Original subtitles

1

Hey guys. So I've got another code challenge for you. And this one,

2

as the title suggests, this is a difficult challenge,

3

and this is something that I expect some of you might need to spend 15,

4

20 minutes on. Others might need to spend 30, 40 minutes on,

5

and it might just be something that you need to have a think about, get stuck,

6

come back to it after a bit of a break or sleep on it,

7

come back to it the next day. It's not easy,

8

but what I'm trying to say is really,

9

really give it all you have before you check the solution video,

10

because it's a really, really interesting challenge.

11

So the idea is that we want you to write a program that works out

12

whether if any given year is a leap year.

13

We know that a normal year has 365 days,

14

but leap years have an extra day in February.

15

And the reason is actually really, really interesting. If you're interested,

16

there's a really good explanation of how leap years work and why we have them at

17

this YouTube link. So just go ahead and copy this URL and check it out.

18

Now, the way that you work out if a given year is a leap year,

19

so let's say we pick one out of thin air, 2020,

20

is that on every year that is evenly divisible by four,

21

so that means divisible by four with no remainder.

22

Then that is a leap year, but except the years

23

that is also evenly divisible by a hundred,

24

unless the year is also evenly divisible by 400.

25

Now 2020 divided by four is equal to 505.

26

So it's evenly divisible by four. There's no remainder.

27

But the rule says that even if it's divisible by four,

28

if it's evenly divisible by a hundred,

29

then that is still not going to be a leap year.

30

So let's try that 2020 divided by a hundred equals 20.2.

31

So this is in fact not evenly divisible by a hundred.

32

So that means it is still a leap year. But then it says

33

unless the years also evenly divisible by 400.

34

So 2020

35

divided by 400 is not evenly divisible.

36

So it is in fact, still a leap year.

37

The trickiest part of this entire challenge

38

I would say is getting your head around these three sentences: a year that is

39

divisible by four with no remainder is a leap year,

40

except if it's also divisible by a hundred. In that case,

41

it's no longer a leap year,

42

unless that year also happens to be evenly divisible by 400 then it is still a

43

leap year. What I recommend doing is a)

44

if English is not your native language

45

then have a look online for the rules for determining a leap

46

year in your own language, because these words, except

47

and unless, are so,

48

so integral to how this algorithm works. And you really,

49

really need to understand exactly what it means.

50

Then what I recommend you doing is to go ahead and create a flow chart,

51

just like what I've done in previous examples.

52

Head over to a website like draw.io,

53

or simply just write it on a piece of paper.

54

Draw it out and visualize what this actually means in terms of your code.

55

Now, if you really get stuck on that,

56

I've also created a flowchart for you.

57

So if you actually get stuck creating the flowchart,

58

there is one there that you can refer to as well.

59

But it's a really good exercise

60

creating your own flowchart based on what it is that you want your program to

61

do, and your algorithm or your code should correctly tell us whether

62

if it's a leap year or not a leap year. So it just has to print out leap year

63

if it is like 2400, and if it's not a leap year,

64

like 1989, then just print, not leap year.

65

This is the time to pause the video and really give this challenge

66

everything you've got. I'll see you back here once you're done.

67

[inaudible]

68

All right. So how did you get on? As I mentioned,

69

this is a really difficult challenge. If you haven't managed to solve it,

70

then try to come back to it a little bit later on and spend more time on it.

71

This is a decisive moment. Don't give up on yourself. Keep going.

72

I'm always going to be here for you with the solution.

73

You're not going to get stuck. It's really,

74

really important to try and solve it yourself. Now,

75

the first thing I mentioned is turning this rule for determining whether if a

76

year is a leap year or not into a flow chart.

77

So I've created one which I linked to in the challenge

78

and it's very simple.

79

We start off by asking ourself is a particular year that we have in mind,

80

um, let's say the year 2000, is it cleanly divisible by four?

81

Which means can you divide it by four with zero remainder?

82

Well, if the case is no, then it already is definitely not a leap year.

83

But if the case is yes, then we have to ask ourselves some more questions.

84

Namely, can we divide it by a hundred with no remainder?

85

Well, if we can't, then it's definitely a leap year, but if we can,

86

then we have to check yet again. Can we divide it by 400 with no remainder?

87

If the answer is yes, then it's a leap year. If the answer is no,

88

then it's not a leap year. Let's try and code that logic out,

89

keep your flow chart up somewhere so that you can write your code while

90

referring to the diagram.

91

We start off by checking whether if the year that came in through the input is

92

divisible by four with no remainder.

93

And you might remember from our previous code exercise

94

that the way we do this is using the modulo. So year

95

modulo four will give us the remainder.

96

So if the remainder is equal to zero, well in that case it's cleanly

97

divisible by four. And you can check this out yourself in the console.

98

Let's say,

99

if you have 8 % 4, 4 goes into 8 cleanly,

100

there's two four's in an eight. The remainder is zero.

101

Whereas 9 % 4, 4 + 4 is 8 remainder 1.

102

So if the remainder from the year divided by four is equal to zero,

103

then it is cleanly divisible by four. So in this case,

104

we'll have to go into yet another if statement.

105

But if it's actually false, then it's definitely not a leap year.

106

So the else statement that matches this

107

if statement is going to tell us for sure that this is not a leap year.

108

But if it is true, then we have to check the next thing. Is it cleanly

109

divisible by a hundred?

110

So we check if the year % 100 is equal to zero.

111

And if the answer is yes,

112

then we have to check that year against another question. But if it's no,

113

then it's definitely a leap year. In this case that will lead to an if statement

114

if it's true, but if it's false,

115

then we can simply print leap year because that's what it's going to be.

116

The final question that we have to ask ourselves about the year is,

117

is it cleanly divisible by 400?

118

So now if year % 400 is equal to zero,

119

well, in this case, if it is true,

120

then it's definitely a leap year.

121

If it's false, then it's definitely not a leap year. So, you can print

122

lear year if it's true, otherwise or if it's false,

123

then we can print not leap year.

124

And that's it.

125

That's all the code that you would need in order to represent this logic.

126

Now there's plenty of other ways you could have written your code,

127

but this is probably the most readable. Now,

128

if you've written some different code

129

but it still uses these rules to work out

130

whether if a given year is a leap year and it's correct,

131

then that's fine as well.

132

There's plenty of ways that you can solve this challenge.

133

The most important thing I want you to take away from this code exercise is that

134

you can turn any sort of logic into a flowchart to be able to visualize it.

135

And once you've got your logic straight,

136

then converting it into code is much, much simpler.

137

If you haven't figured out the logic and then you try to start writing code,

138

then it's going to be a mess.

139

So first, get to the stage of being able to create your own flowcharts and then

140

take those flow charts and turn them into code.

141

If you got stuck on this challenge

142

or if you didn't manage to complete it for some reason,

143

head back to it now and give it some extra attention, because this is really,

144

really important. It's in these moments, these code challenges,

145

where you are actually going to a level up as a programmer.

146

It's not during the tutorials,

147

it's not during the videos where I explain things to you.

148

It's where you actually write the code yourself.

149

I hope you had fun on this code challenge and you managed to complete it,

150

be it with hints or without hints. On the next lesson, we're going to take the

151

if statement one step further and I want to show you how you can have multiple

152

if statements and tests for multiple conditions.

153

So for all of that and more, I'll see you on the next lesson.