Re: [go-nuts] float exactness

['Jack Li' via golang-nuts]

Thanks Robert,


It is great!


        a, _ := decimal.NewFromString("0.1")
        b, _ := decimal.NewFromString("0.2")
        c := a.Add(b)
        fmt.Println("decimal:", c)


        a = decimal.NewFromFloat(0.1)
        b = decimal.NewFromFloat(0.2)
        c = a.Add(b)
        fmt.Println("decimal:", c)


        a, _ = decimal.NewFromString("0.1")
        b, _ = decimal.NewFromString("0.35")
        c = a.Add(b)
        fmt.Println("decimal:", c)


        a = decimal.NewFromFloat(0.1)
        b = decimal.NewFromFloat(0.35)
        c = a.Add(b)
        fmt.Println("decimal:", c)



/*
$ go build && ./main 
decimal: 0.3
decimal: 0.3
decimal: 0.45
decimal: 0.45
$ 

*/




------------------ Original ------------------
From: "Robert Engels"<reng...@ix.netcom.com&gt;; 
Date: 2022年4月9日(星期六) 晚上10:22
To: "Jack Li"<ljh_...@qq.com&gt;; 
Cc: "golang-nuts"<golang-nuts@googlegroups.com&gt;; 
Subject: Re: [go-nuts] float exactness




There are several. 


See github.com/robaho/fixed


As to why, read up on numerical analysis. It’s an interesting topic. 

On Apr 9, 2022, at 8:56 AM, 'Jack Li' via golang-nuts 
<golang-nuts@googlegroups.com&gt; wrote:


Hi group,


1. 
Is there a package can do exact float operations, like Decimal in Python? For 
example:


x := 0.1; y := 0.2; z := x + y;


z will be exact 0.3


2.
Why literal operation is exact, variable is not?


fmt.Println(0.1 + 0.2) // 0.3 exactly


fmt.Println(x + y) // 0.30000000000000004




Thanks




func main() {
        x := 0.1
        y := 0.2
        fmt.Println("x + y &nbsp;:", x &nbsp; + y)
        fmt.Println("literal:", 0.1 + 0.2)


        x = 0.1
        y = 0.35
        fmt.Println("x + y &nbsp;:", x &nbsp; + y)
        fmt.Println("literal:", 0.1 + 0.35)
}


/*
$ go build &amp;&amp; ./main
x + y &nbsp;: 0.30000000000000004
literal: 0.3
x + y &nbsp;: 0.44999999999999996
literal: 0.45
$ 
*/




// Python:


// &gt;&gt;&gt; 0.1 + 0.2
// 0.30000000000000004


// &gt;&gt;&gt; float(Decimal('0.1') + Decimal('0.2'))
// 0.3
// &gt;&gt;&gt;


// &gt;&gt;&gt; 0.35 + 0.1
// 0.44999999999999996


// &gt;&gt;&gt; float(Decimal('0.35') + Decimal('0.1'))
// 0.45
// &gt;&gt;&gt;



 

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