C Examples: Pointers

// FILE I/O EXAMPLE - C
// (c) gigabytemon
// 04/07/2020
// 2862 abc5 66cb a008 dbd9 5bf7 0dda 04f8
// e192 33e4 e689 4efb 24b9 d34f 6c2d 1ee1

/* introduction
 * ------------
 * every variable declaration is a location in your computer's memory.
 * that location has an address, which is a 8-character long hexadecimal number.
 * for example:
 * int i; <-- this 'i' is a location in memory where an integer is stored
 * so 'i' is a symbolic representation of that location in memory.
 * meanwhile, its proper address could be, say, 0xbff5a400
 *
 * don't worry about the hexadecimal. just understand that it's an address.
 * much like a house address. "gigabytemon's house" (the variable) and "the address of gigabytemon's house" (the address) both lead to the same thing.
 * the only difference is that pointers give you the address of the variable, rather than the value stored inside the variable.
 */

 /* syntax
 * ------
 * the general syntax of a pointer is:
 *
 * type *pointer_name
 * pointer_name = &variable
 *
 * like any variable, the pointer must be declared with a type and a name.
 * the '*' tells the compiler that this declaration is for a pointer.
 * the pointer's type must be the same as the variable you are associating it with
 */

/* personal notes
 * --------------
 * pointers are very useful for memory management and manipulation.
 * they can store and manage dynamically allocated blocks of memory.
 *
 * i hate them.
 */

 #include <stdio.h>

 void main()
 {
    //------------------------------------------------------------------------------------------------
    // >> 1) Assigning pointers to variables
    //------------------------------------------------------------------------------------------------
    // variable declaration
    int number = 8;         // lets assume this variable is stored at address 0x1000

    // the pointer we'll use to point to 'number'. currently set to NULL
    int *numberPtr = NULL;      // assume this pointer is stored right after number, i.e. 0x1004

    //      memory table
    /*/----------------------\
     *| Address |  Contents  |
     *|----------------------|
     *|  0x1000 | 0x00000008 |  <--- this is where number = 8 is stored
     *|  0x1004 | 0x00000000 |  <--- this is the where the pointer is stored (currently set to NULL)
     *\----------------------/
     */

    // now we'll assign the pointer to the 'number' variable
    numberPtr = &number;        // 'number' pointer = address of 'number'

    //      memory table
    /*/----------------------\
     *| Address |  Contents  |
     *|----------------------|
     *|  0x1000 | 0x00000008 |  <--- number
     *|  0x1004 | 0x00001000 |  <--- now the pointer's value is set to 'number''s address (0x1000)
     *\----------------------/
     */

    //------------------------------------------------------------------------------------------------
    // >> 2) Retrieving information related to the pointer
    //------------------------------------------------------------------------------------------------
    // to get the address stored in the pointer numberPtr, do this:
    printf("Address stored in numberPtr = %p\n", numberPtr);

    // to get the value at the address pointed to by numberPtr, do:
    printf("Value at that address = %d\n", *numberPtr);
    // this is also called "dereferencing" the pointer
    // note that you use the asterisk '*' to get the value at the address

    //------------------------------------------------------------------------------------------------
    // >> 3) Manipulating values stored at addresses
    //------------------------------------------------------------------------------------------------
    // you can change the value stored at the address that a pointer points to
    // for example:

    *numberPtr = 2;

    //      memory table
    /*/----------------------\
     *| Address |  Contents  |
     *|----------------------|
     *|  0x1000 | 0x00000002 |  <--- 8 has been replaced with 2
     *|  0x1004 | 0x00001000 |  <--- pointer (0x1000)
     *\----------------------/
     */

    //------------------------------------------------------------------------------------------------
    // >> 4) Pointer arithmetic - adding and subtracting
    //------------------------------------------------------------------------------------------------
    // addresses in C are numeric values. because of this, you can perform arithmetic on pointers.
    // the four arithmetic operations you can use on pointers are: ++, +, --, -
    // below are examples for ++. -- also works the same way.

    // for integers (like the above examples), pointer arithmetic works like this:
    numberPtr++;    //pointer increments by 4 because each int is 4 bytes in length
    // note: this is the same as numberPtr = numberPtr + 1;

    //      memory table                     memory table
    /*/----------------------\         /----------------------\
     *| Address |  Contents  |         | Address |  Contents  |
     *|----------------------|   +4    |----------------------|
     *|  0x1004 | 0x00001000 |  ---->  |  0x1004 | 0x00001004 |
     *\----------------------/         \----------------------/
     */

    // for characters:
    char character = 'C';
    char *characterPtr = NULL;
    characterPtr = &c;

    //      memory table
    /*/----------------------\
     *| Address |  Contents  |
     *|----------------------|
     *|  0x1008 |    0x43    |  <--- character (1 byte, 0x43 is the ascii for capital 'C')
     *|  0x1009 | 0x00001008 |  <--- characterPtr
     *\----------------------/
     */

    characterPtr++; //pointer increments by 1, because char is 1 byte
    // note: this is the same as characterPtr = characterPtr + 1;

    //      memory table                     memory table
    /*/----------------------\         /----------------------\
     *| Address |  Contents  |         | Address |  Contents  |
     *|----------------------|   +1    |----------------------|
     *|  0x1009 | 0x00001008 |  ---->  |  0x1009 | 0x00001009 |
     *\----------------------/         \----------------------/
     */

    //------------------------------------------------------------------------------------------------
    // >> 5) Pointer arithmetic - comparisons
    //------------------------------------------------------------------------------------------------
    // pointers can be compared as well, using ==, <, and >
    // you can use this to find out if two address are the same
    // or if they're different, where they are in relation to each other

    if ( numberPtr < characterPtr )
        printf("number's address is located BEFORE character's address");
    else if ( numberPtr > characterPtr )
        printf("number's address is located AFTER character's address");
    else if ( numberPtr == characterPtr )
        printf("number and character both share the SAME address!");

    // the above code should output the first statement, since
    // numberPtr is pointing to 0x00001004, while
    // characterPtr's is pointing to 0x00001009
    // i.e. 1004 < 1009
 }