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This code exercise is a difficult one,

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and you can see that in the description box.

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I've marked it out as a difficult challenge.

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The goal of this exercise is for you to build,

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a program that checks when given any year,

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whether if that year is a leap year.

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A leap year has 366 days instead of the usual 365

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with an extra day in February

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and it's actually kind of interesting,

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the reason behind why we need a leap year.

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And in the description box you'll see a link

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to a video that actually explains

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to you the full reasoning behind it and it's really,

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really fascinating.

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So take a look if you're interested.

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In order to work out whether if a year is a leap year,

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there is a logic to it and it's expressed like this.

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So you take a particular year and it is a leap year

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if it can be divisible by four with no remainder,

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except if that year is also divisible

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by 100 with no remainder, and unless the year

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is also divisible by 400 with no remainder.

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So there's actually three steps that we need to take,

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our year through.

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And it's because these criteria flip whether

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if a year is leap year or not leap year.

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If you take a look at the example in the description box

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and see how I've worked through various years,

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like year 2000 or year 2100 and

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how I've applied these rules into maths to show you

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which one is leap and which one's not leap

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and the logic behind it.

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What I recommend doing is to try it out yourself

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with pen and paper before you write any code,

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just pick a random year out of the top of your head

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and work through these three rules and figure out

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whether if it's a leap year or not a leap year,

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and then just type into Google that year and check is

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this a leap year?

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That's probably the easiest way to just wrap your head

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around these three rules.

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Now, if English is not your first language

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and you struggle to understand what the statements mean,

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then I've actually created a flowchart for you.

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So if you click on the link in the description box,

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it will take you to a flow chart

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which you can use to figure out whether if a year

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is a leap year or not a leap year.

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And this can be really useful

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even if English is your first language because

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it is really tricky.

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The wording for the logic can be confusing,

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but I think it's really important to convert something

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that is logic,

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so in words to a program.

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I can of course reword this to say, "If this,

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then that..." but then that's kind of like cheating

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and writing it for you.

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So don't worry if you get it wrong,

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just try your best at figuring out

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how the logic works, understanding it

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if you still don't understand it use the flow chart

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and then convert that logic into code.

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So that is your challenge.

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The first thing we know is we can get hold

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of a year as an integer into our program on line two

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and we can work with it

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through using various conditional statements to check

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whether if that year matches our criteria.

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The first thing we're going to check is

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whether if that year can be divided

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by four with no remainder,

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and remember in order to figure out what the remainder is

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in programming we use the modulo symbol (%).

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So we've seen this before when we did the odd

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and even exercise, and in this case,

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it's again the same thing we can write.

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if year % 4 == 0:

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then that means it's cleanly divisible by four.

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So that means that we can proceed

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to the next thing to check.

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And if this is not true,

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so here we've got an else statement that matches

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this if condition,

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then in that case it's obviously not a leap year.

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We've already eliminated that year

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because it can't be divisible by four.

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The next step is to drill it down even further.

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So if that year is cleanly divisible by 4,

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well then is it cleanly divisible by 100?

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Because according to our logic,

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if it's divisible by four but also divisible cleanly by 100

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then it's actually not a leap year.

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This is the part that gets a little bit confusing

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for most people because in this case,

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when it's divisible by 4 and divisible by 100,

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then it is actually not a leap year

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and we have to continue checking the final condition.

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So that's why we've got this embedded if and else statement.

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So at this point we've already verified the year

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is cleanly divisible by four

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and we're now checking if it is cleanly divisible by 100

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then we're going to continue checking.

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But if it is not divisible by 100 with no remainder

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and it's divisible by 4 ,

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then it is now confirmed as a leap year,

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and this will become more obvious

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if you actually look at the flow chart that I provided

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in the description box as well.

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This is just one of those peculiarities

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about working out which year is a leap year.

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The final check we have to do is

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in the case when a year is cleanly divisible by 4

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also cleanly divisible by 100.

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Now the final thing is to check,

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well is that year also cleanly divisible by 400?

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Because in that case it flips again

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and it is now a leap year

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if all three of those conditions are met.

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There's many routes to navigate through the flow chart,

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but in the final section here

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if all three criteria is true, divisible cleanly by 4,

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100 and 400, then it is a leap year,

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otherwise, if that last condition fails

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then it is not a leap year.

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That's why we've got three nested if and else statements

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in order to complete this logic in the flow chart

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or in the text version of how to work out a leap year.

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There are many ways of solving this exercise

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and this code solution is not the only way.

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Feel free to Google for how to check for leap year

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in Python, and you'll see many,

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many other methods.

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You could combine different conditions,

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you could use singular if and else statements,

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there are many, many ways of doing the same thing

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so don't be too hard on yourself.

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If your solution passed all the checks

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then that means you got it right

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and every solution is accepted as long

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as it passes all the tests

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because we checked for all the different years

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against all the different conditions.

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And if your code works,

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then that's great.

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Now, as always,

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my solutions optimize for the most readability

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and hopefully the easiest to understand.

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And in this case,

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this one kind of mirrors the flow chart one for one.

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So hopefully you can understand where it comes

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from by comparing it against the flow chart.

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And this is a good tip for programming,

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whenever you get stuck with your logic,

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try and make a flow chart for yourself

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and then use that flow chart to create the code.

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If you had any fixes you need to do go back

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and make that now and make sure you've understood

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how we've converted logic into code.