}
// deleting is necessary after using them
for (int i = 0; i < b; i++)
for (int j = 0; j < c; j++)
delete[] a[i][j];
for (int i = 0; i < b; i++)
delete[] a[i];
delete[] a;
}
As you can see, this is a nightmare to maintain. It's much more easier to use <vector> because the container takes care of the memory allocation for you. If you don't want to use STL, atleast use a[b][c][d], where b, c, d are const / constexpr.
This is how it looks like with <vector>:
#include <iostream>
#include <vector>
int main()
{
int b, c, d;
std::cin >> b >> c >> d;
std::vector<std::vector<std::vector<int>>> threeDim(b, std::vector<std::vector<int>>(c, std::vector<int>(d, 0)));
for (const auto x : threeDim) {
for (const auto y : x) {
for (const auto z : y)
std::cout << z << ' ';
std::cout << '\n';
}
std::cout << '\n';
}
}
You could typedef / using to shorten std::vector if you find it too cumbersome to declare and init the 3d array, or you could resize it via loops.

Answer 2

You can declare a as a ***a[b] (pointer, to a pointer, to a pointer).
int ***a[b];

Answer 3

I suggest using the standard stl, which is safer than using pointers, so I would have to declare the "vector template":
#include <vector>
using std::vector;
The code would look like:
vector<vector<vector<int>>> A;
int b, c, d;
cin >>b>>c>>d;
int i,j,k;
A.resize(b);
for(i=0;i<b;++i){
A[i].resize(c);
for(j=0;j<c;j++){
A[i][j].resize(d);
for(k=0;k<d;k++){
// Optional inputs elements
cout << "Input element " <<i<<","<<j<<","<<k<<": ";
cin >> A[i][j][k];
}
}
}