Why is my loop breaking early? - c

Question

I am writing a function for class that takes an amount of money and tells the user how many coins add up to that amount. I seem to have everything working just fine, except pennies. My loop will sometimes stop short and break before adding the proper amount. It usually will stop 1 penny short, but sometimes it will give me the proper answer (one value I have found that gives the correct answer is .09). I've tried changing the float to a double, and I have the same issue. I am pulling my hair out trying to figure out what I am doing wrong.
void change(float total)
{
int quarters, dimes, nickels, pennies;
quarters = 0;
dimes = 0;
nickels = 0;
pennies = 0;
printf("\nTotal value entered: %.2f", total);
while (total >= .25)
{
quarters += 1;
total -= .25;
}
while (total >= .10)
{
dimes += 1;
total -= .10;
}
while (total >= .05)
{
nickels += 1;
total -= .05;
}
while (total >= .01)
{
pennies += 1;
total -= .01;
}
printf("\nQuarters: %d \nDimes: %d \nNickels: %d \nPennies: %d\n\n", quarters, dimes, nickels, pennies);
}

Answer 1

It's almost certainly caused by the limited precision of floating point numbers.
You'll probably find that you're reaching a point where the remaining value is something like 0.009999942 rather than 0.1 and that's why you're exiting early.
But it can show itself even before you reach pennies, if you end up with something like 0.249999 left, which should be a quarter but precision limits may force down to two dimes and four pennies.
As to solving it, I'd get the floating point value rounded to an integer as quickly as possible (multiplying it by a hundred beforehand of course), then you don't have to worry about floating point precision.
You can do this with something like:
int itotal = total * 100 + 0.2;
then using itotal for your calculations:
while (itotal >= 25) {
quarters++;
itotal -= 25;
}
// and so on ...

Answer 2

I had a similar question a while back for one of my labs. Instead of a a while loop for each coin denominations, i had a single do..while with cascaded if statements. In my case the max cost of an item was $1, and I opted to work in ints, but you can format the final output later.
int price, remainder, quarters, dime, nickel, pennies;
printf("Enter the price of the item you bought:>");
scanf("%d", &price);
remainder = 100 - price;
do {
if (remainder >= 25)
{
quarters++;
remainder -= 25;
}
else if (remainder >= 10)
{
dime++;
remainder -= 10;
}
else if (remainder >= 5)
{
nickel ++;
remainder -=5;
}
else if (remainder >= 1)
{
pennies ++;
remainder -=1;
}
} while (remainder > 0);
printf("\nYour change will be dispensed as:\n Quarters: %d \n Dimes: %d \n Nickel: %d \n Pennies: %d \n\n", quarters, dime, nickel,pennies);
Hope it helps.

Answer 3

Floating point math is not exact. See http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

Answer 4

Floating point math is very complex and subject to many common mis expectations. Try converting your math to using int, and later translate it back.

Answer 5

As you are comparing float which subjected to be full of errors so you can use a roundof function and change the floating point number to integer.
int_total = round( total * 100.0 ); //include math.h
Now, change the loop as,
while (total >= 25)
{
quarters += 1;
total -= 25;
}