12.md

1. Writing a good suite of tests is not always easy, but the process of writing tests often reveals more about a problem then you may at first realize. For example, with our Average function what happens if you pass in an empty list ([]float64{})? How could we modify the function to return 0 in this case?

If we pass an empty list Average returns NaN because we are trying to divide by 0 (the length of the list). We can solve this by checking if the list is empty and returning 0 if so:

func Average(xs []float64) float64 {
    if len(xs) == 0 {
        return 0
    }

    total := float64(0) 
    for _, x := range xs {
        total += x
    } 
    return total / float64(len(xs))
}

2. Write a series of tests for the Min and Max functions you wrote in the previouse chapter.

package math 

import "testing" 

type minTestPair struct {
	values []float64
	min float64
}

type maxTestPair struct {
	values []float64
	max float64
}

var minTests = []minTestPair {
	{ []float64{1, 5}, 1},
	{ []float64{6, 2, 5}, 2},
	{ []float64{9, 22, 3}, 3},
}

var maxTests = []maxTestPair {
	{ []float64{1, 5}, 5},
	{ []float64{6, 2, 5}, 6},
	{ []float64{9, 22, 3}, 22},
}

func TestMin(t *testing.T) {
	for _, pair := range minTests {
		v := Min(pair.values)
		if v != pair.min {
			t.Error(
				"For", pair.values,
				"expected", pair.min,
				"got", v,
			)
		}
	}
}

func TestMax(t *testing.T) {
	for _, pair := range maxTests {
		v := Max(pair.values)
		if v != pair.max {
			t.Error(
				"For", pair.values,
				"expected", pair.max,
				"got", v,
			)
		}
	}
}