L-29 MCS 260 Fri 26 Oct 2001

/* L-29 MCS 260 Fri 26 Oct 2001 : counting positive and negative numbers

The program below reads a sequence of integer numbers from input
and counts the number of positive and negative numbers.
Zeros are ignored.  The reading stops when a character or
EOF is encountered (either this happens when the program
runs with redirection or when the user hits return followed 
by control d on UNIX, or a control z in MS-DOS).

We use a function with call-by-reference to do the counting.

Besides another good use of call-by-reference, we emphasize
the precedence of operators :
      *pos++ is equivalent to *(pos++), which is not what we
want in this application.  Therefore we must use brackets to
update the counter for positive numbers and write (*pos)++.    */

void count ( int n, int *pos, int *neg );
/* increments *pos when n > 0, or *neg when n < 0 */

#include<stdio.h>

int main ( void )
{
   int a, pos_cnt = 0, neg_cnt = 0;

   printf("Give a sequence of integers : ");

   while (scanf("%d", &a) > 0)
      count(a,&pos_cnt,&neg_cnt);

   printf("Number of positive numbers read : %d\n", pos_cnt);
   printf("Number of negative numbers read : %d\n", neg_cnt);

   return 0;
}

void count ( int n, int *pos, int *neg )
{
   if (n > 0)
      (*pos)++;       /* we must use brackets ! */
   else if (n < 0)
      (*neg)++;
   else
      ;               /* we do not count zero */
}

/* L-29 MCS 260 Fri 26 Oct 2001 : variables inside a block

Sometimes we wish to declare and use a variable only when we really
need it.  The program below is an illustration of such a use.   */

#include<stdio.h>

int main ( void )
{
   int n, sum = 0;

   printf("Summing a sequence of integers.\n");
   printf("Give the number of integers in the sequence : ");
   scanf("%d", &n);

   {
      int i;                      /* here we need i for the loop */

      printf("Give %d numbers : ", n);

      for (i = 0; i < n; i++)
      {
         int nb;                  /* nb exists only inside the loop */

         scanf("%d", &nb);
         sum += nb;               /* we do not need nb after the loop */
      }
   }

   printf("The sum of the numbers : %d\n", sum);

   return 0;
}

/* L-29 MCS 260 Fri 26 Oct 2001 : scope rules

The program below is a slight modification of the previous program:
the variable "nb" declared inside the for loop has now name "i",
just like the "i" outside the for loop.  Although this is really not
a good coding practice, the program works just as well.
Inside the for loop, we use the inner "i" as a local variable,
the outer "i" is not visible inside the loop, but that does not
matter since the outer "i" is only used to control the loop.    */

#include<stdio.h>

int main ( void )
{
   int n, sum = 0;

   printf("Summing a sequence of integers.\n");
   printf("Give the number of integers in the sequence : ");
   scanf("%d", &n);

   {
      int i;                      /* outer i controls the loop */

      printf("Give %d numbers : ", n);

      for (i = 0; i < n; i++)
      {
         int i;                   /* this i is different for the other i */

         scanf("%d", &i);
         sum += i;                /* here we lose the inner i */
      }
   }

   printf("The sum of the numbers : %d\n", sum);

   return 0;
}