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#golang §56 Exercise: Errors

A Tour of Go chapter 56

Exercise: Errors

Copy your Sqrt function from the earlier exercises and modify it to return an error value.
Sqrt should return a non-nil error value when given a negative number, as it doesn't support complex numbers.
Create a new type

type ErrNegativeSqrt float64

and make it an error by giving it a

func (e ErrNegativeSqrt) Error() string

method such that ErrNegativeSqrt(-2).Error() returns "cannot Sqrt negative number: -2".
Note: a call to fmt.Print(e) inside the Error method will send the program into an infinite loop. You can avoid this by converting e first: fmt.Print(float64(e)). Why?
Change your Sqrt function to return an ErrNegativeSqrt value when given a negative number.


package main

import (

type ErrNegativeSqrt float64

func (err ErrNegativeSqrt) Error() string {	
	return fmt.Sprintf("cannot Sqrt negative number: %v", float64(err))

func Sqrt(f float64) (float64, error) {
	z := f / 2
	if f < 0 {
		return 0, ErrNegativeSqrt(f)
	var prez float64 = 0
	for math.Abs(z - prez) > 1e-10 {
		prez = z
		z = z - (z * z - f) / (z * 2)
	return z, nil

func main() {