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How can a negative integer be represented as a binary in Go, using the two's complement notation?

For example:

n := int64(-1)
fmt.Printf("%b", n)
fmt.Println(strconv.FormatInt(n, 2))

Both lines print -1. The result shoud be like ffffffffffffffff.

I am sorry but did you mean you wanted to explore how int64 values are stored in the memory? You will need to use the unsafe package to force reinterpret a value in go.

This code does what you want:

package main

import (
 "fmt"
 "unsafe"
)

func main() 
 a := int64(-1)
 fmt.Printf("%xn", *(*[8]byte)(unsafe.Pointer(&a)))

I also wrote a func to programmatically calculate the two's complement notation of a int64 in case that you just wanted to calculate the value:

package main

import (
 "fmt"
 "math/big"
)

func main() 
 a := int64(-1)
 fmt.Printf("%xn", TwoComplement(a))


func TwoComplement(val int64) byte 
 n := big.NewInt(val)

 var r byte
 if n.Cmp(big.NewInt(0)) != -1 
 r = n.Bytes()
 else 
 mask := big.NewInt(1)
 mask.Lsh(mask, 64)

 r = n.Add(n, mask).Bytes()
 

 res := bytes.NewBuffer(byte)
 for i := 0; i < 8-len(r); i++ 
 res.WriteByte(0)
 
 res.Write(r)
 return res.Bytes()