C++ Handle -0.0 from Database

Question

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At present I am working on piece of C++ code in which I need to read data from a database, and if database value is non-zero, then I need to apply some further logic.

But in the database there are values which are being calculated and can come out as -0.0. And this negative zero is being treated as Garbage value in C++ double variable. I have already initialized the value as 0.0 in constructor.

C++

// This list is being populated from Database
for (Sample::List<BalanceSheet>::Iterator i((Sample::List<BalanceSheet> &) Balance.Entries()); i.HaveItem(); ++i) 
{
    if (SomeCondition != "")
    {
        // This is where am getting Garbage values since GetBalance() returns -0.0
        if (i->GetBalance() != 0) 
        {
            DoOperation();
        }
    }
}

Posted 30-Nov-15 7:49am

WENzER

Updated 30-Nov-15 21:55pm

George Jonsson

v3

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Comments

Sergey Alexandrovich Kryukov 30-Nov-15 14:37pm

How can this garbage be related to garbage collection? :-)
—SA

enhzflep 30-Nov-15 18:14pm

Well, if you'd be kind enough to ask a question it will surely expedite the process.

Richard MacCutchan 1-Dec-15 3:14am

If you are using doubles to hold financial values you are in for a whole heap of trouble.

2 solutions

Solution 2

A C/C++ floating point value has always a defined state and is never "garbage". It is up to you handle special states like NaN (not a number), infinity, denormalized numbers and signed zeroes (yes, negative zero is a valid state). To see what your value is exactly print it out using a function that uses printf formatting setting the max. precision for doubles (format "%.15E").

However, the value may be "garbage" from the point of view of your application. Then you should handle such values accordingly. To avoid such values, ensure that only valid values (again from the point of view of your application) are written to the database.

Posted 30-Nov-15 21:09pm

Jochen Arndt

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User 59241 · Accepted Answer · 2015-11-30T12:24:00

This is not about garbage collection but about sound programming practices.
Doubles are not integers - while obvious this has subtle consequences. It is unlikely that the value is actually -0.0 but in fact a very small negative number which can be shown using your debugger. Doing this this would have answered the question for you.

Never test a double in this way:

C#

if (i->GetBalance() != 0) // This is a bug waiting to happen

Read here: https://isocpp.org/wiki/faq/newbie#floating-point-arith[^]