How can we truncate float64 type to a particular precision?

Question

package main

import (
    \"fmt\"
    \"strconv\"
    )

func main() {
    k := 10/3.0
    i := fmt.Sprintf(\"%.2f\", k)
    f,_ := strconv.ParseFloat(i, 2)
    fmt.Pri         


        

Answer 1

The answer by threeve brought me to this issue on GitHub where a solution based on math/big for rounding values is presented - this way the rounding method is used correctly:

package main

import (
    "fmt"
    "math/big"
)

func main() {
    f := new(big.Float).SetMode(big.ToNearestEven).SetFloat64(10/3.0)
    // Round to 2 digits using formatting.
    f.SetPrec(8)
    fmt.Printf("%.2f\n", f)
}

The rounding mode is also respected in threeve's example:

j := 0.045

f := new(big.Float).SetMode(big.AwayFromZero).SetFloat64(j)
// Round to 2 digits using formatting.
f.SetPrec(8)
fmt.Printf("%.2f\n", f)

-> correctly yields 0.05

Also, Go 1.10 has been released and added a math.Round() function, see this excellent answer by icza: Golang Round to Nearest 0.05

package main

import (
    "fmt"
    "math"
)

func main() {

    fmt.Println(Round(10/3.0, 0.01))

}

func Round(x, unit float64) float64 {
    return math.Round(x/unit) * unit
}

However, one should not use float for storing monetary values. (See: Why not use Double or Float to represent currency?) One way around this is using a library that implements decimal like https://github.com/ericlagergren/decimal or https://github.com/shopspring/decimal