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(Instructor)This is coding challenge number two. 2

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And in this one, 3

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we're gonna go back to Julia in case study about dogs. 4

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And this time, they want to convert the ages of the dogs 5

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to human ages, and then calculate their average age. 6

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So your job is to create 7

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this calc average human age function. 8

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And this function accepts an array of dogs ages. 9

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And it does the following four tasks, 10

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or actually just three tasks. 11

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So first, calculate the age of all the dogs 12

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in human years using this formula. 13

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So if the dog is less or equal two years old, 14

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then the human age is simply twice the dog's age. 15

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But if the dog is older than two years, 16

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then the human age is calculated by this formula. 17

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All right. 18

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Then in the next step, exclude all the dogs 19

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that are less than 18 human years old. 20

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And that is essentially the same as keeping the dogs 21

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that are at least 18 years old here okay. 22

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Then, finally, 23

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calculate the average human age of all the adult dogs. 24

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And from the other challenges, you should already know how 25

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to calculate averages. 26

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Now, in this one, you will do it 27

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in a different way, hopefully, 28

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because the goal of this challenge 29

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is of course to use the Map, Filter and Reduce methods. 30

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Finally, then run this function 31

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for both these test data sets. 32

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So as I said, this is a way of training the map filter 33

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and array methods that we just studied earlier. 34

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And so good luck with this challenge. 35

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Take all the time you need and they see you here 36

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once you're ready. 37

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So were you successful in solving this one? 38

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I sure hope so. 39

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And in case there was some doubt, here is my solution. 40

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So this function takes an array of dog Ages. 41

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And then we want to convert all of these ages to human ages. 42

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So what we want here is to create 43

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a new array based on the original array. 44

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And so for that the map method is perfect. 45

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So we will have a new array called Human Ages. 46

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And that will be the result of calling the map method 47

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on the ages array. 48

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Then this callback function gets access to the current age, 49

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so the current element in the array. 50

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And now here what we need to return 51

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is the result of this formula. 52

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So this one here, 53

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and so let's write a ternary operator here. 54

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So if the age is less or equal two, 55

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then what we want to return here is two times the age, 56

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right? 57

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Because that dog age is of course, this age variable here, 58

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because it comes from the ages array, 59

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which itself is an array of dog Ages. 60

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Anyway, if the age is greater than two, 61

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then the result will be 16 plus the age times four. 62

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And so since this has an implicit return here, remember, 63

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it's like writing this here, 64

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okay? 65

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And so it's returning either this value here or actually, 66

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either this value, so twice the age, 67

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or four times the age plus 16. 68

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So that's the callback function, 69

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and this will then produce the array that we're looking for. 70

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Let's take a look. 71

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So human ages, and then we just need to call the function. 72

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And so here is the age converted to human ages. 73

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And it does indeed look correct. 74

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So let's assume it is correct. 75

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Then next up we want to filter for all the dogs 76

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that are at least 18 years old. 77

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And so that will then exclude all the dogs 78

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that are less than 18 years old. 79

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So that's gonna be the adult dogs and so that is human ages. 80

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And now we use the filter method and that makes sense right? 81

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Because it's the filter method who allows us 82

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to set a condition like this one. 83

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So when the age is at least, so great or equal 18, 84

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then put it in the new array which will be the adults array. 85

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And so let's lock that one as well. 86

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So actually, I want both of them here. 87

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So human ages 88

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and the adults. 89

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And so indeed, these two here, 90

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the four and the two are gone. 91

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Alright. 92

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And now finally, we want to calculate the average. 93

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Okay? 94

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And so the average is basically adding all 95

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of these values together, 96

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and then dividing the result by five, 97

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so the length of the array. 98

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So that's essentially what we already did before. 99

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So that's gonna be adults.reduce, 100

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then the accumulator and the current age. 101

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And we want to return the accumulator plus the age. 102

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And the start is gonna be at zero. 103

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So this here will return the sum, 104

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so all of these ages here added together. 105

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That's now all we need to do is to divide 106

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this by the length of this array. 107

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And then we simply want to return 108

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this average value that we just calculated. 109

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So let's just see if this makes sense. 110

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Let's actually save it here in a variable, 111

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instead of logging into directly. 112

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So average one, 113

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and then also 114

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average two. 115

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And then just log both here to the console. 116

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And all right, here they are. 117

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And this looks pretty reasonable to me. 118

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And so our solution here works. 119

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Now, I just wanted to let that we could have done 120

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this average calculation here in a different way. 121

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So in a bit more complex way. 122

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So let me show you one property of an average. 123

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So let's say we want to calculate the average 124

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of two and three. 125

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So we do that 126

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by two plus three. 127

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So we add all of this together and then divided by two, 128

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because we have two values. 129

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And so that is then 2.5. 130

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However, we can also do it in a different way. 131

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So we can divide both of them by two immediately. 132

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So two divided by two, 133

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plus two divided by three, 134

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and this will also be 2.5. 135

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So these here are essentially the exact same thing. 136

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Okay, and so here, we can do this differently. 137

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So let me actually duplicate this line 138

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So that we can keep both here. 139

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So what I mean is that we can actually simply divide 140

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the current age here immediately by the length of the array. 141

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Right? 142

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Here, this is actually three divided by two. 143

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Okay, sorry for that mistake. 144

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And so what we did here was basically 145

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to divide both of the values always immediately by the two. 146

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And so that would be the same as immediately dividing 147

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the age here by the length of the array. 148

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So dividing this by the length of the array. 149

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And now remember that we can get 150

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this length immediately here from this parameter. 151

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Well, not really the length, but the array itself. 152

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So that's the fourth parameter. 153

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And so now we can say array.length, like this. 154

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And so see what happens now here. 155

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And indeed, the values are exactly the same. 156

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And so this way of calculating it 157

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is just the same as this one here, 158

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but we can do it kind of all in one line. 159

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And I also wanted to show you a good use case, 160

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for using this array here. 161

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In this case, we could of course, 162

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also just have written here, adult.length, 163

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but sometimes when we change 164

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these methods together especially 165

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then this is the only way we have 166

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of using the current array. 167

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And we will see a use case of that later. 168

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Now of course, I did not expect you to write it like this, 169

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but I just wanted to show you a good use case 170

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for the current array parameter that we have here. 171

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Okay, and with that, we finish the challenge, 172

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and I see you in the next video.