creating pointer arrays in c++ with help of new keyword - c++

Question

What's the meaning of the code below?
int **matrix = new int*[n]
What's the difference here between matrix and int*[n]?

Answer 1

It is an array of 'n' pointers, for which memory can be allocated and initialized in loop.
If n is 3, it is an array of 3 elements and each is pointer to int, can point to set of array of integer values like below.
matrix[0] -> Ox001 points to array of int [ 1 2 3 4]
matrix[1] -> Ox017 [ 5 6 7 8]
matrix[2] -> Ox024 [ 9 10 11 12]
Sample code like this
int **m = new int*[3];
for(auto i=0; i < 3; i++)
{
m[i] = new int[3];
for(auto j=0; j < 3; j++)
m[i][j] = 0;
}
for(auto i=0; i < 3; i++)
{
m[i] = new int[3];
for(auto j=0; j < 3; j++)
cout << m[i][j];
cout << "\n";
}

Answer 2

for instance there is cricket team and you need
Since you have Cricket* team;, this indicates you have one of two possible
situations:
1) a pointer to a single CricketPlayer (or any derived) type
2) a pointer to an array of CricketPlayer (but not derived) types.
What you want is a pointer to an array of CricketPlayer or derived types. So you
need the **.
You'll also need to allocate each team member individually and assign them to the array:
// 5 players on this team
CricketPlayer** team = new CricketPlayer*[5];
// first one is a bowler
team[0] = new Bowler();
// second one is a hitter
team[1] = new Hitter();
// etc
// then to deallocate memory
delete team[0];
delete team[1];
delete[] team;
In your query,
It can be understood as
int *matrix[]=new int*[n];
SO there are n pointers pointing to n places.
Because
int *foo;
foo=new int[5];
will also create 5 consecutive places but it is the same pointer.
In our case it is array of pointers

Answer 3

You need to notice some thing as follows:
int *p means that p is a pointer that points to an int variable or points to an array of int variables.
int* *p means that p is a pointer that points to an int* variable or points to an array of int* variables.
new int[5] is an array of 5 int variables.
new int*[5] is an array of 5 int* variables.
In this case, matrix is the second type so it can point to an array of int* variables so the statement:int **matrix = new int*[n] is legal

Answer 4

It means that you have declared a double pointer(of type int) called matrix and allocated an array of n int* pointers. Now you can use matrix[i] as an int* pointer (0 <= i < n). Later you might want to allocate memory to individual pointers as well like matrix[i] = new int[size];(size is an int or more appropriately size_t)

Answer 5

matrix is an object of type int **. This type is a pointer to pointer to int.
On the other hand, int * is a type, a pointer to int.
The expression:
new int*[n]
creates n adjacent objects of type int * somewhere in memory and returns a pointer to the first of them. In other words, it allocates and constructs n pointers to int; and returns a pointer to the first. Since the first (and each of them) are int *, the pointer pointing to the first is, in turn, of type int **.

Answer 6

It would be absolutely clear if you remember the operator associativity and precedence.
Going by that int *[n] will be interpreted by compiler : as array of " n pointer to integers".(as [ ] has greater precedence than *, so for easy understanding, compiler interprets it that way.).
Now comming to other side int ** matrix is pointer to pointer.
When you created an array of pointer,you basically have the address of the first location ,here first location stores an pointer to integer.
So,when you are storing the address of the first address of such array you will need pointer to a pointer to point the whole array.

Answer 7

//code for passing matrix as pointer and returning as a pointer
int ** update(int ** mat)
{
mat[0][1]=3;
return mat;
}
int main()
{ int k=4;
int **mat = new int*[k];
for(auto i=0; i < k; i++)
{
mat[i] = new int[k];
for(int j=0;j<k;j++)
{
if(i==j)
mat[i][j]=1;
else
mat[i][j]=0;
}
}
mat=update(mat);
for(auto i=0; i < k; i++)
{
for(auto j=0; j < k; j++)
cout << mat[i][j];
cout << "\n";
}
return 0;
}