According to the go spec <https://tip.golang.org/ref/spec#Constant_expressions>: "Constant expressions are always evaluated exactly; intermediate values and the constants themselves may require precision significantly larger than supported by any predeclared type in the language"
Hence there's a difference between: * doing an exact addition of 0.1 + 0.2 , then converting the exact value 0.3 to a float64; versus * converting 0.1 to float64, converting 0.2 to a float64, and then adding them using float64 precision. https://go.dev/play/p/x7vf_064RUf On Sunday, 10 April 2022 at 22:16:21 UTC+1 wagner riffel wrote: > On Sat Apr 9, 2022 at 3:56 PM CEST, 'Jack Li' via golang-nuts wrote: > > Why literal operation is exact, variable is not? > > > > fmt.Println(0.1 + 0.2) // 0.3 exactly > > fmt.Println(x + y) // 0.30000000000000004 > > > > Both aren't exact because floats can't represent 0.3 exactly, they > differ because literals and constants expressions have arbitrary > precision (http://golang.org/ref/spec/#Constants), so "0.1" and "0.2" > in "0.1 + 0.2" are exact, then when the exact "0.3" result is > converted to a float64 it becomes the closest possible, You can check > this if you ask to print with more precision > > x, y := 0.1, 0.2 > fmt.Printf("%.17f...\n", 0.1+0.2) // 0.29999999999999999... > fmt.Printf("%.17f...\n", x+y) // 0.30000000000000004... > > https://go.dev/play/p/2qSfoCZGaD6 > > -w > -- 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/a487a8bb-caf7-4b74-b81c-93d3d689a5ecn%40googlegroups.com.
