L-27 MCS 260 Mon 22 Oct 2001

Below are the programs we used in class to introduce pointers.

/* L-27 MCS 260 Mon 22 Oct 2001 : addressing and derefencing

C has a fine mechanism for dealing with variables: we can
declare a pointer to an int variable.  With the addressing
operator we store in the pointer the address of some int.
Here is the picture :

   int *p,i = 17;       p            i
   p = &i;              +------+     +------+
                        |  &i -|---> |  17  |
                        +------+     +------+

The program below shows how to manipulate variables indirectly
with the dereferencing operator.                               */

#include<stdio.h>

int main(void)
{
   int *p, i = 17;

   p = &i;

   printf("Some variable i has value %d = %d.\n", i, *p);
   printf("The address of i is %x = %x.\n", &i, p);

   *p = 3;

   printf("Via dereferencing we changed i into %d = %d.\n", i, *p);

   return 0;
}

In the program below we introduce call by reference :

/* L-27 MCS 260 Mon 22 Oct 2001 : call by reference

To illustrate call by reference, the program below uses
a function "add" to sum a sequence of user given integers.  */

void add ( int *sum, int n );
/* adds n to the sum */

#include<stdio.h>

int main(void)
{
   int i, s = 0;

   printf("Give sequence of integers (terminate with zero) : ");

   while (scanf("%d",&i) > 0)
   {
      if (i == 0) break;
      add(&s,i);
   }

   printf("The sum : %d.\n", s);

   return 0;
}

void add ( int *sum, int n )
/* we pass the sum via call by reference,
   while n is passed via call by value */
{
   *sum += n;
}

The function in the following example has as output two values: one as return value of the function and the other as called by reference.

/* L-27 MCS 260 Mon 22 Oct 2001 : call by reference

The function below illustrates interest compounding.  */

float interest ( float *amount, float rate );
/* returns the yield on the given amount after one year with
   the given interest rate; this yield is added to the amount */

#include<stdio.h>

int main(void)
{
   float principal, rate, yield;

   printf("Give dollar amount : ");
   scanf("%f", &principal);
   printf("Give interest rate as percentage : ");
   scanf("%f", &rate);

   yield = interest(&principal,rate/100);

   printf("The yield : $%.2f.\n", yield);
   printf("Amount after compounding : $%.2f.\n", principal);

   return 0;
}

float interest ( float *amount, float rate )
{
   float y = *amount * rate;

   *amount += y;

   return y;
}