2D arrays in pointer fashion issues

[momotaro]

I have a piece of code that is supposed to fill a 2D array and print it out using pointers, the problem in stead I get only a part of it correct!
here is the code:

#include <stdio.h>
 
#define MAXROW 2
#define MAXCOL 2
 
main()
{
    int i, j, T[MAXROW][MAXCOL];
 
//filling
    for(i = 0; i < MAXROW; i++)
    {
        for(j = 0; j < MAXCOL; j++)
        {
            printf("Please enter your value (%d,%d): \n", i, j);
            scanf("%d", T+i+(j*MAXROW));
        }
    }
 
//prompting
    for(i = 0; i < MAXROW; i++)
    {
        for(j = 0; j < MAXCOL; j++)
            printf("%d\t", T[i][j]);
        printf("\n");
    }
    getch();
}

example:

I fill it with 1 2 3 4 I get: 1 -xxxxxx 3 -xxxxxx

Please help

[donbock]

Refer to Arrays Revealed for general information about 2D arrays.

[JonathanS]

in your scanf, your T should be T[0] (so that your pointer is correct). To correct your other mistake, draw your array out on a piece of paper from the point of view of how it is stored in memory.

[momotaro]

Jonathans thx for the help but you didn't get my pointer :p I mean my point :d
am using JUST pointers

thx anyway

[JonathanS]

T[0] is a pointer since this is a 2-D array . If you want to you could substitute &T[0][0] (see the Arrays Revealed that Don had recommended).

I was commenting before that you have the calculation i+j*MAXROW, but that's not how arrays are laid out in memory, where the column is offset into it's particular row.

[Banfa]

Do you mean that you don't want to do this?

scanf("%d", &T[i][j]);

Which is the obvious solution.

Your problem with

T+i+(j*MAXROW)

is that the arithmetic is wrong.

T[MAXROW][MAXCOL]

is not MAXCOL array of MAXROW arrays but the other way around MAXROW array of MAXCOL arrays.

Put another way if you have an array

char T[2][3];

the memory map of elements in the array looks something like

-------------------------
|T[0][0]|T[0][1]|T[0][2]|
-------------------------
|T[1][0]|T[1][1]|T[1][2]|
-------------------------
 

but the way you have coded it it would have to be like this

-----------------
|T[0][0]|T[1][0]|
-----------------
|T[0][1]|T[1][1]|
-----------------
|T[0][2]|T[1][2]|
-----------------
 

to work

[vipin sharma]

hello,

I have slightly modified given code. When I am using T[i] +j to display address, i am getting correct addresses of array T in output.

But when I am using T + i*MAXCOL +j (mathematically this is correct), I am getting incorrect answer. In the output instead of getting 4 bytes increment one by one to base address I am seeing 8 bytes increment.

Can anyone explain why it is acting differently?

#include <stdio.h>
 
#define MAXROW 2
#define MAXCOL 2
 
int main()
{
    int i, j, T[MAXROW][MAXCOL];
 
 
 
    for(i = 0; i < MAXROW; i++)
        {
            for(j = 0; j < MAXCOL; j++)
                printf("%x\t", &T[i][j]);
            printf("\n");
        }
 
//filling
 
    printf("\nUSING T[i]+j\n");
    for(i = 0; i < MAXROW; i++)
    {
        for(j = 0; j < MAXCOL; j++)
        {
            printf("Address of  (%d,%d): ", i, j);
            printf("%x\t", T[i]+j);
        }
    printf("\n");
    }
 
    printf("\nUSING T+i*MAXCOL +j\n");
    for(i = 0; i < MAXROW; i++)
    {
        for(j = 0; j < MAXCOL; j++)
        {
            printf("Address of  (%d,%d): ", i, j);
            printf("%x\t", T+ i*MAXCOL +j);
        }
    printf("\n");
    }
/*
//prompting
    for(i = 0; i < MAXROW; i++)
    {
        for(j = 0; j < MAXCOL; j++)
            printf("%d\t", T[i][j]);
        printf("\n");
    }
//    getch();
*/
    return 0;
}
 

OUTPUT

bfbeb10c    bfbeb110    
bfbeb114    bfbeb118    
 
USING T[i]+j
Address of  (0,0): bfbeb10c    Address of  (0,1): bfbeb110    
Address of  (1,0): bfbeb114    Address of  (1,1): bfbeb118    
 
USING T+i*MAXCOL +j
Address of  (0,0): bfbeb10c    Address of  (0,1): bfbeb114    
Address of  (1,0): bfbeb11c    Address of  (1,1): bfbeb124    

[Banfa]

vipin sharma to answer your question you need to ask yourself what type is T interpreted as when used as you have done in pointer arithmetic.

Here's a clue, it is not int *.