如何计算浮点数的小数位？

Question

I want to check if a float32 has two decimal places or not. My javascript way to do this would look like:

step  := 0.01
value := 9.99

if int(value/step) % 1 == 0 {
    printf("has two decimal places!")
}

The above example also works. However it will not work when step is incorrect as go then cannot properly cast from float64 to int.

Example:

step  := 0.1
value := 9.99

if int(value/step) % 1 == 0 {
    printf("has two decimal places!")
}

Compiler Error: constant 9.99 truncated to integer

When we use dynamic values it will just return true for every case.

So what is the appropriate way to count decimal places?

Answer 1

int value % 1 is always zero!

I suggest an alternative way:

value := float32(9.99)
valuef := value*100
extra := valuef - float32(int(valuef))
if extra < 1e-5 {
    fmt.Println("has two decimal places!");
}

http://play.golang.org/p/LQQ8T6SIY2

Update

package main

import (
    "math"
)

func main() {
    value := float32(9.9990001)

    println(checkDecimalPlaces(3, value))
}

func checkDecimalPlaces(i int, value float32) bool {
    valuef := value * float32(math.Pow(10.0, float64(i)))
    println(valuef)
    extra := valuef - float32(int(valuef))

    return extra == 0
}

http://play.golang.org/p/jXRhHsCYL-