How do I isolate the cents from a dollar.cent amount?

Question

New to programming. Taking class in C. Below is my attempt at a program that will print "What is your hourly wage?" then read the dollar.cent amount and calculate salary. Then print the salary in the form, "Your total income over a year is X dollars and Y cents.

To isolate dollars from the dollar.cent amount, I truncated the value by converting to an int from a double. I wasn't sure how to get the cents, so I figured I could subtract the dollars from the dollars.cents (*100) and I'd have the number of cents.

I run the program and it works fine, except I don't get the amount of cents I expect.

If the user enters 18.33 as the hourly wage. Then I get 31826 for total Dollars, 31836.40 for total Income. But when I subtract them and multiply by 100, I am given 39 cents instead of 40.

int main(void) {

double totalIncome       = 0.0;
int totalDollars         = 0;
int totalCents           = 0;
double hourlyWage        = 0.0;
int hoursPerWeek         = 40;
const int WEEKS_PER_YEAR = 52;

printf("What is your hourly wage? ");
scanf("%lf", &hourlyWage);

totalIncome = hourlyWage * hoursPerWeek * WEEKS_PER_YEAR;
totalDollars = totalIncome; //converts to int from double
totalCents = 10 * (totalIncome - totalDollars);

printf("Your total income over a year is %d dollars and %d cents", totalDollars, totalCents);

return 0;

}

If you compile with `-Wall`, do you get any warnings? I think that how you're assigning totalCents might lead to a loss of precision. But I'm not 100% sure. Compile with that flag, and check. — Ricky Mutschlechner, Jan 17 '17 at 03:26
I came up with this hack solution... totalCents = ((totalIncome + 0.005) - totalDollars) * 100 ; — Alex Butterfield, Jan 17 '17 at 03:30
If you use GCC as your compiler, using `gcc -Wall …` will add a lot more helpful diagnostics, basically pointing out errors in your program that you should fix before running it. If you use `gcc -Wall -Werror …`, you'll have to fix them as they'll be errors, not warnings. If you're using MS Visual Studio, you should check whether you get better warnings if you use `cl /W3` — if not, check the manual. — Jonathan Leffler, Jan 17 '17 at 03:34
@AlexButterfield `int totalCents = ((totalIncome + 0.005) - totalDollars) * 100` fails for many negative numbers and large numbers, fails for many edge conditions. Consider `totalIncome = 123.9999`. You idea will end up with 123 dollars and 100 cents. C has a good library in `` with many _round_ functions -use those — chux - Reinstate Monica, Jan 17 '17 at 06:21

cthl · Accepted Answer · 2017-01-17T03:47:53.713

4

The problem is that 100*(totalIncome - totalDollars) is not exactly 40, but 3.999999999941792e+01, thus casting it to int yields 39. This is a good example why one should never use floating points for computations with currencies.

BTW: You can simply avoid this issue altogether using something like scanf("%d.%d", &hourlyDollars, &hourlyCents);

edited Jan 17 '17 at 03:47

answered Jan 17 '17 at 03:38

cthl

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2
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Love this solution, but if the user puts 10 as opposed to 10.00 what will happen there? Will this still work? – Alex Butterfield Jan 17 '17 at 06:01
Yes, it will actually just read 10 into `hourlyDollars`. However, this might not be the most reliable solution to cover all cases, but your initial solution did not perform any checks either. – cthl Jan 17 '17 at 06:23

score 1 · Answer 2 · edited May 23 '17 at 12:01

To take a floating point money to floating point cents to the nearest whole number, use round() or rint()

double x;
double cents = rint(x * 100.0);

To take a floating point money to nearest integer cents, use lround() or llround()

long long cents = llround(x * 100.0);

To break into FP dollars into whole dollars and whole cents, scale, round and then separate.

double x;
double cents = rint(x * 100.0);
double cent =  fmod(cents, 100.0);
double dollar = (cents - cent)/100;

Avoid mixing types to support money. Use long long or double. Each has its short comings. For learner programs start with wide integer types of the smallest denomination (cents).

How do I isolate the cents from a dollar.cent amount?

2 Answers2