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C++ Discussion Thread
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Oct 2nd, 2009

Dynamic Memory allocation

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01 Why i am not able to do this

C++ Syntax (Toggle Plain Text)
int* px;
px = new int[2][2];
int* px;
px = new int[2][2];

other the px = new int[2*2]; is there any other method

02 Why i am not able do access through pointer dx by dx[1][2] other than by using dx[1*7+2]

cpp Syntax (Toggle Plain Text)
static float mat[5][7] =
	{
	        -5, -4,  0,  0,  0,  0,  0,
		 6,  4,  1,  0,  0,  0, 24,
		 1,  2,  0,  1,  0,  0,  6,
		-1,  1,  0,  0,  1,  0,  1,
		 0,  1,  0,  0,  0,  1,  2
	};
	float *dx=&sim_max[0][0];
	cout<<dx[1][0];
static float mat[5][7] =
	{
	        -5, -4,  0,  0,  0,  0,  0,
		 6,  4,  1,  0,  0,  0, 24,
		 1,  2,  0,  1,  0,  0,  6,
		-1,  1,  0,  0,  1,  0,  1,
		 0,  1,  0,  0,  0,  1,  2
	};
	float *dx=&sim_max[0][0];
	cout<<dx[1][0];

03 why i am not able to directly initialize array of new dynamically alloc. mem to

C++ Syntax (Toggle Plain Text)
float *dx= new float[5*7]
	(
		-5, -4,  0,  0,  0,  0,  0,
		 6,  4,  1,  0,  0,  0, 24,
		 1,  2,  0,  1,  0,  0,  6,
		-1,  1,  0,  0,  1,  0,  1,
		 0,  1,  0,  0,  0,  1,  2
	);
float *dx= new float[5*7]
	(
		-5, -4,  0,  0,  0,  0,  0,
		 6,  4,  1,  0,  0,  0, 24,
		 1,  2,  0,  1,  0,  0,  6,
		-1,  1,  0,  0,  1,  0,  1,
		 0,  1,  0,  0,  0,  1,  2
	);

Last edited by Rhohitman; Oct 2nd, 2009 at 3:38 pm.

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Rhohitman

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Oct 2nd, 2009

Re: Dynamic Memory allocation

Well Dynamic 2D arrays take a little more complexity.This is mainly because when you declare something like this

C++ Syntax (Toggle Plain Text)
int *p;
p= new int [2][2];
int *p;
p= new int [2][2];

What you are trying to do is assigning a pointer array to a pointer itself.
THe new int [2][2] converts itself into a pointer array in this case it is something like int * [2] So In order to Dynamically Create a 2D array you will need to do something like this.

C++ Syntax (Toggle Plain Text)
 int *p,**q;
   q=new int* [2];
   q[0]=new int [2];
   q[1]=new int [2];
   delete []q[0];
   delete [] q[1];
   delete [] q;
 int *p,**q;
   q=new int* [2];
   q[0]=new int [2];
   q[1]=new int [2];
   delete []q[0];
   delete [] q[1];
   delete [] q;

So now you have a 2d array whose elements you can access.

Sky Diploma

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Oct 2nd, 2009

Re: Dynamic Memory allocation

I've always found it simpler to allocate a one-dimensional array, and then calculating the row and column manually, it's an easy approach, and it works.

tux4life

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Oct 2nd, 2009

Re: Dynamic Memory allocation

Thanx Sky Diploma
here things got too much complicated

C++ Syntax (Toggle Plain Text)
Temp2=T1->dx[i*T1->n+T1->n-1] / T1->dx[i*T1->n+pivot_j];
Temp2=T1->dx[i*T1->n+T1->n-1] / T1->dx[i*T1->n+pivot_j];

i guess i have to use the loop for assigning the value inside it..

Last edited by Rhohitman; Oct 2nd, 2009 at 4:30 pm.

Rhohitman

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Oct 2nd, 2009

Re: Dynamic Memory allocation

Yes, Btw, Dont forget deleting each one of the dynamic pointers . Before you delete the pointer to pointer.
And yea, Loops could assign values.
Another solution would be to use vectors.

Sky Diploma

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