Pointer to variable length array

**dungeon** · 08-05-2020

Refer below code for variable length array:

Code:

#include <stdio.h>


int main()
{
    int row, col;


    printf("Enter row: ");
    scanf("%d", &row);


    printf("Enter column: ");
    scanf("%d", &col);


    int X[row][col], i, j;


    printf("Enter the numbers:\n");


    for(i=0; i<row; i++)
    {
        for(j=0; j<col; j++)
        {
            scanf("%d", &X[i][j]);
        }
    }


    printf("You have entered...\n");


    for(i=0; i<row; i++)
    {
        for(j=0; j<col; j++)
        {
            printf("%d\n", X[i][j]);
        }
    }


    return 0;
}

Now in order to retrieve the array values using an UDF (display), I can write following code in main:

Code:

int (*p)[c];
p=X;
void display(X);

But how do I write the function as compiler doesn't allow me to define something like:

Code:

void  display(int (*p)[c])
{
    //some code
}

Would appreciate any guidance in this regard.

**laserlight** · 08-05-2020

Try:

Code:

void  display(int row, int col, int x[row][col])

I rarely use VLAs so I could be mistaken, but I believe you could leave out the row in the brackets as it's the usual array decay to pointer, but you do need col.

**dungeon** · 08-05-2020

Originally Posted by laserlight

Try:

Code:

void  display(int row, int col, int x[row][col])

I rarely use VLAs so I could be mistaken, but I believe you could leave out the row in the brackets as it's the usual array decay to pointer, but you do need col.

Thanks... but do you mean to say that we can't achieve this task (in C) using a pointer to the array?

**laserlight** · 08-05-2020

x would still be a pointer to the first element of the variable length array

**dungeon** · 08-05-2020

Originally Posted by laserlight

x would still be a pointer to the first element of the variable length array

true but... there is difference between passing a pointer to the first element & passing a pointer to the (whole) array

**laserlight** · 08-05-2020

Originally Posted by dungeon

there is difference between passing a pointer to the first element & passing a pointer to the (whole) array

Yes, but unless you're dealing with an array of array of arrays, there's usually no point passing a pointer to an entire array of arrays. Hence, what you want here is a pointer to the first element of the array of arrays, i.e., an array itself.

EDIT:
I read that again and my apologies, it's probably more confusing.

Firstly, remember that when you pass an array as an argument, it is converted to a pointer to its first element. If you pass a 2D array, the same thing happens, except that an element is itself an array.

When I wrote that "x would still be a pointer to the first element of the variable length array", that's what I meant: x would be a pointer to the first element of the variable length array, and since the array is a 2D array, that first element would also be an array.

**dungeon** · 08-05-2020

Originally Posted by laserlight

Yes, but unless you're dealing with an array of array of arrays, there's usually no point passing a pointer to an entire array of arrays. Hence, what you want here is a pointer to the first element of the array of arrays, i.e., an array itself.

EDIT:
I read that again and my apologies, it's probably more confusing.

Firstly, remember that when you pass an array as an argument, it is converted to a pointer to its first element. If you pass a 2D array, the same thing happens, except that an element is itself an array.

When I wrote that "x would still be a pointer to the first element of the variable length array", that's what I meant: x would be a pointer to the first element of the variable length array, and since the array is a 2D array, that first element would also be an array.

Thanks...

made it work using below function prototype & function call in main

Code:

    void display(int, int, int[][col]);
    display(row, col, X);

and below function definition

Code:

void display(int row, int col, int A[][col])
{
    int i, j;


    for(i=0; i<row; i++)
    {
        for(j=0; j<col; j++)
        {
            printf("%d\n", *(*(A+i)+j));
        }
    }
}

**laserlight** · 08-05-2020

That's good to hear. For ease of reading, *(*(A+i)+j)) should be written as A[i][j]

**dungeon** · 08-05-2020

Originally Posted by laserlight

That's good to hear. For ease of reading, *(*(A+i)+j)) should be written as A[i][j]

Sure will consider this henceforth... I read that accessing array elements using pointer is much faster than using subscript but as of now it doesn't matter much to me...
PS: As array dimensions need to be explicitly passed on along with pointer to the array, I must declare the function prototype inside main function (and that too only after initializing variables 'row' & 'col'). Please do share even if at some later stage, if you happen to come across some means to figure out the dimensions using just pointer to the array...

**laserlight** · 08-05-2020

Originally Posted by dungeon

I read that accessing array elements using pointer is much faster than using subscript

They are identical except for spelling.

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