Basic Pointers CSCI 112: Programming in C What the #$@# is a - - PowerPoint PPT Presentation

Basic Pointers

CSCI 112: Programming in C

What the #$@# is a pointer?

Pointers are variables Instead of holding values, they hold the address of another variable. They “point” to where that other variable is stored.

Some new syntax

• A pointer variable is declared using a *
• A pointer variable “points” to specific type
• int pointer, char pointer, etc.
• A pointer to an int:
• A pointer to a char:

int *intPointer; char *charPointer;

What does a pointer do?

• A pointer variable holds an address of another variable
• Ex: an int* holds address of an int
• We don’t assign a value to a pointer—we assign it an address!

What does a pointer do?

• A pointer variable holds an address of another variable
• Ex: an int* holds address of an int
• We don’t assign a value to a pointer—we assign it an address!
• Remember the address of operator from scanf? (&)

int myInt; // Declare an int variable int *myIntPointer; // Declare an int pointer variable myIntPointer = &myInt; // Point "myIntPointer" to the memory location of // "myInt"

…

0x01 0x02 0xAE 0xAF We start out with a bunch of unallocated memory cells. The hexadecimal numbers above each cell represent the address.

…

0x01 0x02 0xAE 0xAF

int x;

x Declare an int variable, and allocate a space in memory for it.

34

…

0x01 0x02 0xAE 0xAF

int x; x = 34;

x Assign 34 to x. The value 34 goes into the memory cell for x.

34

…

0x01 0x02 0xAE 0xAF

int x; x = 34; int *point;

x Declare an int pointer called point. Allocate space in memory for this variable. *point

34 0x01

…

0x01 0x02 0xAE 0xAF

int x; x = 34; int *point; point = &x;

x Assign the address of x to point. We can now think of *point as being a “link” to x *point

34 0x01

…

0x01 0x02 0xAE 0xAF

int x; x = 34; int *point; point = &x;

x Assign the address of x to point. We can now think of *point as being a “link” to x *point

*point is “pointing” to where x is stored!

Pointer operations: the “*” symbol

• There are three uses for it:
• Multiplication
• Indicating that a variable is of a pointer type
• Unary: “follows the pointer” (often called “dereferencing”)

int x = 6 * 7; int *p = &x; int z = *p;

Working with pointers

In (1), we assign a value to an int variable x, then “point” the variable point to the address of x.

// 1 int x = 34; int *point = &x; // 2 int *point2 = point; // 3 int y = *point;

Working with pointers

In (1), we assign a value to an int variable x, then “point” the variable point to the address of x.

// 1 int x = 34; int *point = &x; // 2 int *point2 = point; // 3 int y = *point; Quick tip: to print the value of a pointer variable, use the “%p” Quick tip: to print the value of a pointer variable, use the “%p” format specifier! format specifier!

Working with pointers

In (2), we create another int pointer, point2, and set it equal to point. This means *point and *point2 both “point” to the same place…x!

// 1 int x = 34; int *point = &x; // 2 int *point2 = point; // 3 int y = *point;

A picture is worth 436 words

34 0x01

…

0x01 0x02 0xAE 0xAF

int x; x = 34; int *point; point = &x; int *point2; point2 = point;

x point and point2 both reference the same address/variable (x) *point *point2

Working with pointers

In (3), we declare a new int variable y, and assign the value “pointed to” by point to it.

// 1 int x = 34; int *point = &x; // 2 int *point2 = point; // 3 int y = *point;

Wait, what?

• A pointer variable itself only holds an address (of the variable it points

to)

• But using the unary operator (*) we can “follow the pointer” and

retrieve the value of the variable being pointed to!

• So if point is pointing to x, the above snippet is actually assigning the

value of x to y (which is of the int type)

int y = *point;

A picture is worth 436 words

34 0x01

…

0x01 0x02 0xAE 0xAF

int x; x = 34; int *point; point = &x; int y;

x point is an int pointer to x We declare the int variable y, and space is allocated in memory for it. *point y

A picture is worth 436 words

34 34 0x01

…

0x01 0x02 0xAE 0xAF

int x; x = 34; int *point; point = &x; int y; y = *point;

x We dereference point, and assign its value to y. END RESULT: y == x *point y

Dereferencing works for assignment, too

int x; int *myPointer; // This next line assigns 34 to the variable myPointer points to // Equivalent to writing "x = 34;" *myPointer = 34;

Use case: output parameters

• We’re used to functions returning a value

int x = my_function();

Use case: output parameters

• We’re used to functions returning a value
• We can also pass a pointer as an argument to function, and the

function can write the result into that variable:

int x = my_function(); void increment_int(int *value) { *value = *value + 1; } int x = 6; increment_int(x); // x now equals 7