Golang四舍五入保留两位小数
Sprintf
四舍六入:
value, _ := strconv.ParseFloat(fmt.Sprintf("%.2f", 9.824), 64)
fmt.Println(value) //9.82
value, _ = strconv.ParseFloat(fmt.Sprintf("%.2f", 9.826), 64)
fmt.Println(value) //9.83
第三位为5且5之后有有效数字,满足五入:
value, _ := strconv.ParseFloat(fmt.Sprintf("%.2f", 9.8251), 64)
fmt.Println(value) //9.83
value, _ = strconv.ParseFloat(fmt.Sprintf("%.2f", 9.8351), 64)
fmt.Println(value) //9.84
第三位为5且5之后没有有效数字:
网上有人说,第二位为奇数则进位,第二位为偶数则舍去,例如:
value, _ := strconv.ParseFloat(fmt.Sprintf("%.2f", 9.825), 64)
fmt.Println(value) //9.82
value, _ = strconv.ParseFloat(fmt.Sprintf("%.2f", 9.835), 64)
fmt.Println(value) //9.84
但是:
value, _ := strconv.ParseFloat(fmt.Sprintf("%.2f", 9.815), 64)
fmt.Println(value) //9.81 居然舍去了
value, _ = strconv.ParseFloat(fmt.Sprintf("%.2f", 9.845), 64)
fmt.Println(value) //9.85 居然进位了
所以,如果想满足正常的四舍五入逻辑,最好不要使用Sprintf处理。
math.Trunc
fmt.Println(math.Trunc(9.815*1e2+0.5)*1e-2) //9.82
fmt.Println(math.Trunc(9.825*1e2+0.5)*1e-2) //9.83
fmt.Println(math.Trunc(9.835*1e2+0.5)*1e-2) //9.84
fmt.Println(math.Trunc(9.845*1e2+0.5)*1e-2) //9.85
以上结果显示符合四舍五入,但是偶尔会出现精度问题:
fmt.Println(math.Trunc(3.3*1e2+0.5)*1e-2) //3.3000000000000003
fmt.Println(math.Trunc(3.3000000000000003*1e2+0.5) * 1e-2) //3.3000000000000003
同样使用Trunc,稍作调整:(此方案被大家提醒后发现确实存在问题,文末有修改方案)
n10 := math.Pow10(2)
fmt.Println(math.Trunc((9.815+0.5/n10)*n10) / n10) //9.82
fmt.Println(math.Trunc((9.825+0.5/n10)*n10) / n10) //9.83
fmt.Println(math.Trunc((9.835+0.5/n10)*n10) / n10) //9.84
fmt.Println(math.Trunc((9.845+0.5/n10)*n10) / n10) //9.85
fmt.Println(math.Trunc((3.3+0.5/n10)*n10) / n10) //3.3
fmt.Println(math.Trunc((3.3000000000000003+0.5/n10)*n10) / n10) //3.3
符合四舍五入规则。
如果要固定显示两位小数,需转换为string类型,前提是传入的数值,已经做过两位小数处理,否则依旧有进位问题:
value := strconv.FormatFloat(3, 'f', 2, 64)
fmt.Println(value) //3.00
value = strconv.FormatFloat(3.3, 'f', 2, 64)
fmt.Println(value) //3.30
value = strconv.FormatFloat(9.815, 'f', 2, 64)
fmt.Println(value) //9.81 被舍去
value = strconv.FormatFloat(9.82, 'f', 2, 64)
fmt.Println(value) //9.82
内容修改
错误示例:
n10 := math.Pow10(2)
fmt.Println(math.Trunc((129.975+0.5/n10)*n10) / n10) // 129.97
fmt.Println(math.Trunc((34423.125+0.5/n10)*n10) / n10) // 34423.12
代码修改:
package main
import (
"fmt"
"github.com/shopspring/decimal"
)
func main() {
v1, _ := decimal.NewFromFloat(9.824).Round(2).Float64()
v2, _ := decimal.NewFromFloat(9.826).Round(2).Float64()
v3, _ := decimal.NewFromFloat(9.8251).Round(2).Float64()
fmt.Println(v1, v2, v3)
v4, _ := decimal.NewFromFloat(9.815).Round(2).Float64()
v5, _ := decimal.NewFromFloat(9.825).Round(2).Float64()
v6, _ := decimal.NewFromFloat(9.835).Round(2).Float64()
v7, _ := decimal.NewFromFloat(9.845).Round(2).Float64()
fmt.Println(v4, v5, v6, v7)
v8, _ := decimal.NewFromFloat(3.3).Round(2).Float64()
v9, _ := decimal.NewFromFloat(3.3000000000000003).Round(2).Float64()
v10, _ := decimal.NewFromFloat(3).Round(2).Float64()
fmt.Println(v8, v9, v10)
v11, _ := decimal.NewFromFloat(129.975).Round(2).Float64()
v12, _ := decimal.NewFromFloat(34423.125).Round(2).Float64()
fmt.Println(v11, v12)
}
结果如下:
9.82 9.83 9.83
9.82 9.83 9.84 9.85
3.3 3.3 3
129.98 34423.13
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