how to force to save all decimal points of bigFloat in file, instead of rounding

Let's assume I have a very accurate input number(string format), and after math/big manipulation, I want to convert to string format again.

package main

import (
	"fmt"
	"math/big"
	"os"
)

func check(e error) {
	if e != nil {
		panic(e)
	}
}


func main() {	
    // edit
    s := "3.1415926123456789123456789123456789123456789"
	var n, _ = new(big.Float).SetString(s)
	// var n = big.NewFloat(3.1415926123456789123456789123456789123456789)
	fmt.Println(n) // 3.1415926123456788 

	N := n.String()
	fmt.Println(N)  // 3.141592612

	d1 := []byte(N)
	err := os.WriteFile("./dat1.txt", d1, 0644) // 3.141592612
	check(err)
}

How to save a big Float like 3.1415926123456789123456789123456789123456789 into a file? I want to keep all the decimal points, or at least as much as possible

You can parse and store your input "precisely", but you must increase the precision (the default precision doesn't cover that). Use Float.SetPrec() for that (requires a bit-count).

When generating text representation, use Float.Text(), again, with sufficiently large precision (requires a decimal digit-count). If you don't know the required digit-precision, as per the doc, you may use a negative value to have the smallest number of decimal digits that is needed for the Float's mantissa bits.

For example:

s := "3.1415926123456789123456789123456789123456789"
fmt.Println(s)

n := big.NewFloat(0)
n.SetPrec(200)
n.SetString(s)

N := n.Text('f', 50)
fmt.Println(N)

N = n.Text('f', -1)
fmt.Println(N)

This will output (try it on the Go Playground):

3.1415926123456789123456789123456789123456789
3.14159261234567891234567891234567891234567890000000
3.1415926123456789123456789123456789123456789

I've just looked what String method does and its documentation (https://pkg.go.dev/math/big#Float.String).

> String formats x like x.Text('g', 10)...

So let's go to that method doc.

> func (x *Float) Text(format byte, prec int) string

> Text converts the floating-point number x to a string according to the given format and precision prec.

> The precision prec controls the number of digits ... A negative precision selects the smallest number of decimal digits necessary to identify the value x uniquely

All you need is just reading the doc.