Commit: 13bbe14cddfbb4e8ecabf84c37906a48d7159fd6 - myprojects-hg-reborn (hg)

Commit MetaInfo

• Révision: 13bbe14cddfbb4e8ecabf84c37906a48d7159fd6 (tree)
• l'heure: 2008-10-21 02:02:25
• Auteur: iselllo
• Commiter: iselllo

Message de Log

An example taken from the C book that allows me to swap two numbers in C.

Change Summary

• add: C-codes/swap_test.c

Modification

diff -r 9209b2e13fe8 -r 13bbe14cddfb C-codes/swap_test.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/C-codes/swap_test.c	Mon Oct 20 17:02:25 2008 +0000
@@ -0,0 +1,101 @@
+#include <stdio.h>
+
+void swap_wrong(int x, int y)
+{
+  int temp;
+  temp=x;
+  x=y;
+  y=temp;
+
+}
+
+/* Some comments: the following function takes two pointers as arguments. Or better: 
+the argument ip is defined as a pointer to an integer variable. 
+
+
+From the C book:
+The idea is that the the parameters are declared to be pointers inside the function itself and
+then the calling program passes pointers to the values to be changed.
+
+
+The pointers are generated in the calling program directly from the variables using the
+referencing operator "&".
+
+
+See pages 95 and following of the C book.
+
+
+    */
+
+
+void swap_right(int *px, int *py)
+{
+  int temp;
+
+  temp= *px;  /* store in temp [plain integer variable] the value the integer pointer px
+   points at   */
+  
+  *px= *py;   /* read the value stored in the memory location the pointer py points at and copy
+	       that value in the memory location the pointer px points at.*/
+
+  *py= temp;  /* store the value of them in the memory location the pointer py points at  */
+
+/* Moral of the story: there are the integer variables x and y and their pointers px and py.
+The variables x and y are stored somewhere in memory.
+ The function swaps_right accesses the locations where a and b are stored and their values via the 
+pointers to x and y. It then swaps the values stored at the memory locations corresponding
+to a and b. The net result is that the function can change the values of the variables x and y     */
+
+}
+
+
+main()
+{
+  int a;
+  int b;
+
+
+  a=10;
+  b=20;
+
+
+
+  swap_wrong(a,b);
+
+  printf("before swapping \n");
+
+  
+  printf("a is\n");
+  printf("%d\n ", a);        
+
+  printf("b is\n");
+
+  printf("%d\n ", b);        
+
+  printf("after swapping without pointers \n");
+
+  
+  printf("a is\n");
+  printf("%d\n ", a);        
+
+  printf("b is\n");
+
+  printf("%d\n ", b);        
+
+
+  swap_right(&a,&b);
+
+
+  printf("after swapping USING pointers \n");
+
+  
+  printf("a is\n");
+  printf("%d\n ", a);        
+
+  printf("b is\n");
+
+  printf("%d\n ", b);        
+
+  
+
+}