You've fallen into a common trap. Base 2 floating point values cannot represent most decimal values precisely. This is why you should never, ever, do a simple equality test involving a F.P. value derived from a calculation. You always have to apply an epsilon to define a range within which the two F.P. values should be considered equal. The %v formatter does something similar. The base 10 value 0.29 is actually 0.28999999999999998002 when stored base 2 (that's from printing a long double var on a x86_64 platform).
Welcome to floating point pitfalls. Here be dragons! On Sat, Jan 25, 2020 at 7:14 PM Jason E. Aten <j.e.a...@gmail.com> wrote: > > https://play.golang.org/p/87bDubJxjHO > > I'd like to truncate a float64 to just 2 decimal places (in base 10), but > math.Trunc is not helping me here... ideally I put 0.29 in and I get 0.29 > out. > Suggestions? Playground examples appreciated. > > package main > > import ( > "fmt" > "math" > ) > > // truncate off everything after the last two decimals, no rounding. > func decimal2(x float64) float64 { > return math.Trunc(x*100) / 100 > } > > func main() { > x := 0.29 > y := decimal2(x) > fmt.Printf("x=%v -> y = %v", x, y) // prints x=0.29 -> y=0.28 > } > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/96528d64-a670-45ba-ad4c-0701dcd0d78d%40googlegroups.com > <https://groups.google.com/d/msgid/golang-nuts/96528d64-a670-45ba-ad4c-0701dcd0d78d%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- Kurtis Rader Caretaker of the exceptional canines Junior and Hank -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/golang-nuts/CABx2%3DD8JvQETOfkO63FWveyS7AbpKpLM1uVQsB6QH_mDNGajgw%40mail.gmail.com.
