11/11/05 CISC105 Lecture Notes


(1) Defining a struct

(1a) One reason to do it is so we can avoid
     parallel arrays.

     With an array of structs, we can represent
     a table with any number of columns,
     using only a single array.

     Each element in the array will be 
     a "struct" that contains several fields.

     (The fields are also called "members
     of the struct")

For example:


#define TEAM_NAME_SIZE 32

struct PackerStats_S
{
  char teamName[TEAM_NAME_SIZE];
  int pointsScored;
  int pointsAgainst;
};

	This struct contains three fields:

	teamName
	pointsScored
	pointsAgainst

If I declare a single variable:

   PackerStats_S homeOpener;

Now I have a variable:

homeOpener

   +---------------------------------------------+
   |          +----------------------------+     |
   . teamName |Bears                       |     |
   |          +----------------------------+     |
   |              +---------------+              |
   . pointsScored |30             |              |
   |              +---------------+              |
   |              +---------------+              |
   . pointsAgainst|50             |              |
   |              +---------------+              |
   +---------------------------------------------+

I can access the fields in this struct by
using variables such as:

strncpy(homeOpener.teamName,"Bears",TEAM_NAME_SIZE);
homeOpener.pointsScored = 30;
homeOpener.pointsAgainst = 50;

printf("The opponent for the home opener was %s\n",
      homeOpener.teamName);

if (homeOpener.pointsScored < homeOpener.pointsAgainst)
  printf("The Packers lost, %d to %d\n", homeOpener.pointsScored,
                                       homeOpener.pointsAgainst     );

else if (homeOpener.pointsScored > homeOpener.pointsAgainst)
  printf("The Packers won, %d to %d\n",homeOpener.pointsScored,
                                       homeOpener.pointsAgainst     );

else /* they are equal */
  printf("It was a tie game, %d to %d\n", homeOpener.pointsScored,
                                       homeOpener.pointsAgainst     );


If I declare an array:

#define MAX_NUM_GAMES 20

  PackerStats_S greenBayStats[MAX_NUM_GAMES];


This array means that we have 20 elements,
each of which contains

    teamName
    pointsScored
    pointsAgainst

So it looks like this in memory:

      greenBayStats[0]
   +---------------------------------------------+
   |          +----------------------------+     |
   . teamName |                            |     |
   |          +----------------------------+     |
   |              +---------------+              |
   . pointsScored |               |              |
   |              +---------------+              |
   |              +---------------+              |
   . pointsAgainst|               |              |
   |              +---------------+              |
   +---------------------------------------------+
      greenBayStats[1]
   +---------------------------------------------+
   |          +----------------------------+     |
   . teamName |                            |     |
   |          +----------------------------+     |
   |              +---------------+              |
   . pointsScored |               |              |
   |              +---------------+              |
   |              +---------------+              |
   . pointsAgainst|               |              |
   |              +---------------+              |
   +---------------------------------------------+
   
   .
   .
   .
      greenBayStats[19]
   +---------------------------------------------+
   |          +----------------------------+     |
   . teamName |                            |     |
   |          +----------------------------+     |
   |              +---------------+              |
   . pointsScored |               |              |
   |              +---------------+              |
   |              +---------------+              |
   . pointsAgainst|               |              |
   |              +---------------+              |
   +---------------------------------------------+

If I want to assign the first element in this
struct to contain: Panthers,20,23

    strncpy(greenBayStats[0].teamName,"Panthers",TEAM_NAME_SIZE);
    greenBayStats[0].pointsScored = 20;
    greenBayStats[0].pointsAgainst = 23;
 

We can't just do:
   
    greenBayStats[0].teamName = "Panthers";

because teamName is an array of characters, and you
can't copy an entire array with an assignment statement.



(2) Why the _S

(3) A note about naming...