How to create a multi-dimension array in C Plus Plus

I need some help to create a multi-dimension array in C Plus Plus, but the user have to input the size of the array n x n.
The array has to be in simple C Plus Plus code with no class creation.
I try to make some thing like this, but dont work.

int main ()
{
    int a, b;
    int matriz[a][b];
    cout<<"Define the size of the matrix.\n";
      cin >> a;
      b=a;
}
But because i´m a noob on programming i can´t make it work. Please can you give me some help or directions.
Appreciated.
LVL 12
David Paris VicenteSystems and Comunications Administrator Asked:
Who is Participating?
 
mrjoltcolaCommented:
Now, I will show you the trick I was referring to, which is something C programmers may do more than C++, but it is more memory and cache efficient than an array of pointers to arrays, and is one less level of indirection, because it allocates a single contiguous array. I'm not recommending you go this route, I would use an stl component, myself, but its worth knowing how to do this as an alternative.

void matrix()
{
  int a,b,c=1;
  int *matrix;
  cout<< "Insert the size of the Matrix.\n";
  cin>>a;
  b=a;

  matrix= new int[a * b];
  for (int i=0; i<a;++i)
  {
    for (int j=0; j<a; ++j)
    {
      matrix[i*a + j]= c;
      c++;
      cout <<matrix[i * a + j]<<" ";
    }
    cout << endl;
  }

  delete [] matrix;
}
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mrjoltcolaCommented:
Read the numbers in first, then create the array.
You are doing it backwards.
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rendaduiyanCommented:
1) create a array of size n, but they are pointers.
2) foreach pointer in this array, create a array to be pointed by this pointer.
int ** pa = new int [n];
for()
....
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Infinity08Commented:
>>     int matriz[a][b];

You can't create a static array with runtime dimensions. You'll need to use a dynamically allocated array.

Take a look at this tutorial :

        http://www.cplusplus.com/doc/tutorial/dynamic.html
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errangCommented:
Would this work?

int main (int argc, char **argv)
{
    int a = atoi(argv[1])
    int b = atoi(argv[2]);
    int matriz[a][b];
}

With this you'd have to enter the dimensions of the array as a command line argument.
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Infinity08Commented:
Right ... C++, not C ... Ignore my previous post please :)
0
 
itsmeandnobodyelseCommented:
>>>> Right ... C++, not C ... Ignore my previous post please :)

Most nowadays C++ compilers won't allow definition of an array with non-const size parameters (though a next standard might require that)

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Infinity08Commented:
>> Most nowadays C++ compilers won't allow definition of an array with non-const size parameters

It seems I have been spoiled by g++ which supports it as an extension.

Variable length arrays are allowed in C99, but are not covered by the C++ standard. So, my initial response was correct after all (I should rely on my intuition a bit more lol).

You have to use dynamically allocated memory for variable length arrays in C++, unless you want to make use of a compiler extension like g++ has.
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David Paris VicenteSystems and Comunications  Administrator Author Commented:
Yes it´s in C++.
errang, that don´t work .
I´m trying to make the game Tic Tac Toe but the user have to input the size of the matrix.
If the matrix is static is easy but they want the matrix dynamic.
I made this when i saw some examples, but i don´t understand.

#include <iostream>
using namespace std;
 
int main ()
{
int i=0,a,b;
int **matrix;
cout<< "Insert the size of the Matrix.\n";
cin>>a;
b=a;

matrix= new int*[a];
for (i; i<a;i++)
{
matrix[i]= new int [b];
}

system ("PAUSE");
return 0;
}
And how can i send some information to inside, i cant visualize in my mind how this works.
Thank you all.
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Infinity08Commented:
>> I made this when i saw some examples, but i don´t understand.

That looks ok. It creates a dynamically allocated 2-dimensional array. For more information, see the tutorial I mentioned earlier.

You can access the dynamically allocated array, in the same way as you access a normal array, ie. matrix[i][j] will get the value at row i, column j.
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itsmeandnobodyelseCommented:
>> I made this when i saw some examples, but i don´t understand

>>>> matrix= new int*[a];

that creates an array of pointers which are supposed to point to the first element of a row.

>>>> matrix[i]= new int [b];

that creates an array of int for row with index i and the pointer to that array was assigned to the rows array:

matrix             0  1  2  ... b-1
[row 0]    ->   [] [] []  ...  []
[row 1]    ->   [] [] []  ...  []
....
[row a-1] ->   [] [] []  ...  []

So actually the matrix is an array of pointers  (the left column in the diagram) and the second index works horizontal for each row (pointer).

for a fixed-sized matrix it is different:

const int a = 4;
const int b = 5;
int matrix[a][b] = { 0 };

0  1  2 3 4
[] [] [] [] []
[] [] [] [] []
[] [] [] [] []
[] [] [] [] []

Here we have no additional pointer array but only one int array of 20 elements accessible as 4 rows with 5 columns each.

Nevertheless for both we have the same expression when accessing one single element:

    matrix[2][3] = 12345;



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David Paris VicenteSystems and Comunications  Administrator Author Commented:
I´m gone try.
Thanks
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mrjoltcolaCommented:
Another approach is to just allocate it in a linear 1 dimensional array, and write an "access(arr, row, col)" method. Arrays are laid out like that anyway.

int *matrix = new int[ ROWSIZE * COLSIZE ];

int access(int * matrix, int row, int col) {
    return matrix[ ROWSIZE * row + col];
}
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David Paris VicenteSystems and Comunications  Administrator Author Commented:
I believe is this that i want>

void matriz()
{
int a,b,c=1;
int **matrix;
cout<< "Insert the size of the Matrix.\n";
cin>>a;
b=a;

for (int i=0; i<a;++i)
{
matrix= new int*[a];
for (int j=0; j<a; ++j)
{
matrix[i]= new int [b];
matrix[i][j]= c;
c++;
cout <<matrix[i][j]<<" ";
}
cout << endl;
}
}
Maybe it´s not the write way but it work.
Can you give me your feedback please.
If you agree with this way i will close this question and give the points.
Thank you all
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mrjoltcolaCommented:
Thats not what you want because the statement:

   matrix = new int *[a];

is _inside_ a for loop. So it will overwrite matrix each time. Move that statement above the loop.

After that, I think it looks good.
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Infinity08Commented:
>> I believe is this that i want>

As mrjoltcola already pointed out, there are a few problems with that code, memory leaks not being the least of it. But note that you posted correct code earlier here : http:#24059085. Why change that ?
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David Paris VicenteSystems and Comunications  Administrator Author Commented:
Thanks to you all.
Antoher Question it is possible to call another function inside of a function?
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