Solution written in Go for challenge 2012-10-12 on ProgrammingPraxis.com

challenge-2012-10-12.go

// The MIT License (MIT)
// Copyright (c) 2012 Christian Siegert
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

// Exercise:
// 	In the study of probability, the Birthday Paradox states that in a group of
// 	23 people, there is a 50% chance that two will have the same birthday; in a
// 	group of 57 people, the odds rise to 99%. Your task is to simulate the
// 	birthday paradox over many trials and verify the odds.
// 	http://programmingpraxis.com/2012/10/12/birthday-paradox/

package main

import (
	"fmt"
	"math/rand"
)

func main() {
	fmt.Printf("Chance of same birthday in group with 23 people: %.0f%%\n", getChanceOfSameBirthdayInGroup(23)*100)
	fmt.Printf("Chance of same birthday in group with 57 people: %.0f%%\n", getChanceOfSameBirthdayInGroup(57)*100)
}

func getChanceOfSameBirthdayInGroup(groupSize uint) float32 {
	n := 10000
	numberOfSameBirthdays := 0

	for i := 0; i < n; i++ {
		if isSameBirthdayInGroup(groupSize) {
			numberOfSameBirthdays++
		}
	}

	return float32(numberOfSameBirthdays) / float32(n)
}

func isSameBirthdayInGroup(groupSize uint) bool {
	birthdays := make(map[int]uint8)

	for i := uint(0); i < groupSize; i++ {
		birthday := rand.Intn(365)
		birthdays[birthday]++

		if birthdays[birthday] == 2 {
			return true
		}
	}

	return false
}

Console output:

$ go run challenge-2012-10-12.go
Chance of same birthday in group with 23 people: 50%
Chance of same birthday in group with 57 people: 99%