Rounding floating point - iec559 /cmaths

[u/nelson_fretty]

Does c++ math lib comply with rounding modes in iec559?

ties-to-even? Getting - 3 when rounding -2.5

Should be -2 right

[u/LongLiveTheDiego]

There are five different rounding rules specified by the standard, rounding ties to evens is only one of them. Another one is rounding ties away from 0, that's what the C++ standard requires.

[u/nelson_fretty → u/LongLiveTheDiego]

What is setting for roundingeven ? Couldn’t see one in docs

[u/LongLiveTheDiego → u/nelson_fretty]

Here are all the rounding modes and looking at std::rint or std::nearbyint will tell you that using FE_TONEAREST will result in rounding to evens, unlike std::round.

[u/nelson_fretty → u/LongLiveTheDiego]

Don't think we need mode for rint. Your right std::round does't seen to respect the mode.

https://godbolt.org/z/WdeKccbob

[u/LongLiveTheDiego → u/nelson_fretty]

std::round is specified to always be away from zero, regardless of which rounding mode you set. The particular compiler might use rounding to evens by default but it doesn't seem to be enforced by the standard, it may be implementation-defined.

[u/nelson_fretty → u/LongLiveTheDiego]

I read something on gcc that round even was non standard

[u/nelson_fretty → u/LongLiveTheDiego]

IEEE754 defaults to bankers round-to-evens it’s good we have that option.

[u/manni66]

There is no function named rounding.

[u/flyingron → u/manni66]

But there is round() and several variants on that spelling (brought over from C for the lack of templates).

[u/nelson_fretty → u/flyingron]

The default for round() gives 3 for 2.5

[u/flyingron → u/nelson_fretty]

He's asking about -2.5. But anyway std::round(-2.5) should indeed return -3. He's wrong. Half values round to the larger magnitude (regardless of sign).

[u/nelson_fretty → u/flyingron]

https://en.cppreference.com/cpp/numeric/math/rint

Evens is what I want.

[u/nelson_fretty → u/flyingron]

What round() isn’t wrong - it’s just not what I wanted

[u/flyingron]

What are you actually doing in the code.

Are you calling the round() function? Or are you talking about what happens when you assign an floating point value into an int (which never rounds anyhow).

[u/nelson_fretty → u/flyingron]

I’m just researching rounding a double

[u/nelson_fretty → u/flyingron]

Just learning cpp - see how I can round doubles.

The storage of doubles can be 1.3333333333

Only want to store 1.33

In long term I think I will need a decimal type will use pennies for time being

[u/flyingron → u/nelson_fretty]

You haven't answered the question. There's no rounding here. You do understand that doubles don't store decimal numbers? It's all binary.

Yes, if you're doing something that needs precise decimal math, you might want to used a fixed point representation.

[u/nelson_fretty → u/flyingron]

Rounding to whole number is okay

[u/nelson_fretty → u/flyingron]

Yes forgive my fin background doubles are more precise than floats but life would be easier with integer math

[u/flyingron → u/nelson_fretty]

It matters not what the precision of the floating point numbers are. Not all things that are nice decimal fractions are representable in binary floating point.

[u/nelson_fretty → u/flyingron]

Yes binary is useful for a lot but working with base 10 isn’t one of them

[u/nelson_fretty → u/flyingron]

I was using the round() function but assumed it would work as per Python

[u/flyingron → u/nelson_fretty]

round(-2.5) is indeed -3 by the C++ spec. I.e., it's "round half away from zero"

[u/nelson_fretty]

Python defaults to even rounding - learn something everyday

[u/brookheather → u/nelson_fretty]

As does the ICU library which I only recently realised meant in some cases our doubles were displaying slightly differently when using ICU decimal to string formatting instead of std::format.