C++ Money Class - storing and rounding cents

Question

thanks in advance. I'm writing a C++ assignment for class where we're creating our own money/currency class. I'm having trouble figuring out why my passing of a float isn't giving me enough precision.

Implementation:

private:
    long int DOLLARS;
    short int CENTS;

Currency::Currency(float tmpMoney)
{
// cout << "tmpMoney is: " << tmpMoney << endl;
    int tmpDollar;
    float tmpCent;
    tmpDollar = static_cast<int>(tmpMoney);
    tmpCent = (tmpMoney-tmpDollar)*100;
    DOLLARS = tmpDollar;
    CENTS = tmpCent;
    }

Main Program:

Currency c1(2342.59);
Currency c2(2342.515); // there is no half cent coin, round it to 2342.52

If I output 'tmpMoney' it just gives me (for c2) 2345.51. I'm not sure how to round .515 if the value doesn't even go that far.

If you can avoid it, don't use floating point for money. You typically want to use an integer number of the smallest unit you care about (apparently cents in this case). — Jerry Coffin, Sep 17 '14 at 19:43

score 3 · Answer 1 · edited May 23 '17 at 12:15

It's a bad idea to make a currency type be constructible from floating-point type.

7 decimal digits is in general beyond float precision. You can still get desired output by:

float tmpMoney = 2342.515;
cout << setprecision(7) << tmpMoney << endl;
// 2342.515

But the internal representation is far from perfect:

cout << setprecision(10) << tmpMoney << endl;
// 2342.514893

If the number is large enough, you'll lose more:

float tmpMoney = 123456789.12;
cout << setprecision(12) << tmpMoney << endl;
// 123456792

So you may decide to use double instead, but you should be really careful because for large enough numbers you'll get the same errors:

double tmpMoney = 3333333333333333.42; // 17 digits 
cout << setprecision(18) << tmpMoney << endl;
// 3333333333333333.5

If there is a chance that you'll have such numbers, don't initialize Currency with double either.

I would advise you to have just a constructor like Currency(int dollars, int cents).

You can also check this question for some insights.

Olayinka · Accepted Answer · 2014-09-17T20:09:44.317

1

    #include <cmath>

    CENTS = round(tmpCent);

Due to floating point representation, this may not always give the right result. The closest you can get is have a margin of error epsilon, say

    #define EPS 0.000001

then you can do

    CENTS = round(tmpCent + EPS);

Note that this will accept values that are represented as 0.499999 <= x < 0.5

And it's preferable to use double and not float to keep the precision as close as possible.

edited Sep 17 '14 at 20:09

answered Sep 17 '14 at 19:43

Olayinka

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That doesn't work. tmpMoney's value at the start of the constructor is "2342.51" so it doesn't include a third number after the decimal point. – ThePeter Sep 17 '14 at 19:47
1

@DancinPeter use double not float. – Olayinka Sep 17 '14 at 19:48
I've written my entire thing based on float. I tried changing it, but I'm getting TONS of ambiguity issues. – ThePeter Sep 17 '14 at 19:52
1

@DancinPeter the problem is floating point precision. change the 5 to 6 and it will work just right. The number isn't represented as you see it. – Olayinka Sep 17 '14 at 19:55
I'm not the one testing the program. My teacher is. So if he decides he wants to test .515 then I'm screwed. – ThePeter Sep 17 '14 at 19:58

C++ Money Class - storing and rounding cents

2 Answers2