Structure Example

Here's another example with nested structures that shows

• Functions that take and return structures
• That assignments involving structures copy data

The Code

/*************************************** 
 ** File: euclid.c
 ** Author: Tim Finin
 ** Date: 03/08/05
 ** email: finin@cumbc.edu
 ** Modified: 9/25/05
 ** By: Sue Evans
 ** Modified to meet current 201 standards
 ** 
 ** This program verifies that assigning 
 ** one structure to another in C *copies* 
 ** all of the values. It is not like assignment 
 ** for objects.
 **
 *************************************/

#include <stdio.h>

/* structures */

/* a 2D point has an x and y coordinate */
struct point { 
   int x; 
   int y; 
}; 

/* a line has a start and end point */
struct line {
   struct point startPoint;
   struct point endPoint;
};

/* function prototypes */

void PrintGreeting (void);

struct line ReverseLine(struct line theLine);

void PrintLine (struct line theLine);



int main (void) {

   PrintGreeting();

   struct point p1 = {-1, -2};
   struct point p2 = {1, 2};
   struct line line1;
   struct line line2;

   printf("Initial values: p1 is (%d,%d), p2 is (%d,%d)\n", 
           p1.x, p1.y, p2.x, p2.y);

   p1 = p2;

   printf("After p1=p2: p1 is (%d,%d), p2 is (%d,%d)\n",
           p1.x, p1.y, p2.x, p2.y);

   p2.x = 100;
   p2.y = 200;

   printf("After modifying p2: p1=(%d,%d), p2=(%d,%d)\n",
           p1.x, p1.y, p2.x, p2.y);

   p1.x = -100;
   p1.y = -200;

   printf("After modifying p1: p1=(%d,%d), p2=(%d,%d)\n",
           p1.x, p1.y, p2.x, p2.y);

   printf("\nLines...\n");

   line1.startPoint = p1;
   line1.endPoint = p2;

   line2 = ReverseLine(line1);

   printf("line1:"); 
   PrintLine(line1);
   printf(" line2:"); 
   PrintLine(line2);
   printf("\n");
   
   line1 = line2;

   printf("After line1 = line2, line1:");
   PrintLine(line1);
   printf(" line2:"); 
   PrintLine(line2);
   printf("\n");

   line2.endPoint.x = 999;

   printf("After line2.endPoint.x = 999,\nline1:");
   PrintLine(line1);
   printf(" line2:"); 
   PrintLine(line2);
   printf("\n");

   return 0;
}


/*************************************** 
 ** PrintLine prints a line w/o newlines
 ** 
 ** INPUTS: a line structure.
 ** OUTPUT: void
 **
 *************************************/

void PrintLine (struct line l) {
   printf("(%d,%d)=>(%d,%d)",
	  l.startPoint.x, l.startPoint.y, 
	  l.endPoint.x, l.endPoint.y);
}


/*************************************** 
 ** ReverseLine reverses a line struct, i.e.,
 ** the start and end points are swaped.
 ** 
 ** INPUTS: a line structure.
 ** OUTPUT: a line structure
 **
 *************************************/

struct line ReverseLine(struct line l) {
   struct line reversed;

   reversed.startPoint = l.endPoint;
   reversed.endPoint = l.startPoint;

   return reversed;
}

/*************************************** 
 ** PrintGreeting prints an initial greeting message
 ** and explanation of the program
 ** 
 ** INPUTS: void
 ** OUTPUT: void
 **
 *************************************/

void PrintGreeting (void) {
   printf("Euclidian fun\n\n");
   printf("This example uses both struct points and struct lines, making\n");
   printf("use of nested structures to model points & line segments.\n");
   printf("It also shows functions that take structs as arguments and\n");
   printf("that return structs.\n\n");
}

The Output

% a.out


Euclidian fun

This example uses both struct points and struct lines, making
use of nested structures to model points & line segments.
It also shows functions that take structs as arguments and
that return structs.

Initial values: p1 is (-1,-2), p2 is (1,2)
After p1=p2: p1 is (1,2), p2 is (1,2)
After modifying p2: p1=(1,2), p2=(100,200)
After modifying p1: p1=(-100,-200), p2=(100,200)

Lines...
line1:(-100,-200)=>(100,200) line2:(100,200)=>(-100,-200)
After line1 = line2, line1:(100,200)=>(-100,-200) line2:(100,200)=>(-100,-200)
After line2.endPoint.x = 999, 
line1:(100,200)=>(-100,-200) line2:1(100,200)=>(999,-200)
%