CODEWITHSUNDEEP
c++cmultidimensional-arrayimplicit-conversionexplicit-conversion
Sanjeev
Sanjeev

Reputation: 125

Why can't we directly assign the address of a 2D array to a pointer?

Why can't we directly assign the address of a 2D array to a pointer?

Is there any way we can assign it in a single line, not with the help of a for loop?

Is there any other better approach?

// Array of 5 pointers to an array of 4 ints 

int (*Aop[5])[4];

for(int i = 0;i<5;i++) 
{
    Aop[i] = &arr2[i];  //Works fine
} 
 
//Why this doesn't work int 
int (*Aop[5])[4] = &arr2[5][4]

Upvotes: 0

Views: 204

Answers (2)

Vlad from Moscow
Vlad from Moscow

Reputation: 311156

This declaration

int (*Aop[5])[4];

does not declare a pointer. It is a declaration of an array of 5 pointers to one-dimensional arrays of the the int[4].

Arrays do not have the assignment operator. However you could initialize it in its declaration as for example

int (*Aop[5])[4] = { &arr2[0], &arr2[1], &arr2[2], &arr2[3], &arr2[4] };

or

int (*Aop[5])[4] = { arr2, arr2 + 1, arr2 + 2, arr2 + 3, arr2 + 4 };

Here is a demonstrative program.

#include <stdio.h>

int main(void) 
{
    enum { M = 5, N = 4 };
    
    int arr2[M][N] =
    {
        { 1, 1, 1, 1 },
        { 2, 2, 2, 2 },
        { 3, 3, 3, 3 },
        { 4, 4, 4, 4 },
        { 5, 5, 5, 5 }
    };
    
    int (*Aop[M])[N] = { arr2, arr2 + 1, arr2 + 2, arr2 + 3, arr2 + 4 };
    
    for ( size_t i = 0; i < M; i++ )
    {
        for ( size_t j = 0; j < N; j++ )
        {
            printf( "%d ", ( *Aop[i] )[j] );
        }
        putchar( '\n' );
    }
    
    return 0;
}

The program output is

1 1 1 1 
2 2 2 2 
3 3 3 3 
4 4 4 4 
5 5 5 5

If you need to declare a pointer to the array arr2 then you should bear in mind that array designators are implicitly converted (with rare exceptions) to pointers to their first elements. The array arr2 has elements of the type int[4]. So a pointer to an object of this type will have the type int ( * )[4]. So you can write

int (*Aop )[4] = arr2;

Here is another demonstrative program that uses pointers in for loops to output elements of the array arr2.

#include <stdio.h>

int main(void) 
{
    enum { M = 5, N = 4 };
    
    int arr2[M][N] =
    {
        { 1, 1, 1, 1 },
        { 2, 2, 2, 2 },
        { 3, 3, 3, 3 },
        { 4, 4, 4, 4 },
        { 5, 5, 5, 5 }
    };
    
    int (*Aop )[N] = arr2;
    
    for ( int ( *p )[N] = Aop; p != Aop + M; ++p )
    {
        for ( int *q = *p; q != *p + N; ++q )
        {
            printf( "%d ", *q );
        }
        putchar( '\n' );
    }
    
    return 0;
}

Again the program output is

1 1 1 1 
2 2 2 2 
3 3 3 3 
4 4 4 4 
5 5 5 5

Upvotes: 2

Ben Voigt
Ben Voigt

Reputation: 283941

These work just fine to declare a pointer to the entire array:

int (*Ap)[4] = arr2;

int (*Bp)[5][4] = &arr2;

The size of an array only appears in its declaration, and NOT when passing it around. arrs[5][4] is an out-of-bounds array access (out of bounds by a whole row and a whole column, so you can't even use legally it as the past-the-end reference by avoiding lvalue-to-rvalue conversion)

Upvotes: 0

Related Questions