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

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as the title suggests, this is a difficult challenge,

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

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

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

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

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

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but what I'm trying to say is really,

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

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because it's a really, really interesting challenge.

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

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whether if any given year is a leap year.

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We know that a normal year has 365 days,

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but leap years have an extra day in February.

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

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

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

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

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

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

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so that means divisible by four with no remainder.

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Then that is a leap year, but except the years

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that is also evenly divisible by a hundred,

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unless the year is also evenly divisible by 400.

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Now 2020 divided by four is equal to 505.

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

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

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if it's evenly divisible by a hundred,

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

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

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

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

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unless the years also evenly divisible by 400.

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So 2020

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divided by 400 is not evenly divisible.

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So it is in fact, still a leap year.

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The trickiest part of this entire challenge

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

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divisible by four with no remainder is a leap year,

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

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it's no longer a leap year,

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

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leap year. What I recommend doing is a)

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if English is not your native language

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

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year in your own language, because these words, except

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and unless, are so,

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

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really need to understand exactly what it means.

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

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just like what I've done in previous examples.

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Head over to a website like draw.io,

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or simply just write it on a piece of paper.

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

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Now, if you really get stuck on that,

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I've also created a flowchart for you.

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So if you actually get stuck creating the flowchart,

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

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But it's a really good exercise

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

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

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

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

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like 1989, then just print, not leap year.

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

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

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[inaudible]

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

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

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

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

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

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

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really important to try and solve it yourself. Now,

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

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

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

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and it's very simple.

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

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

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

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

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

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

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

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

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

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

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

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referring to the diagram.

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

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divisible by four with no remainder.

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And you might remember from our previous code exercise

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

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modulo four will give us the remainder.

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

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

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Let's say,

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

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

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

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

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

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

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

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So the else statement that matches this

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

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

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divisible by a hundred?

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

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And if the answer is yes,

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

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

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

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

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

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is it cleanly divisible by 400?

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So now if year % 400 is equal to zero,

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well, in this case, if it is true,

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then it's definitely a leap year.

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

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

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then we can print not leap year.

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And that's it.

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

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

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but this is probably the most readable. Now,

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if you've written some different code

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but it still uses these rules to work out

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

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then that's fine as well.

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

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

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

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And once you've got your logic straight,

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then converting it into code is much, much simpler.

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

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then it's going to be a mess.

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

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take those flow charts and turn them into code.

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If you got stuck on this challenge

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

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

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

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

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It's not during the tutorials,

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

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It's where you actually write the code yourself.

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

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

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

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if statements and tests for multiple conditions.

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