arithmetic operations print wrong result

Question

I have made a simple program to print out numbers by using arithmetic operations. Every thing works fine, except last operator. It prints result as : 1 which is wrong and the expected result should be 1.7. What is wrong with my program I have made. Why does it print like this?

#include <iostream>
#include <exception>


class Money
{
public:
    Money(float amount = 0) : m_amount(amount){}

    // logic operations
    bool operator==(const Money& other) const
    {
        return m_amount == other.m_amount;
    }

    // arithmetic operations
    Money operator*=(const Money& other)
    {
        m_amount *= other.m_amount;
        return *this;
    }

    Money operator/=(const Money& other)
    {
        if (other.m_amount == 0)
            throw std::invalid_argument("Division by zero");

        m_amount /= other.m_amount;
        return *this;
    }

    friend std::ostream& operator<<(std::ostream& os, const Money& money)
    {
        return os << '$' << money.m_amount;
    }

private:
    float m_amount;
};

int main()
{
    Money my_money(1.7f);

    std::cout << (my_money *= my_money) << '\n';
    std::cout << (my_money /= my_money) << '\n'; // <-- wrong it should be 1.7
}

`my_money /= mu_money` is equivalent to `mymoney = my_money/mymoney` which is equal to 1 — Brahim, Nov 06 '15 at 10:44
please dont post so much irrelevant code, to see the "problem" two methods would have been sufficient — 463035818_is_not_an_ai, Nov 06 '15 at 10:47
Also, such a big class for something which is actually a float. — Mr Lister, Nov 06 '15 at 10:54
And are you sure you want to be able to multiply two money classes? I mean, it's not possible in real life; you'd end up with squared money. — Mr Lister, Nov 06 '15 at 10:59
@Brahim .. thanks .., i was confused with those arithmetic operations. its quite clear now. — MORTAL, Nov 06 '15 at 10:59
@MrLister .. no ,, its my practice code on arithmetic operations. — MORTAL, Nov 06 '15 at 11:01
@MORTAL write down your maths analytically first, and **after** that turn it into code. — decltype_auto, Nov 06 '15 at 11:06
@MORTAL Especially for money you shouldn't use floating point. I would recommend to redesign your class to handle cents instead of dollars. See here: http://stackoverflow.com/questions/2100490/floating-point-inaccuracy-examples — Simon Kraemer, Nov 06 '15 at 11:10
@decltype_auto .. absolutely i'm going on to do that from now and on. — MORTAL, Nov 06 '15 at 11:10
@MORTAL: forgive me - Friday is my wisenheimer day: if you start programming numerics, it doesn't hurt to learn a little (modern, 95) Fortran. It still may look punch-cardly, but compiles and runs lightning fast, and one can learn a lot about vectorization and side-effect free loop optimization from that, which also enhances the feeling for modern C++ algorithms. wisenheimer mode: OFF — decltype_auto, Nov 06 '15 at 11:17

score 2 · Accepted Answer · answered Nov 06 '15 at 10:47

2

my_money /= my_money

is in maths talk

x = a/a, x,a ∈ ℝ\{0}

and because the (ℝ,+,⋅) constitute a field, that has to yield the one-element of the multiplicative sub-group (ℝ, ⋅), and that is 1.

answered Nov 06 '15 at 10:47

decltype_auto

1,706
10
19

Like the posh maths bit, that's got to be worth a plus one. – Bathsheba Nov 06 '15 at 10:52
@Bathsheba When I once a counter-proofed a theoretical physicist's attempt to invalidate Maxwell's, Lorentz's and Einstein's work with a single stroke, he started blaming me for being a dumb numeric only; that's when I made it a habit to get analytical at the earliest possible stage. ;) – decltype_auto Nov 06 '15 at 11:00

arithmetic operations print wrong result

1 Answers1