Go-货币计算

Question

I am trying to do monetary calculations with Go.

Looking around the standard library, big/math seems to be the one I can make use of.
And I ran some codes to check how it works.

First I used Float.SetFloat64()

var f = 18.9
var f2 = 1.65
bf := new(big.Float)
bf.Add(bf, new(big.Float).SetFloat64(f))
bf.Add(bf, new(big.Float).SetFloat64(f2))
result, _ := bf.Float64()
fmt.Println(result)

This gives me the result: 20.549999999999997
Just like the calculation with float64 type.
Since I'm dealing with money, the result must be 20.55.

Instead, when using Float.SetString()

var s = fmt.Sprintf("%f", 18.9)
var s2 = fmt.Sprintf("%f", 1.65)
bf := new(big.Float)
bf2, _ := new(big.Float).SetString(s)
bf3, _ := new(big.Float).SetString(s2)
bf.Add(bf, bf2)
bf.Add(bf, bf3)
result, _ := bf.Float64()
fmt.Println(result)

This gives me the result: 20.55
It SEEMs I can use this for my purpose (but I am not sure..).

My questions are

1. Why the difference beteween using Float.SetFloat64() and Float.SetString()?

2. Are there any pitfalls when using Float.SetString() for monetary calculations?

Thanks in advance, and forgive my English.

[EDIT]

I know float types should be avoided for representing money value.
but the choice of the type is not really under my control..

I want to know the answers to either(or both) of the above two questions.

Or the reason why Float.SetString() gives me the seemingly correct results is also helpful.

Answer 1

Never, ever, never use float for currency. Its an imprecise type, so you end up having to round calculations off and you end up losing (or gaining) a penny here and there. You may think that's ok but your accountant will be able to tell you its not. You have to be perfectly accurate.

So either use an specialised arbitrary-precision decimal type, or use an integer and hold pennies (or fractions of a penny) and display it appropriately. (just like how you hold dates as seconds since 1970, yet display them as dd-mm-yyyy).

PS. Never use floats for currency. Really.

PPS. Do not use floats for currency, not ever never ever.