Pointers in C

I know when I'm giving a C Programming course that I'll come to the section on pointers and I'll have to slow right down - those extra * and & characters take a little explaining and getting used to.

Here are the key points.

A variable declared with a * in front of it holds a reference to (i.e. the address of) another variable. The type of the OTHER variable is stated inthe declaration. Thus: float *boat; declares a variable called boat that holds an ADDRESS, and at that address you'll find a float variable.

A reference to a variable preceeded with an & calls up the ADDRESS of a variable rather than its contents - so it's suitable for assigning to a variable of the type described in the previous paragraph.

And a reference to a variable preceeded with an * calls up the CONTENTS of the address pointed to by the variable - in effect it's the opposite of &.

Example:

int main(int argc, char ** argv) {

 

       int bill = 25;   /* hold int */

       int ben = 43;

 

       int *fpotmen;  /* holds POINTER TO an int  */

 

       fpotmen = &bill;   /* ADDRESS OF bill */

 

       *fpotmen = 65 - *fpotmen;

       printf("Retires in %d years\n",*fpotmen); /* CONTENTS OF fpotmen */

 

       fpotmen = &ben;   /* ADDRESS OF ben */

 

       *fpotmen = 65 - *fpotmen;

       printf("Retires in %d years\n",*fpotmen); /* CONTENTS OF fpotmen */

 

}

WHY?

Because you can assign lots of different variable addresses in turn to a single pointer variable, teh use common code to process a whole lot (array) of data. You'll note that I've illustrated this in the example above by exactly duplicating two of the lines of code, though in a real life application I would prefer not to duplicate but rather to write them into a separate function.

Let's run that code and see when Bill and Ben retire ...

[trainee@daffodil cxx]$ gcc -o panda panda.c

[trainee@daffodil cxx]$ ./panda

Retires in 40 years

Retires in 22 years

[trainee@daffodil cxx]$

(written 2007-04-19)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

C207 - C and C based languages - Pointers and references
  [1478] Some new C programming examples - files, structs, unions etc - (2007-12-19)
  [1497] Training Season Starts again! - (2008-01-07)
  [2005] Variables and pointers and references - C and C++ - (2009-01-23)
  [2572] The what and why of C pointers - (2010-01-13)
  [2670] Pointers to Pointers to Pointers - what is the point? - (2010-03-10)
  [3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
  [3121] New year, new C Course - (2011-01-05)
  [3238] Bradshaw, Ben and Bill. And some C and C++ pointers and references too. - (2011-04-09)
  [3242] How to return 2 values from a function (C++ and C) - more uses of pointers - (2011-04-10)
  [3386] Adding the pieces together to make a complete language - C - (2011-08-11)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [4128] Allocating memory dynamically in a static language like C - (2013-06-30)
  [4560] Variables, Pointers and References - C and C++ - (2015-10-29)

Back to
As I came back from Tesco

Previous and next
or
Horse's mouth home

Forward to
Two by One by Wiltshire

Some other Articles

It can take more that one plus one to get two.
Private Java Course - A customer's pictures
Speed Networking - a great evening and how we arranged it
Two by One by Wiltshire
Pointers in C
As I came back from Tesco
Object Oriented Model - a summary of changes from PHP4 to PHP5
Course, right place, right time
Gordon Dodge, R.I.P.
Helsinki - what comes naturally