Summary

In this lesson, you learned that:

• Every variable is stored in memory and has an address. &x is the address of the variable x, and %p prints it.
• A pointer is a variable that holds an address: int *p = &x;. Its type says what it points to: int *, double *, char *.
• *p is the value p points to (dereferencing). You can read it, printf("%d\n", *p);, and change it, *p = 20;. Without the star, p = &y; changes where the pointer points.
• In a declaration, * makes a pointer. Before a pointer, it gets the value from the address. Between two values, it multiplies. & and * are opposites.
• A function with a pointer parameter can change the caller's variable: void add_bonus(int *points, int bonus), called as add_bonus(&score, 5). A forgotten & in the call is a compile error; a forgotten * inside the function is a bug without any error.
• Through pointers, a function can swap two variables or give back several results at once (output parameters).
• The name of an array is the address of its first element, so int *p = numbers; and p[1] work. A parameter int numbers[] is a pointer, the same as int *numbers. That's why a function changes the caller's array and needs its count.
• NULL is a pointer that points nowhere. A function that returns a pointer, such as the address of an element it found, returns NULL when there is none. Check a pointer that can be NULL before you use it: if (p != NULL). Reading through NULL stops the program here with RuntimeError: unreachable.
• const int *numbers is a pointer that the function only reads through. Text in quotes gets a const char * pointer: const char *name = "Anna";.

In the next lesson, on structures, you'll learn to group values that belong together, such as a student's name and points, into one variable. Pointers will come in handy there too.