Basics of Integer Double Pointers
=====================================

In this section, you are going to learn

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                How to use Double Pointers ?

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		Basics of Double Pointers

		* :ref:`1 Double Pointer, 1 Single Pointer : Simple variables <basic_ptr_int_dp_sp_ex1>`
		* :ref:`1 Double Pointer, 1 Single Pointer : With Single pointer pointing to arrays <basic_ptr_int_dp_sp_ex2>`
		* :ref:`1 Double Pointer, 1 Single Pointer : With Single pointer heap allocation <basic_ptr_int_dp_sp_ex3>`
		* :ref:`1 Double Pointer : With two heap allocations : Create 1x1 array <basic_ptr_int_dp_sp_ex4>`
		* :ref:`1 Double Pointer : With two heap allocations : Create 1x10 array <basic_ptr_int_dp_sp_ex5>`
		* :ref:`1 Double Pointer : Pointing to array of single pointers : Static <basic_ptr_int_dp_sp_ex6>`
		* :ref:`1 Double Pointer : Pointing to array of single pointers : Dynamic <basic_ptr_int_dp_sp_ex7>`
		* :ref:`1 Double Pointer : Pass by Reference <basic_ptr_int_dp_sp_ex9>`

.. _basic_ptr_int_dp_sp_ex1:

.. tab-set::

        .. tab-item:: 1 Double Pointer, 1 Single Pointer : Simple variables

		* Step 1 : Define a integer

		.. code-block:: c

			int c = 65;

		* Step 2 : Define a Single Pointer

		.. code-block:: c

		        int *sp = &c;

		OR

		.. code-block:: c

		        int *sp;

		        sp = &c;

		* Step 3 : Define a Double Pointer

		.. code-block:: c

		        int **dp = &sp;

		OR

		.. code-block:: c

		        int **dp;

		        dp = &sp;

		* Step 4 : Access user data (in this case integer) using single pointer

		.. code-block:: c

		        printf("c = %d\n", *sp);

		* Step 5 : Access user data (in this case integer) using double pointer

		.. code-block:: c

		        printf("c = %d\n", **dp);

		* Step 6 : Use ``*dp`` to point to new user data (in this case variable ``d``)

		.. code-block:: c

		       *dp = &d;

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			Remember below equations

			* dp = &sp
			* \*dp = sp
			* \*dp = &c
			* Hence changing ``*dp`` changes ``sp`` as well

		* Step 7 : Now user data can be accessed using \*sp, \*\*dp which prints value of integer ``d``

		.. code-block:: c

		        printf("d = %d\n", d);
		        printf("d = %d\n", *sp);
		        printf("d = %d\n", **dp);

		* See full program below

		.. code-block:: c

			#include <stdio.h>

			int main(void)
			{
			        int c = 65;
			        int *sp = &c;
			        int **dp = &sp;

			        printf("c = %d\n", c); 
			        printf("*sp = %d\n", *sp);
			        printf("**dp = %d\n", **dp);

			        int d = 100;

			        *dp = &d;

			        printf("d = %d\n", d);
			        printf("*sp = %d\n", *sp);
			        printf("**dp = %d\n", **dp);

			        return 0;
			}

		* Output is as below

		.. code-block:: c

			c    = 65
			*sp  = 65
			**dp = 65

			d    = 100
			*sp  = 100
			**dp = 100

.. _basic_ptr_int_dp_sp_ex2:

.. tab-set::

        .. tab-item:: 1 Double Pointer, 1 Single Pointer : With Single pointer pointing to arrays

		* Step 1 : Define a single dimension array of integers

		.. code-block:: c

                        int arr[5] = {1, 2, 3, 4, 5};

		* Step 2 : Define a single pointer

		.. code-block:: c

			int *sp = arr;

		OR

		.. code-block:: c

			int *sp;

			sp = arr;

		OR

		.. code-block:: c

			int *sp;

			sp = &arr[0];

		* Step 3 : Define a double pointer

		.. code-block:: c

			int **dp = &sp;

		OR

		.. code-block:: c

			int **dp;

			dp = &sp;

		* Step 4 : Access user data (in this case array of integers) using single pointer variable ``arr``

		.. code-block:: c

                        for (int i = 0 ; i < sizeof(arr)/sizeof(arr[0]); i++)
                        {
                                printf("arr[%d] = %d\n", i, arr[i]);
                        }

		* Step 5 : Access user data (in this case array of integers) using single pointer variable ``sp``

		.. code-block:: c

                        for (int i = 0 ; i < sizeof(arr)/sizeof(arr[0]); i++)
                        {
                                printf("sp[%d] = %d\n", i, sp[i]);
                        }

		* Step 6 : Access user data (in this case array of integers) using double pointer variable ``dp``

		.. code-block:: c

                        for (int i = 0 ; i < sizeof(arr)/sizeof(arr[0]); i++)
                        {
                                printf("(*dp)[%d] = %d\n", i, (*dp)[i]);
                        }

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        	        Note ``*dp`` should be called as single pointer because of below equations

			* dp = &sp;
			* \*dp = sp;

		* See full program below

		.. code-block:: c

			#include <stdio.h>

			int main(void)
			{
                                int arr[5] = {1, 2, 3, 4, 5};
			        int *sp = arr;
			        int **dp = &sp;

                                // Access user data (in this case array of integers) using single pointer variable ``arr``
                                for (int i = 0 ; i < sizeof(arr)/sizeof(arr[0]); i++)
                                {
                                        printf("arr[%d] = %d\n", i, arr[i]);
                                }

                                // Access user data (in this case array of integers) using single pointer variable ``sp``
                                for (int i = 0 ; i < sizeof(arr)/sizeof(arr[0]); i++)
                                {
                                        printf("sp[%d] = %d\n", i, sp[i]);
                                }

                                // Access user data (in this case array of integers) using double pointer variable ``dp``
                                for (int i = 0 ; i < sizeof(arr)/sizeof(arr[0]); i++)
                                {
                                        printf("(*dp)[%d] = %d\n", i, (*dp)[i]);
                                }

			        return 0;
			}

		* Output is as below

		.. code-block:: c

                        arr[0] = 1
                        arr[1] = 2
                        arr[2] = 3
                        arr[3] = 4
                        arr[4] = 5

                        sp[0] = 1
                        sp[1] = 2
                        sp[2] = 3
                        sp[3] = 4
                        sp[4] = 5

                        (*dp)[0] = 1
                        (*dp)[1] = 2
                        (*dp)[2] = 3
                        (*dp)[3] = 4
                        (*dp)[4] = 5

.. _basic_ptr_int_dp_sp_ex3:

.. tab-set::

        .. tab-item:: 1 Double Pointer, 1 Single Pointer : With Single pointer heap allocation

		* Step 1 : Define a single pointer

		.. code-block:: c

			int *sp;

		* Step 2 : Allocate heap memory to single pointer

		.. code-block:: c

			sp = malloc(5 * sizeof(int));

		* Step 3 : Copy User data to heap memory

		.. code-block:: c

                        sp[0] = 100;
                        sp[1] = 200;
                        sp[2] = 300;
                        sp[3] = 400;
                        sp[4] = 500;

		* Step 4 : Define a double pointer

		.. code-block:: c

			int **dp;
			dp = &sp;

		* Step 5 : Access User data using single pointer variable ``sp``

		.. code-block:: c

			printf("sp[3] = %d\n", sp[3]);

		* Step 6 : Access User data using double pointer variable ``dp``

		.. code-block:: c

			printf("(*dp)[3] = %d\n", (*dp)[3]);

		* See full program below

		.. code-block:: c

			#include <stdio.h>
			#include <stdlib.h>
			#include <string.h>

			int main(void)
			{
			        int *sp;

			        sp = malloc(5 * sizeof(int));
                                memset(sp, 0, 5 * sizeof(int));

                                sp[0] = 100;
                                sp[1] = 200;
                                sp[2] = 300;
                                sp[3] = 400;
                                sp[4] = 500;

			        int **dp;
			        dp = &sp;

			        //Access individual integer
                                printf("sp[3] = %d\n", sp[3]);
                                printf("(*dp)[3] = %d\n", (*dp)[3]);

                                // Access user data (in this case array of integers) using single pointer variable ``sp``
                                for (int i = 0 ; i < 5; i++)
                                {
                                        printf("sp[%d] = %d\n", i, sp[i]);
                                }

                                // Access user data (in this case array of integers) using double pointer variable ``dp``
                                for (int i = 0 ; i < 5; i++)
                                {
                                        printf("(*dp)[%d] = %d\n", i, (*dp)[i]);
                                }

				free(sp);

			        return 0;
			}

		* Output is as below

		.. code-block:: c

                        sp[3]    = 400
                        (*dp)[3] = 400

                        sp[0] = 100
                        sp[1] = 200
                        sp[2] = 300
                        sp[3] = 400
                        sp[4] = 500

                        (*dp)[0] = 100
                        (*dp)[1] = 200
                        (*dp)[2] = 300
                        (*dp)[3] = 400
                        (*dp)[4] = 500

.. _basic_ptr_int_dp_sp_ex4:

.. tab-set::

        .. tab-item:: 1 Double Pointer : With two heap allocations : Create 1x1 array

		* Step 1 : Define a double pointer

		.. code-block:: c

			int **dp;

		* Step 2 : Allocate memory to a double pointer

		.. code-block:: c

			dp = malloc(sizeof(int *));

		* Step 3 : Allocate memory to a single pointer

		.. code-block:: c

			*dp = malloc(sizeof(int));

		* Step 4 : Store user data

		.. code-block:: c

			**dp = 65;

		* Step 5 : Read user data

		.. code-block:: c

			printf("**dp = %d\n", **dp);

		* Step 6 : Free memory in opposite flow of allocation

		.. code-block:: c

			free(*dp);

			free(dp);

		* See full program below

		.. code-block:: c

			#include <stdio.h>
			#include <string.h>
			#include <stdlib.h>

			int main(void)
			{
			        int **dp;

			        dp = malloc(sizeof(int *));

			        *dp = malloc(sizeof(int));

			        **dp = 65;

			        printf("**dp = %d\n", **dp);

			        free(*dp);

			        free(dp);

			        return 0;
			}

		* Output is as below

		.. code-block:: c

                        **dp = 65

.. _basic_ptr_int_dp_sp_ex5:

.. tab-set::

        .. tab-item:: 1 Double Pointer : With two heap allocations : Create 1x10 array

		* Step 1 : Define a double pointer

		.. code-block:: c

			int **dp;

		* Step 2 : Allocate memory to a double pointer

		.. code-block:: c

			dp = malloc(sizeof(int *));

		* Step 3 : Allocate memory to a single pointer

		.. code-block:: c

			*dp = malloc(5 * sizeof(int));

		* Step 4 : Copy User data to heap

		.. code-block:: c

                        (*dp)[0] = 100;
                        (*dp)[1] = 200;
                        (*dp)[2] = 300;
                        (*dp)[3] = 400;
                        (*dp)[4] = 500;

		* Step 5 : Read user data from heap

		.. code-block:: c

			for (int i = 0; i < 5; i++) {
				printf("(*dp)[i] = %d\n", (*dp)[i]);
			}

		* Step 6 : Free memory in opposite flow of allocation

		.. code-block:: c

			free(*dp);

			free(dp);

		* See full program below

		.. code-block:: c
	
			#include <stdio.h>
			#include <string.h>
			#include <stdlib.h>

			int main(void)
			{
			        int **dp;

			        dp = malloc(sizeof(int *));

			        *dp = malloc(5 * sizeof(int));
			        memset(*dp, 0, 5 * sizeof(int));

                                (*dp)[0] = 100;
                                (*dp)[1] = 200;
                                (*dp)[2] = 300;
                                (*dp)[3] = 400;
                                (*dp)[4] = 500;

			        for (int i = 0; i < 5; i++) {
			                printf("(*dp)[%d] = %d\n", i, (*dp)[i]);
			        }

			        free(*dp);

			        free(dp);

			        return 0;
			}

		* See full program below

		.. code-block:: c

                        (*dp)[0] = 100
                        (*dp)[1] = 200
                        (*dp)[2] = 300
                        (*dp)[3] = 400
                        (*dp)[4] = 500

.. _basic_ptr_int_dp_sp_ex6:

.. tab-set::

        .. tab-item:: 1 Double Pointer : Pointing to array of single pointers : Static

		* Step 1 : Define 3 Single dimension integer arrays

		.. code-block:: c

			int arr0[5] = {1, 2, 3, 4, 5};
			int arr1[5] = {10, 20, 30, 40, 50};
			int arr2[5] = {100, 200, 300, 400, 500};

		* Step 2 : Define array of single pointers

		.. code-block:: c

			int *sp_arr[] = {arr0, arr1, arr2};

		* Step 3 : Use ``sp_arr`` to change contents of single dimension arrays

		.. code-block:: c

                        sp_arr[0][2] = 9;   // Same as arr0[2]
                        sp_arr[1][2] = 99;  // Same as arr1[2]
                        sp_arr[2][2] = 999; // Same as arr2[2]

		* Step 4 : Use ``sp_arr`` to access contents of single dimension arrays

		.. code-block:: c

                        for (int i = 0; i < 3; i++)
                        {
                                for (int j = 0; j < 5; j++)
                                {
                                        printf("sp_arr[%d][%d] = %d\n", i, j, sp_arr[i][j]);
                                }
                                printf("\n");
                        }

		* Step 5 : Define a double pointer

		.. code-block:: c

			int **dp;

			dp = sp_arr;

		* Step 6 : Use ``dp`` to change contents of single dimension arrays

		.. code-block:: c

                        dp[0][2] = 9;   // Same as arr0[2]
                        dp[1][2] = 99;  // Same as arr1[2]
                        dp[2][2] = 999; // Same as arr2[2]

		* Step 7 : Use ``dp`` to access contents of single dimension arrays

		.. code-block:: c

                        for (int i = 0; i < 3; i++)
                        {
                                for (int j = 0; j < 5; j++)
                                {
                                        printf("dp[%d][%d] = %d\n", i, j, dp[i][j]);
                                }
                                printf("\n");
                        }

		* See full program below

		.. code-block:: c

                        #include <stdio.h>
                        #include <stdlib.h>
                        #include <string.h>

                        int main(void)
                        {
                                int arr0[5] = {1, 2, 3, 4, 5};
                                int arr1[5] = {10, 20, 30, 40, 50};
                                int arr2[5] = {100, 200, 300, 400, 500};
                                int *sp_arr[] = {arr0, arr1, arr2};

                                // Use ``sp_arr`` to change contents of single dimension arrays
                                sp_arr[0][2] = 9;   // Same as arr0[2]
                                sp_arr[1][2] = 99;  // Same as arr1[2]
                                sp_arr[2][2] = 999; // Same as arr2[2]

                                // Use ``sp_arr`` to access contents of single dimension arrays
                                printf("------------ Access arrays using sp_arr -----------\n");
                                for (int i = 0; i < 3; i++)
                                {
                                        for (int j = 0; j < 5; j++)
                                        {
                                                printf("sp_arr[%d][%d] = %d\n", i, j, sp_arr[i][j]);
                                        }
                                        printf("\n");
                                }

                                // Define a double pointer
                                int **dp;
                                dp = sp_arr;

                                // Use ``dp`` to change contents of single dimension arrays
                                dp[0][2] = 9;   // Same as arr0[2]
                                dp[1][2] = 99;  // Same as arr1[2]
                                dp[2][2] = 999; // Same as arr2[2]

                                // Use ``dp`` to access contents of single dimension arrays
                                printf("------------ Access arrays using dp -----------\n");
                                for (int i = 0; i < 3; i++)
                                {
                                        for (int j = 0; j < 5; j++)
                                        {
                                                printf("dp[%d][%d] = %d\n", i, j, dp[i][j]);
                                        }
                                        printf("\n");
                                }

                                return 0;
                        }

		* See full program below

		.. code-block:: c

                        ------------ Access arrays using sp_arr -----------
                        sp_arr[0][0] = 1
                        sp_arr[0][1] = 2
                        sp_arr[0][2] = 9
                        sp_arr[0][3] = 4
                        sp_arr[0][4] = 5

                        sp_arr[1][0] = 10
                        sp_arr[1][1] = 20
                        sp_arr[1][2] = 99
                        sp_arr[1][3] = 40
                        sp_arr[1][4] = 50

                        sp_arr[2][0] = 100
                        sp_arr[2][1] = 200
                        sp_arr[2][2] = 999
                        sp_arr[2][3] = 400
                        sp_arr[2][4] = 500

                        ------------ Access arrays using dp -----------
                        dp[0][0] = 1
                        dp[0][1] = 2
                        dp[0][2] = 9
                        dp[0][3] = 4
                        dp[0][4] = 5

                        dp[1][0] = 10
                        dp[1][1] = 20
                        dp[1][2] = 99
                        dp[1][3] = 40
                        dp[1][4] = 50

                        dp[2][0] = 100
                        dp[2][1] = 200
                        dp[2][2] = 999
                        dp[2][3] = 400
                        dp[2][4] = 500

.. _basic_ptr_int_dp_sp_ex7:

.. tab-set::

        .. tab-item:: 1 Double Pointer : Pointing to array of single pointers : Dynamic

		* Step 1 : Define a double pointer

		.. code-block:: c

			int **dp;

		* Step 2 : Allocate heap memory : Create 2 single pointers ``dp[0]``, ``dp[1]``

		.. code-block:: c

			dp = malloc(2 * sizeof(int *));

		* Step 3 : Allocate heap memory : Create 3 single dimension integer arrays of size 32 integers each

		.. code-block:: c

			dp[0] = malloc(3 * sizeof(int));
			dp[1] = malloc(3 * sizeof(int));

		* Step 4 : Store user data using ``dp``

		.. code-block:: c

                        dp[0][0] = 1; dp[0][1] = 2; dp[0][2] = 3;
                        dp[1][0] = 10; dp[1][1] = 20; dp[1][2] = 30;

		* Step 5 : Access user data using ``dp``

		.. code-block:: c

                        for (int i = 0 ; i < 2; i++)
                        {
                                for (int j = 0; j < 3; j++)
                                {
                                        printf("dp[%d][%d] = %d\n", i, j, dp[i][j]);
                                }
                        }

		* Step 6 : Free 3 integer arrays

		.. code-block:: c

			free(dp[0]);
			free(dp[1]);

		* Step 7 : Free 3 single pointers

		.. code-block:: c

			free(dp);

		* See full program below

		.. code-block:: c

			#include <stdio.h>
			#include <stdlib.h>
			#include <string.h>

			int main(void)
			{
			        int **dp;

			        dp = malloc(2 * sizeof(int *));
			        dp[0] = malloc(3 * sizeof(int));
			        dp[1] = malloc(3 * sizeof(int));

                                // Store user data using ``dp``
                                dp[0][0] = 1; dp[0][1] = 2; dp[0][2] = 3;
                                dp[1][0] = 10; dp[1][1] = 20; dp[1][2] = 30;

                                // Access user data using ``dp``
                                for (int i = 0 ; i < 2; i++)
                                {
                                        for (int j = 0; j < 3; j++)
                                        {
                                                printf("dp[%d][%d] = %d\n", i, j, dp[i][j]);
                                        }
                                }

			        free(dp[0]);
			        free(dp[1]);

			        free(dp);

			        return 0;
			}

		* Output is as below

		.. code-block:: c

                        dp[0][0] = 1
                        dp[0][1] = 2
                        dp[0][2] = 3
                        dp[1][0] = 10
                        dp[1][1] = 20
                        dp[1][2] = 30

.. _basic_ptr_int_dp_sp_ex9:

.. tab-set::

        .. tab-item:: 1 Double Pointer : Pass by Reference

		* Step 1 : Define a double pointer

		.. code-block:: c

			int **dp;

		* Step 2 : Pass double pointer by reference

		.. code-block:: c

			fun(&dp);

		* Step 3 : Allocate memory inside function ``fun``

		.. code-block:: c

			void fun(int ***dp_r)
			{
			        *dp_r = malloc(2 * sizeof(int *));
			        (*dp_r)[0] = malloc(3 * sizeof(int));
			        (*dp_r)[1] = malloc(3 * sizeof(int));
			}

		* Step 4 : Use single pointers dp[0], dp[1] in caller to store user data

		.. code-block:: c

                        dp[0][0] = 1; dp[0][1] = 2; dp[0][2] = 3;
                        dp[1][0] = 10; dp[1][1] = 20; dp[1][2] = 30;

		* Step 5 : Use single pointers dp[0], dp[1] in caller to access user data

		.. code-block:: c

                        for (int i = 0 ; i < 2; i++)
                        {
                                for (int j = 0; j < 3; j++)
                                {
                                        printf("dp[%d][%d] = %d\n", i, j, dp[i][j]);
                                }
                        }

		* Step 6 : Free 3 arrays of integers after use

		.. code-block:: c

			free(dp[0]);
			free(dp[1]);

		* Step 7 : Free 3 Single pointers after use

		.. code-block:: c

			free(dp);

		* See full program below

		.. code-block:: c

			#include <stdio.h>
			#include <stdlib.h>
			#include <string.h>

			void fun(int ***dp_r)
			{
			        *dp_r = malloc(2 * sizeof(int *));
			        (*dp_r)[0] = malloc(3 * sizeof(int));
			        (*dp_r)[1] = malloc(3 * sizeof(int));
			}

			int main(void)
			{
			        int **dp;

			        fun(&dp);

                                // Store user data using ``dp``
                                dp[0][0] = 1; dp[0][1] = 2; dp[0][2] = 3;
                                dp[1][0] = 10; dp[1][1] = 20; dp[1][2] = 30;

                                // Access user data using ``dp``
                                for (int i = 0 ; i < 2; i++)
                                {
                                        for (int j = 0; j < 3; j++)
                                        {
                                                printf("dp[%d][%d] = %d\n", i, j, dp[i][j]);
                                        }
                                }

			        free(dp[0]);
			        free(dp[1]);

			        free(dp);

			        return 0;
			}

		* See full program below

		.. code-block:: c

                        dp[0][0] = 1
                        dp[0][1] = 2
                        dp[0][2] = 3
                        dp[1][0] = 10
                        dp[1][1] = 20
                        dp[1][2] = 30

.. card:: See Also

        * Current Module

                * :doc:`../basic_ptr`

        * Previous Module

                * :doc:`../../variable_and_ptr/variable_and_ptr`

        * Next Module

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        * Other Modules

                * :doc:`../../malloc_ptr/malloc_ptr`
                * :doc:`../../typecasting_n_ptr/typecasting_n_ptr`
                * :doc:`../../funcs_n_ptrs/funcs_n_ptrs`
                * :doc:`../../memcpy_ptr/memcpy_ptr`
                * :doc:`../../const_ptr/const_ptr`
                * :doc:`../../void_ptr/void_ptr`
                * :doc:`../../array_of_ptr/array_of_ptr`
                * :doc:`../../ptr_to_array/ptr_to_array`
                * :doc:`../../function_ptr/function_ptr`
                * :doc:`../../pre_incr_ptr/pre_incr_ptr`
                * :doc:`../../post_incr_ptr/post_incr_ptr`
                * :doc:`../../pre_decr_ptr/pre_decr_ptr`
                * :doc:`../../post_decr_ptr/post_decr_ptr`