Why is This Not Executing Correctly?

Question

//A code to find Total of Electric Bill...

/* 
   Upto 100 Units - Rs 5 pu
   upto 200 units - Rs 5.5 pu
   upto 500 units - Rs 6.5 pu
   above 500 units - Rs 7 pu
*/

#include<stdio.h>
int
main ()
{
  int a;
  float b=0.0;
  printf ("Enter Your Consumption in Units: ");
  scanf ("%d", &a);
  if (a > 0)
    b=b+(a*5);
  if (a > 100)
    b=b+((a-100)*5.5);
  if (a > 200)
    b=b+((a - 200) * 6.5);
  if (a>500)
    b=b+((a - 500) * 7);
  printf("\n Your Bill is Rs. %f",b);
  return 0;
}

On giving 750 as Input, the expected answer is 4750, but it is showing 12650. also What would be a smarter method of doing this rather than using and and operators within each if statement?

Answer 1

With a equal 750 all if-statements will be true so your code will calculate 750 x 5 + (750-100) x 5.5 + (750-200) x 6.5 + (750-500) x 7 which is not what you want.

You could change the order of the if-statements and reduce a with the amount paid so far.

To keep it in the spirit of your code, it could look like:

  if (a>500)
  {
    b=b+((a - 500) * 7);
    a = 500;
  }

  if (a > 200)
  {
    b=b+((a - 200) * 6.5);
    a = 200;
  }

  if (a > 100)
  {
    b=b+((a-100)*5.5);
    a = 100;
  }
  
  b=b+(a*5);

A more "generic" and easier to maintain approach could be:

struct price_range
{
    unsigned limit;
    double price;
};

const struct price_range pr[] =
{
    {  0, 5.0},
    {100, 5.5},
    {200, 6.5},
    {500, 7.0}
};    

double calculate_price(unsigned amount)
{
    size_t i = sizeof pr / sizeof pr[0];
    double p = 0;    
    do
    {
        --i;
        if (amount > pr[i].limit)
        {
            p += (amount - pr[i].limit) * pr[i].price;
            amount = pr[i].limit;
        }
    } while (i > 0);
    return p;
}