Conversion from 3D array to 1D array fails

Question

The following program converts 3D array into 1D array with help of pointers . Some conversion error is coming . The line in which error is coming contains assignment operator with pointer to pointer to int type on both sides .

#include<iostream>
using namespace std ;
int main()
{
// REPLACING 3D ARRAY WITH A 2D ARRAY AND THEN INTO A 1D ARRAY USING               POINTERS  .  
int abc [2][2][3] ;
int **p[3][2] ;
int *c[6] ;
//          // abc gives address of first element of 3d array ie first 2d   array .

// abc is pointer to pointer to pointer to int type .

int i , j ;     // * abc represents address of first 1d array of first 2d array .
for (i=0 ; i<=2 ; i++) // *abc +1:address of second 1d array of first 2d 
{                            // array .
for (j=0 ; j<=1 ; j++)
{
p[i][j] =  *(abc+i )  + j ; // conversion error comes here.

} 
}

for (i=0 ; i<=5 ; i++) 
{
for (j=0 ; j<=1 ; j++ )     
{
c[i] = *p[i][j] ;   
}

}

// entering array elements .
for (i=0 ; i<=5 ; i++)
{
cin>>* c[i] ;   

}

// required array elements .
for (i=0 ;i<=5 ;i++)
{
cout<<*c[i]<<"    "; // 3d array elements are accessed using 1d array  
}                                                        // of pointers .
}

BTW, your p and c arrays are arrays of pointers. Is this intentional? — Thomas Matthews
– Thomas Matthews, Commented Jan 12, 2017 at 15:36
To convert multidimensional arrays into say 1d array or 2d array , we have to use pointers . This increases speed as pointer arithmetic is often faster than array indexing . — steve
– steve, Commented Jan 12, 2017 at 16:16
Can you prove that pointer arithmetic is faster than array indexing? Many processor instructions can load data using an index a lot faster than using pointer arithmetic. Some processors can perform loading from memory via pointer and an offset in one instruction. I don't see how using pointers is any faster. — Thomas Matthews
– Thomas Matthews, Commented Jan 12, 2017 at 16:27
Take the answer from my example and print the assembly language listing. Print the assembly language listing from your code using pointers. Compare. Next, profile the execution speed of both. — Thomas Matthews
– Thomas Matthews, Commented Jan 12, 2017 at 16:29

Thomas Matthews · Accepted Answer · 2017-01-12 15:44:36Z

2

One method is to use nested for loops.

Verify that your 1D array is large enough to contain the 3D slots.

int a[2][2][2];
int c[2 * 2 * 3];
unsigned int index = 0;
for (unsigned int i = 0; i < 2; ++i)
{
  for (unsigned int j = 0; j < 2; ++j)
  {
    for (unsigned int k = 0; k < 3; ++k)
    {
      c[index++] = a[i][j][k];
    }
  }
}

Note: no pointers were required nor harmed in the above example.

answered Jan 12, 2017 at 15:44

Thomas Matthews

58.1k18 gold badges105 silver badges165 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Conversion from 3D array to 1D array fails

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related