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carrayspointersmatrix
Josh
Josh

Reputation: 3321

Pointing to a Matrix

I wanted to declare a pointer which would point to a matrix and retrieve a value back from the matrix:

float *p;
float ar[3][3];

[..]//give values to ar[][]

p = ar;

//Keep on printing values in the 3 X 3 matrix
for (int i = 0; i < 10; i++)
{
p = p + i;
cout << *p << ", ";
}

Upvotes: 0

Views: 1362

Answers (2)

AusCBloke
AusCBloke

Reputation: 18532

EDIT2: I'm an idiot I accidentally had float **matrix instead of float (*matrix)[3]. caf had the right answer all along.

Is this what you want?

#include <stdio.h>
#include <stdlib.h>

void print_matrix(float (*matrix)[3], size_t rows, size_t cols)
{
   int i, j;
   for (i = 0; i < rows; i++)
      for (j = 0; j < cols; j++)
         printf("%f ", matrix[i][j]);
}

int main(void)
{
   float ar[][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} };
   print_matrix(ar, 3, 3);

   return EXIT_SUCCESS;
}

EDIT: you can also have:

float *row1, *row2, *row3;
row1 = ar[0];
row2 = ar[1];
row3 = ar[2];
...
float row1_total = row1[0] + row1[1] + row2[2];

Upvotes: 0

caf
caf

Reputation: 239341

I suspect that you are after:

p = &ar[0][0];

which can also be written:

p = ar[0];

although your for loop then needs to use p = p + 1; rather than p = p + i;.

You can also use a pointer to an array, if you want your loop to be able to access the members of the matrix by row and column:

float (*p)[3];

p = ar;

for (int i = 0; i < 3; i++)
    for (j = 0; j < 3; j++)
    {
        cout << p[i][j] << ", ";
    }

Upvotes: 3

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