%.2f exception

[w22296437978]

[robpike]

The printed result is correct due to rounding. 0.505 will be rounded up to 0.51. The print routines round, they don't truncate.

Just for fun, I tried it in C as well, and printf("%.2f\n", 1823.505) prints 1823.51 in C too.

Working as intended.

[w22296437978]

由于四舍五入，打印结果正确。0.505将四舍五入为0.51。打印例行程序不会截断。

只是为了好玩，我也在C语言中进行了尝试，并且也在C语言中进行了printf("%.2f\n", 1823.505)打印1823.51。

按预期工作。

but 2.445 => 2.44

[w22296437978]

How could this be?

[robpike]

To avoid bias, rounding a number ending in the digit 5 alternates between rounding up and rounding down, which means that overall all rounded numbers, half round up and half round down.

1 2 3 4 round down.
6 7 8 9 round up
5 rounds up half the time and down half the time.

See this program:

package main

import (
	"fmt"
)

func main() {
	for _, x := range ([]float64{0.005, 0.015, 0.025, 0.035, 0.045, 0.055, 0.065, 0.075, 0.085, 0.095}) {
		fmt.Printf("%.2f\n", x)
	}
}

which prints

0.01
0.01
0.03
0.04
0.04
0.06
0.07
0.07
0.09
0.10

[ianlancetaylor]

https://cr.yp.to/2005-590/goldberg.pdf