Pointer for 2D array seems to be 3D in C Post: 302999243

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Top Forums Programming Pointer for 2D array seems to be 3D in C Post 302999243 by yifangt on Thursday 15th of June 2017 07:07:42 PM

06-15-2017

Registered User

Pointer for 2D array seems to be 3D in C

I am struggling with the pointer to 2D-array (cf: 2D array of pointers). Can anybody help me elaborate how the pointer x moves in the memory to access the individual of y[2][6], especially the high lighted lines?
I have talked to one of the curators of the forum, but I am still not quite clear.
Here is my code:

Code:

#include<stdio.h>

int main(void)
{
    int (*x)[2][6];                 //pointer for integers array in size of 2x6 (2 rows x 6 columns),
                                    //.i.e the array is always with size of 12?
//    int (*a[8])[5];                 //Line 9: a is a pointer array of size 8, each for integer array of size 5 

    int y[2][6] = {{11,12,13,14,15,16},
                   {21,22,23,24,25,26}};    //2D array of integers
    int *z;                      //pointer to integer
    int i;

    z = y[0];
    for(i = 0;i<6;i++)
        printf("%d ",z[i]);
    printf("\n");

    x = &y;    // More complicated situation for me!
    x = y;     // Warning: incompatible pointer type.
//    x[0][0] = y[0][0];     // won't work
        printf("   (x[0][0]): %p\n",  x[0][0]);
        printf("  *(x[0][0]): %d\n",*(x[0][0]));        //Q1a
        printf("  x[0][0][0]: %d\n",x[0][0][0]);        //Q1b
        printf("*(x[0][0]+1): %d\n",*(x[0][0]+1));      //Q1c
//        printf("*(x[0][0]+2): %d\n",*(x[0][0]+2));
//        printf("*(x[0][0]+3): %d\n",*(x[0][0]+3));
//        printf("*(x[0][0]+4): %d\n",*(x[0][0]+4));
        printf("*(x[0][0]+5): %d\n",*(x[0][0]+5));

        printf("    x[0][1]: %p\n",   x[0][1]);        //Q2a
        printf("    *x[0][1]: %d\n",*(x[0][1]));
        printf("  x[0][1][0]: %d\n",x[0][1][0]);       //Q2b 
        printf("*(x[0][1]+1): %d\n",*(x[0][1]+1));     //Q2c 
//        printf("*(x[0][1]+2): %d\n",*(x[0][1]+2));
//        printf("*(x[0][1]+3): %d\n",*(x[0][1]+3));
        printf("*(x[0][1]+4): %d\n",*(x[0][1]+4));
        printf("*(x[0][1][4]): %d\n",*(x[0][4]));
        
    printf("&y: %p\n", &y);
    printf(" y: %p\n",  y);
  
    printf(" x: %p\n",  x);
    printf("&x: %p\n", &x);
    
    return 0;
}

1) Although y and &y are the same, but x = y issues warning;
2) Q1a/Q2a is the part I think I understand which is the first element of each row of y.
3) but Q1b/c, and Q2b/c turns out to be 3-D to me.
Can anybody give me a diagram how pointer x moves in the memory for each member of y?
4) Line 9: int (*a[8])[5]; is related, and I put it here for future reference but skip it at this moment.

Thanks a lot!

yifangt

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LEARN ABOUT BSD

alphasort

SCANDIR(3)						     Library Functions Manual							SCANDIR(3)

NAME

       scandir, alphasort - scan a directory

SYNOPSIS

       #include <sys/types.h>
       #include <sys/dir.h>

       scandir(dirname, namelist, select, compar)
       char *dirname;
       struct direct *(*namelist[]);
       int (*select)();
       int (*compar)();

       alphasort(d1, d2)
       struct direct **d1, **d2;

DESCRIPTION

       Scandir reads the directory dirname and builds an array of pointers to directory entries using malloc(3).  It returns the number of entries
       in the array and a pointer to the array through namelist.

       The select parameter is a pointer to a user supplied subroutine which is called by scandir to select which entries are to  be  included	in
       the  array.  The select routine is passed a pointer to a directory entry and should return a non-zero value if the directory entry is to be
       included in the array.  If select is null, then all the directory entries will be included.

       The compar parameter is a pointer to a user supplied subroutine which is passed to qsort(3) to sort the completed array. If this pointer is
       null, the array is not sorted.  Alphasort is a routine which can be used for the compar parameter to sort the array alphabetically.

       The memory allocated for the array can be deallocated with free (see malloc(3)) by freeing each pointer in the array and the array itself.

SEE ALSO

       directory(3), malloc(3), qsort(3), dir(5)

DIAGNOSTICS

       Returns -1 if the directory cannot be opened for reading or if malloc(3) cannot allocate enough memory to hold all the data structures.

4.2 Berkeley Distribution					September 17, 1985							SCANDIR(3)