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Pointers and Indirection
• In our last lesson, we discovered a problem with
representing data structures that are not linear.
We need some way to map these data
structures to the computer's linear memory. One
solution is to use pointers. Pointers are memory
locations that are stored in memory cells. By
using a pointer, one memory cell can "point" to
another memory cell by holding a memory
address rather than data. Let's see how it works.
created on 29/10/2008
yahaya.wordpress.com
1
Pointers and Indirection
In the diagram above, the memory cell at address 2003 contains a pointer,
an address of another cell. In this case, the pointer is pointing to the
memory cell 2005 which contains the letter 'c'. This means that we now
have two ways of accessing the letter 'c' as stored data. We can refer to
the memory cell which contains 'c' directly or we can use our pointer to
refer to it indirectly. The process of accessing data through pointers is
known as indirection. We can also create multiple levels of indirection
using pointers. The diagram below shows an example of double
indirection. Notice that we must follow two pointers this time to reach the
stored data.
created on 29/10/2008
yahaya.wordpress.com
2
Pointers and Indirection
• As you can see, pointers can become very
complex and difficult to use with many levels of
indirection. In fact, when used incorrectly,
pointers can make data structures very difficult
to understand. Whenever we use pointers in
constructing data structures, we have to
consider the tradeoff between complexity and
flexibility. We will consider some examples of
this
tradeoff in theyahaya.wordpress.com
next few lessons.
created
on 29/10/2008
3
Pointers and Indirection
• The idea of pointers and indirection is not exclusive to
computer memory. Pointers appear in many different
aspects of computer use. A good example is hyperlinks
in web pages. This links are really pointers to another
web page. Perhaps you have even experienced "double
indirection" when you went to visit a familiar web site and
found the site had moved. Instead of the page you
expected, you saw a notice that the web pages had been
moved and a link to the new site. Rather than clicking a
single link, you had to follow two links or two pointers to
reach the web page.
created on 29/10/2008
yahaya.wordpress.com
4