The document discusses dynamic memory allocation in C++ using arrays and pointers. It provides 5 examples of dynamically allocating 2D arrays on the heap using new/delete, pointers, and vectors. The examples allocate 2D arrays of varying sizes, initialize the elements, and output the results. Dynamic allocation allows arrays to be created where the size is only known at runtime, as opposed to static allocation where the size is fixed at compile time.

1. Dynamic Allocation in C++
allan914.120418 (update: 120420)

2. Example 1
#include<iostream>
using namespace std;
int main()
{
int* a;
int n;
cin >> n;
a=new int [n];
for(int i=0;i<n;i++) a[i]=i+1;
for(int i=0;i<n;i++) cout << a[i] << " ";
delete [] a;
return 0;
}

3. Example 1: Result
Sample Input
10
 Sample Output
1 2 3 4 5 6 7 8 9 10

4. Example 2
#include<iostream>
using namespace std;
int main() {
int** a; int n,m;
cin >> n; cin >> m;
a=new int* [n];
for(int i=0;i<n;i++) a[i]=new int [m];
for(int i=0;i<n;i++) for(int j=0;j<m;j++) a[i][j]=i+j+1;
for(int i=0;i<n;i++) {
for(int j=0;j<m;j++) cout << a[i][j] << " ";
cout << endl;
}
for(int i=0;i<n;i++) delete [] a[i];
delete [] a;
return 0;
}

5. Example 2: Result
 Sample Input
5
4
Sample Output
1234
2345
3456
4567
5678

6. Example 3
#include<iostream>
using namespace std;
int main() {
int* a;
int n,m;
cin >> n;
cin >> m;
a=new int [n*m];
int** b;
b=new int* [n];
for(int i=0;i<n;i++) b[i]=&(a[n*i]);
for(int i=0;i<n;i++) for(int j=0;j<m;j++) b[i][j]=i+j+1;
for(int i=0;i<n;i++) {
for(int j=0;j<m;j++) cout << b[i][j] << " ";
//cautious with CodeBlocks !
cout << endl;
}
delete [] b;
delete [] a;
return 0;
}

7. Example 3: Result
 Sample Input
5
4
Sample Output
1234
2345
3456
4567
5678

8. Example 4
// Headers emitted, cautious with CodeBlocks !
void* new2d(int n,int m,int size) {
void **a = new void* [m*sizeof(void*) + n*m*size];
for(int j=0;j<m;j++) a[j] = ((char *)(a+m)) + j*n*size;
return a;
}
int main() {
int** a;
int n,m;
cin >> n;
cin >> m;
a = (int**)new2d(n,n,sizeof(int));
for(int i=0;i<n;i++) for(int j=0;j<m;j++) a[i][j]=i+j+1;
for(int i=0;i<n;i++) {
for(int j=0;j<m;j++) cout << a[i][j] << " ";
cout << endl;
}
delete [] a;
return 0;
}

9. Example 4: Result
 Sample Input
5
4
Sample Output
1234
2345
3456
4567
5678

10. Example 5
#include<iostream>
#include<vector>
using namespace std;
int main() {
int n,m;
cin >> n;
cin >> m;
vector<vector<int> > a(n, vector<int>(m));
for(int i=0;i<n;i++) for(int j=0;j<m;j++) a[i][j]=i+j+1;
for(int i=0;i<n;i++) {
for(int j=0;j<m;j++) cout << a[i][j] << " ";
cout << endl;
}
return 0;
}

11. Example 5: Result
 Sample Input
5
4
Sample Output
1234
2345
3456
4567
5678