Two Dimensional Array Implementation Using Double Pointer
c, double-pointer, multidimensional-array
Solution
`*(ptr+i)` is equals to `ptr[i]` and `*(ptr+1)` is `ptr[1]`.
You can think, a 2-D array as array of array.
- `ptr` points to complete 2-D array, so `ptr+1` points to next 2-D array.
In figure below `ptr` is 2-D and number of columns are `3`
Original figure made by Mr. Kerrek SB, here , you should also check!
+===============================+==============================+====
|+---------+----------+--------+|+----------+---------+--------+|
||ptr[0,0] | ptr[0,1] | ptr[0,2]|||ptr[1,0] |ptr[1,1] | ptr[1,2]|| ...
|+---------+----------+--------+++----------+---------+--------++ ...
| ptr[0] | ptr[1] |
+===============================+===============================+====
ptr
`*(*ptr+1) = *( ptr[0] + 1 ) = ptr[0][1]`
Understand following:
`ptr` points to complete 2-D.
`*ptr = *(ptr + 0) = ptr[0]` that is first row.
`*ptr + 1 = ptr[1]` means second row
`*(*ptr+1) = *(*(ptr + 0) + 1 ) = *(ptr[0] + 1) = ptr[0][1]`
Array 0 Elements:
1 2 3 4
And GDB Output:
(gdb) p *(*ptr+1)
$1 = 2
that is correct `2` this can be read using `ptr[0][1]`.
Problem
Please consider the following code: ``` #include <stdio.h> #include <stdlib.h> #define NUM_ARRAYS 4 #define NUM_ELEMENTS 4 #define INVALID_VAL -1 int main() { int index = INVALID_VAL; int array_index = INVALID_VAL; int **ptr = NULL; ptr = malloc(sizeof(int*)*NUM_ARRAYS); if (!ptr) { printf ("\nMemory Allocation Failure !\n\n"); exit (EXIT_FAILURE); } for (index=0; index<NUM_ARRAYS; index++) { *(ptr+index) = malloc(sizeof(int)*NUM_ELEMENTS); if (!*(ptr+index)) { printf ("\nMemory Allocation Failure !\n"); exit (EXIT_FAILURE); } } /* Fill Elements Into This 2-D Array */ for (index=0; index<NUM_ARRAYS; index++) { for (array_index = 0; array_index<NUM_ELEMENTS; array_index++) { *(*(ptr+index)+array_index) = (array_index+1)*(index+1); } } /* Print Array Elements */ for (index = 0; index<NUM_ARRAYS; index++) { printf ("\nArray %d Elements:\n", index); for (array_index = 0; array_index<NUM_ELEMENTS; array_index++) { printf (" %d ", *(*(ptr+index)+array_index)); } printf ("\n\n"); } return 0; } ``` There is no problem with my code. It works fine. ``` Output: Array 0 Elements: 1 2 3 4 Array 1 Elements: 2 4 6 8 Array 2 Elements: 3 6 9 12 Array 3 Elements: 4 8 12 16 ``` I have a question about pointer arithmetic: `*(ptr+0)` = Pointer to COMPLETE BLOCK (First Array) `*(ptr+1)` = Pointer to COMPLETE BLOCK (Second Array). But what is: `(*ptr+1)`? GDB Output: ``` (gdb) p *(*ptr+1) $1 = 2 (gdb) p *(*ptr+2) $2 = 3 (gdb) p *(*ptr+3) $3 = 4 (gdb) p *(*ptr+4) $4 = 0 ``` I am getting confused on this. Please provide me some explanation to resolve this doubt.