This informal discussion sprung from Suzi's post describing a problem with this program [code reproduced with permission]:
#include <stdio.h>
char line[50];
float amount; /* the amount of $ as entered by the user */
int total; /* amount * 100, converted to an interger */
int dollars; /* # of dollars needed to make change */
int quart; /* # of quarters needed */
int dime; /* # of dimes needed, etc. */
int nick;
int penn;
int main()
{
while (1) {
/* inputting dollar amount */
printf("Enter the amount of money to make change for: $");
fgets(line, sizeof(line), stdin);
if (sscanf(line, "%f", &amount) <= 0)
break;
while (amount <= 0) {
printf("Please enter an amount greater than 0: $");
fgets(line, sizeof(line), stdin);
if (sscanf(line, "%f", &amount) <= 0)
break;
}
total = amount * 100;
/* determining how many of each coin is needed */
dollars = total / 100;
total %= 100;
quart = total / 25;
total %= 25;
dime = total / 10;
total %= 10;
nick = total / 5;
penn = total % 5;
/* printing the change needed. Program should only print a type of
* coin if it is needed to make change (i.e. it should not print
* "0 quarters". */
printf("To make change for $%.2f, you need:\n", amount);
if (dollars > 1)
printf("%d dollars\n", dollars);
if (dollars == 1)
printf("%d dollar\n", dollars);
if (quart > 1)
printf("%d quarters\n", quart);
if (quart == 1)
printf("%d quarter\n", quart);
if (dime > 1)
printf("%d dimes\n", dime);
if (dime == 1)
printf("%d dime\n", dime);
if (nick == 1)
printf("%d nickel\n", nick);
if (penn > 1)
printf("%d pennies\n", penn);
if (penn == 1)
printf("%d penny\n", penn);
}
return (0);
}
It didn't behave correctly when there was change of a single penny (1 cent, or 0.01 dollars) involved.
Mary made a long post in response which eventually identified a floating point error, a quite subtle bug.
Online references
Solutions
Different solutions to Suzi's problem were suggested:
- Suzi's solution [manually rounding to correct for floating point errors]
- Christopher's solution [using round or lround]
- Conor's solution [correcting manual rounds for negative numbers] and his correction
- Lorne's solution [cutting floating point numbers out of the equation altogether]