Pointers - the basics

I did a bit on pointers today. The comments in the code below explain how the program works.

Original Code:

/* 
 * This program demonstrates the basics of pointers.
 * It demonstrates:
 * 1) What pointers do
 * 2) Pointing to an array
 * 3) Pointers in function parameters
 */
#include<stdio.h>
void basicPointerStuff();
void pointerArithmetic();
void pointersInParameters1();
void pointersInParameters2();

int main()
{
    basicPointerStuff();
    pointerArithmetic();
    pointersInParameters1();
    
    return 0;
}

/* 
 * This function demonstrates the VERY basics of pointers.
 * They are variables which POINT to another variable.
 * If you do Java, they somewhat resemble a reference variable pointing to an object...
 */
void basicPointerStuff()
{
    int x = 57;
    int *ptr = &x; // The POINTER variable
    printf("Value of x: %d\n", x);
    printf("Value of ptr: %p\n", ptr);
    /* 
     * A reference variable in Java contains bits that tell the program how to access the object it is referring to.
     * It's sort of the same for pointers in C.
     */
    printf("What does ptr point to? %d\n", *ptr); // Watch out for the asterisk!
}

/*
 * In this function, ptr points to an array. 
 * If you increment ptr, it points to the next element in the array.
 */
void pointerArithmetic()
{
    int x[5] = {1, 2, 3, 4, 5};
    int *ptr = x; // Note that there is NOT an "&"
    printf("%d\n", *ptr);
    printf("%d\n", ++*ptr);
    printf("%d\n", ++*ptr);
}

/*
 * C, like Java, is PASS BY VALUE.
 * That means that if a function uses a value passed to it via its brackets and changes it, the variable with the original value is NOT affected.
 * But since pointers POINT to a variable, a function can CHANGE the value of a variable in a different function using a pointer.
 * This part of the program demonstrates how this is done.
 */
void pointersInParameters1()
{
    int x = 5;
    int *ptr = &x;
    printf("Value of x: %d\n", x);
    pointersInParameters2(ptr);
    printf("Now x is: %d\n", x);
    printf("(Cool init?!)\n");
}

void pointersInParameters2(int *num)
{
    *num += 5;
    /*
     * Although num is not the same pointer as ptr, they both POINT to the variable x.
     * So you can directly change the value of x, as shown above.
     */
}

Output:

Value of x: 57

Value of ptr: 0x7fff62858eac

What does ptr point to? 57

1

2

3

Value of x: 5

Now x is: 10

(Cool init?!)