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adding an integer to a pointer while creating it (pointer).

Posted on 2006-04-10
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Last Modified: 2010-04-15
Can someone please explain me what the folowing code is doing ? I have a problem at , why the pointer is being subtracted from the initial value while returning it. What does it achieve ? And the same (being added) while freeing it.

/*
 * Allocates memory for an double vector from [beg...end]
 */
double  *Dec_Vector(int beg, int end)
{
  double       *vec;
 
  if( (end - beg +1) <= 0)
     {printf("In Dec_Vector, Chk the parameters\n");exit(1); }

  vec = (double *)malloc((end-beg+1)*sizeof(double));
  if (vec == NULL) Abort("Error allocating memory in Dec_vector, Aborting ...\n");
  return(vec-beg);
 
}

/*
 * Free a dec vector
 */
void Free_Dec(double *vec, int beg, int end)
{
  free(vec+beg);
}
For a 2D array ....

/*
 * Allocate memory for a double matrix [begrow...endrow][begcol...endcol]
 */
double  **Dec_Matrix(int begrow, int endrow, int begcol, int endcol)
{
  double       **mat;
  int             i;
 

  mat = (double **)malloc( (endrow-begrow+1)*sizeof(double*));
  if (mat == NULL) Abort("Error allocating memory in Dec_Matrix, Aborting ...\n");
  mat -= begrow;
  for (i=begrow; i<=endrow; i++)
    {
       mat[i] = (double *)malloc((endcol-begcol+1)*sizeof(double));
       if (mat[i] == NULL) Abort("Error allocating memory in Dec_Matrix, Aborting ...\n");
       mat[i] -= begcol;
    }
  return(mat);
 
}

/*
 * Free a Dec matrix
 */
void Free_Decmatrix(double **mat, int begrow, int endrow, int begcol, int endcol)
{
  int             i;


  for(i=begrow; i<=endrow; i++)
      free(mat[i]+begcol);
  free(mat+begrow);

}


 And the array is referenced in the main program as below with their starting int values....
 *aa = Dec_Matrix(1,nclone,0,nprobe);
  for (i=1; i<=nclone; i++)
        for (j=0; j<=nprobe; j++)
    ----- here goes some code


 I  dont understand that why not just return the array without subtracting and indices references be just from 0 to its length...

thanks a lot for explaing this code snippet ..

Susanta
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Comment
Question by:code4susant
5 Comments
 
LVL 48

Accepted Solution

by:
AlexFM earned 1200 total points
ID: 16420029
Maybe this is done to use it by such way:

double* d = Dec_Vector(10, 20);

d[10] = 1.0;
d[11] = 2.0;
...
d[20] = 0.0;

If beg > 0, Dec_Vector returns pointer to incorrect memory location which should not be accessed directly. But using this pointer in index in  (beg, end) range is OK.
0
 
LVL 14

Assisted Solution

by:hoomanv
hoomanv earned 400 total points
ID: 16420849
AlexFM is correct

as you know d[10] is equivalent to *(d+10)

it just forces you to access the vector/matrix elements by indices in the specified range
0
 
LVL 16

Assisted Solution

by:PaulCaswell
PaulCaswell earned 400 total points
ID: 16420916
Hi code4susant,

Although I agree that this is a strange technique, there are some neat uses of it.

double v = Dec_Vector(-1,1);
int i;

for ( i = -1; i <= 1; i++ ) v[i] = ...;

See how it turns the sometimes unnatural need to index from 0 upwards to a much more natural walk through using a range that feels right?

Paul
0
 
LVL 16

Expert Comment

by:PaulCaswell
ID: 16420955
Hi code4susant,

Better still, say you are coding a Tic-Tac-Toe game.

double  ** board = Dec_Matrix ( -1, 1, -1, 1 );

You can now use board[-1][-1] to access one corner and board[1][1] for the opposite corner. That feels far more natural to me that the two corners being board[0][0] and board[2][2]!

Paul
0
 

Author Comment

by:code4susant
ID: 16421436
Thank you all for such a prompt reply !  This is life at its best !

AflexFM hit the point instantly,hoomany spelled it out and thank you Paul for motivating its use . Overall , it was an excellent reply !

thanks again,
bye,
code4susant
0

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