Announcements

1
Announcements

• Tests
• Test 4 Monday 22th September (ZOO1)
• Test 5 Monday 13th October (ZOO1)
• Tuts
• Tut 3 8th ,9th, 15th and 16th September
• Labs
• Lab 6 22nd , 23rd ,29th and 30th September
• Lab 7 6th , 7th , 13th and 14th October

2
Pointers

• So far we have used variables in our programs to
  hold information
• These are locations of a specific size in memory
  where we can store data
• Each location in our computer has a unique number
  associated with it
• This is called an address

3
Declaring Variables

• When we declare a variable we do it as follows
• int myint
• We have reserved a space in memory that is large
  enough to hold and integer we have also given
  that space a name myint

4
Pointers

• It is often useful to refer to a variable
  indirectly
• We can do this by creating a new data type which
  holds the address of the variable
• This is called a pointer
• To create a pointer we do it as follows
• int mypointer
• The mypointer variable has been declared as
  holding the address of an integer

5
Pointers
Pointer
6
Example

• If is say the following
• the first C example in these notes is on page 1
• I have given you a pointer to a C example
• If I have an integer stored at memory address
  0x223F then I can use this number to find that
  integer

7
The Operator

• The operator is a unary operator
• This means that it only operates on one item
• It associates itself with the name to the right
  of it
• Therefore in the following example pointer is a
  pointer and integer is a normal variable
• int pointer, integer

8
Address Size

• On a Pentium based PC all pointers are 32-bits
  long regardless of what they are pointing to
• This means that a pointer to an integer is the
  same size as a pointer to a char or a float
• Despite this C treats pointers to different
  variable types as different types of pointers
• This is because when we write into the location
  pointed to by the pointer, the number of storage
  locations affected will depend on the type of
  pointer

9
The Operator

• If we have an example and we wish to know where
  it is stored we can use the operator
• This is illustrated in the following example
• int myint
• int intpointer
• intpointer myint
• In this example we have declared an integer and
  something which can point to an integer
• We have then told the pointer to point to the
  integer by assigning the pointer (which holds an
  address) to the address of the integer
• The unary operator will give us the address of
  the space in memory where the variable is stored

10
Using Pointers

• If we wish to refer to the thing pointed to by
  the pointer we can do it by prefacing the name of
  the pointer with
• A full example is shown in the following slide
• In this example an integer and a pointer to an
  integer are declared
• After this we make the pointer point to the
  integer
• We then assign the integer a number and print out
  the contents of the item being pointed to by the
  pointer

11
Example

• include ltstdio.hgt
• include ltstdlib.hgt
• int main(void)
• int myint
• int intpointer
• intpointer myint
• myint 3456
• printf(i,intpointer) //This will print
  3456
• system(PAUSE)

12
Null Pointers

• When you first create a pointer it does not point
  to anything
• If you store something in the referenced location
  you have no control over where in memory it is
  stored
• We are able to create a pointer which does not
  point to anything
• This is called a Null Pointer