math: add a Round function

[gopherbot]

by bunge@google.com:

Would it be possible to add a math.Round(n float64, digits uint) float64 function that
rounds a floating point number to the specified number of digits? This has been
discussed on golang-nuts here:
https://groups.google.com/d/topic/golang-nuts/ITZV08gAugI/discussion

I am not entirely convinced by the required digit argument. Perhaps we should have:

Round(n float64) float64 {
  return RoundDigits(n, 0)
}

[remyoudompheng]

Comment 1:

What would this function be supposed to do?

[rsc]

Comment 2:

Floating point numbers don't have digits. 
The bar for being useful needs to be pretty high to be in the Go math package. We
accepted most of what's in the C library but we're not going to take all the routines
from other libraries. The Javascript round is a 1-liner: int(f+0.5).

Status changed to WontFix.

[gopherbot]

Comment 3 by bunge@google.com:

Another option is to round half away from zero, so this would be:
if n < 0 {
  return math.Ceil(n-0.5)
}
return math.Floor(n+0.5)
This is arguably no less complex than math.Abs, although I assume the latter is in the
Go library due to its inclusion in the C library.
If I have some time I will code up a Go rounding module with different rounding
strategies.

[chespinoza]

Comment 4:

Some news about this Issue?

[rsc]

Comment 5:

The issue is marked "won't fix".

[matthewrankin]

@rsc As a new gopher, not seeing a round function in the math package also tripped me up. The one-liner int(f+0.5) only works for positive values, which I assume is one reason JavaScript provides the Math.round() method to properly round numbers to the nearest integer.

I'd like to see this issue reconsidered, but in the meantime I'll use the following function. I'm only a new gopher, so I respect the core contributors' decision. But for those of us coming from Python, JavaScript, or Ruby, this may be a tripping hazard, albeit a small one.

func Round(val float64) int {
    if val < 0 {
        return int(val-0.5)
    }
    return int(val+0.5)
}

For those gophers wanting to round a float64 to a float64 with a given precision, here are two gists:

• https://gist.github.com/indytechcook/5706434
• https://gist.github.com/DavidVaini/10308388

[minux]

FYI, there is also a one-liner version of round(float64) int:
http://play.golang.org/p/iTYytKn6HA

func round(f float64) int {
return int(f + math.Copysign(0.5, f))
}

Unfortunately, math.Copysign couldn't be inlined by the
current gc implementation.

[matthewrankin]

@minux Thanks for the info. I have much to learn about Go.

[jameshartig]

The function above breaks for 0.49999999999999997

Updated function: http://play.golang.org/p/m3d40AQVS6

func round(f float64) int {
    if math.Abs(f) < 0.5 {
        return 0
    }
    return int(f + math.Copysign(0.5, f))
}