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 under

C207 - C and C based languages - Pointers and references
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  [3386] Adding the pieces together to make a complete language - C - (2011-08-11)
  [3242] How to return 2 values from a function (C++ and C) - more uses of pointers - (2011-04-10)
  [3238] Bradshaw, Ben and Bill. And some C and C++ pointers and references too. - (2011-04-09)
  [3121] New year, new C Course - (2011-01-05)
  [3004] Increment operators for counting - Perl, PHP, C and others - (2010-10-18)
  [2670] Pointers to Pointers to Pointers - what is the point? - (2010-03-10)
  [2572] The what and why of C pointers - (2010-01-13)
  [2005] Variables and pointers and references - C and C++ - (2009-01-23)
  [1497] Training Season Starts again! - (2008-01-07)
  [1478] Some new C programming examples - files, structs, unions etc - (2007-12-19)

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