golang - modulus using math big package

Reading up the documentation - http://golang.org/pkg/math/big/

Mod sets z to the modulus x%y for y != 0 and returns z. If y == 0, a division-by-zero run-time panic occurs. Mod implements Euclidean modulus (unlike Go); see DivMod for more details.

10%4 = 2 but I get 8 with this (using the math/big package to do the same thing) - http://play.golang.org/p/_86etDvLYq

package main

import "fmt"
import "math/big"
import "strconv"

func main() {
    ten := new(big.Int)
	ten.SetBytes([]byte(strconv.Itoa(10)))
	
	four := new(big.Int)
	four.SetBytes([]byte(strconv.Itoa(4)))
	
	tenmodfour := new(big.Int)
	tenmodfour = tenmodfour.Mod(ten, four)
	
	fmt.Println("mod", tenmodfour)
		
}

I most likely got something wrong. Where's the mistake?

It's because SetBytes is not doing what you think! Use SetInt64 instead.

ten := new(big.Int)
ten.SetBytes([]byte(strconv.Itoa(10)))

four := new(big.Int)
four.SetBytes([]byte(strconv.Itoa(4)))

fmt.Println(ten, four)

Result:

12592 52

And indeed, 12592%52 == 8

If you want to use numbers bigger than what int64 lets you manipulate, you can also use the SetString function:

n := new(big.Int)
n.SetString("456135478645413786350", 10)

Just an addition to julienc's answer, if you were to use SetBytes, you have to convert the number to bytes like this :

func int2bytes(num int) (b []byte) {
	b = make([]byte, 4)
	binary.BigEndian.PutUint32(b, uint32(num))
	return
}
func main() {
	ten := new(big.Int)
	ten.SetBytes(int2bytes(10))

	four := new(big.Int)
	four.SetBytes(int2bytes(4))

	fmt.Println(ten, four)
	
	tenmodfour := new(big.Int)
	tenmodfour = tenmodfour.Mod(ten, four)
	
	fmt.Println("mod", tenmodfour)
}