How to round up in C-CodePudding

#define _CRT_SECURE_NO_WARNINGS

#include <stdio.h>

int main(void)
{
    const double TAX = 0.13;
    double small = 17.96;

    double subTotal = small * 6;
    double taxes = subTotal * TAX;

    printf("j   SubTotal: %9.4lf Tax: %9.4lf \n", (double)subTotal, (double)taxes);

  return 0;
}

The output value is SubTotal: 107.7600 Tax: 14.0100 should come out.

My output is SubTotal: 107.7600 Tax: 14.0088

What should I do?

CodePudding user response：

The solution is to not ever use floating point types for currency.

If you have a decimal type available, use it.

If you don't, write one.

Or, at a minimum, use long integers to store cents.

CodePudding user response：

Do not use floating point numbers for currency because they are tricky and inexact in many cases. Here is a working solution that gives you the exact output you want, but you should know it is strange to print more than two digits after the decimal point if those extra digits are always zero.

The 99 in the code below is the trick you need to make the calculation always round up, instead of rounding down which is the normal behavior of unsigned integer division.

#include <stdio.h>
#include <stdint.h>

void print_money_with_strange_format(uint64_t amount)
{
  printf("%d.d00", amount / 100, amount % 100);
}

int main(void)
{
  uint64_t tax_rate = 13;
  uint64_t small = 1796;
  uint64_t subtotal = small * 6;
  uint64_t tax = (subtotal * tax_rate   99) / 100;

  printf("SubTotal: ");
  print_money_with_strange_format(subtotal);
  printf(" Tax: ");
  print_money_with_strange_format(tax);
  printf("\n");
}

CodePudding user response：

If you want to round to two decimal places, multiply by one hundred add one half, truncate to an int and then divide by one hundred. Like,

double round(double v) {
    return ((int)(v * 100   .5)) / 100.;
}

Then you can call that like

double taxes = round(subTotal * TAX);

And I get (with no other changes)

j   SubTotal:  107.7600 Tax:   14.0100

CodePudding user response：

It’s a weird requirement to display four decimal places while rounding to two, but you can satisfy it by displaying two decimal places and hard-coding the characters 00 after it:

printf("j   SubTotal: %9.2f00 Tax: %9.2f00 \n", subTotal, taxes);