2D array manipulation using a pointer in C

Question

Let us say I have a function which manipulates a 2D array which receives a pointer to the 2D array from the main function as its parameter.

Now, I want to modify(assume add 10 to each element) each element of the 2D array.

I am interested in knowing about traversing through the 2D array with a single pointer given to me and return the pointer of the newly modified array.

Rough Structure

Assume pointer a contains the initial address of the 2D array.

int add_10(int *a)
{
    int i, j,
        b[M][N] = {0};

    for(i = 0; i < M; i++)
        for(j = 0; j < N; j++)
            b[i][j] = 10 + a[i][j];
}

Try searching "2d array c" on stack overflow. You might find a few answers. — Brian Roach
– Brian Roach, Commented Oct 31, 2011 at 0:48
It depends on whether you have pointer to an array (int (*a)[N])), or a pointer to a pointer to int (int**); you'll have to declare your function accordingly. The syntax a[i][j] is the same in both cases, though. — Kerrek SB
– Kerrek SB, Commented Oct 31, 2011 at 0:50

Mateen Ulhaq · Accepted Answer · 2011-10-31 03:26:26Z

0

int* add_10(const int *dest,
            const int *src,
            const int M,
            const int N)
{
    int *idest = dest;

    memmove(dest, src, M * N * sizeof(int));

    for(int i = 0; i < (M * N); ++i)
        *idest++ += 10;

    return dest;
}

edited Oct 31, 2011 at 3:26

answered Oct 31, 2011 at 0:54

Mateen Ulhaq

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7 Comments

Dave Over a year ago

This breaks if he's using 2d arrays, or pointer-to-array types, rather than an array of arrays.

Mateen Ulhaq Over a year ago

@Dave ...Is *(*(dest + i) + j) = 10 + *(*(source + i) + j); better? I would have thought both versions were correct.

Dave Over a year ago

No, you're using a fundamentally different type. int k[5][5] is a contiguous block of 25 ints. It it not an array of arrays.

Dave Over a year ago

There is no function that would be guaranteed to work for both; but the op asked about 2d arrays.

Mateen Ulhaq Over a year ago

@Dave So... Is this better? :)

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