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Welcome back.

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This video we are going to look at Catlin's When Expression's, which is a replacement force, which

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statements from other programming languages such as C, C++, C, Sharp Java and many others.

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So if you have never seen any such statements yet, no worries, because it's something that is similar

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to what we are going to build in this video.

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And it's just what is used in other programming languages.

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But it's a more compact and more powerful tool than to switch statements off those other programming

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languages.

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So let's go ahead and use it.

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And therefore, I'm going to go ahead and put away our idea.

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Already said, hey, why don't you use it when expression here?

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So when we use this, if H so-and-so had such a better way, would be to simply use a when expression

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or simplify this expression.

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Of course I could do that directly here and the E would do everything for me, but I want to build one

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myself.

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So I want to build a when expressing myself.

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Therefore I create a new variable called season and it's going to be season three or I'm going to set

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the season two three.

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And then I have a one expression Soyuzes when keyword then the brackets and then the variable that I

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want to look at, because here I specifically look at a variable.

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Let's say I look at the season and depending on what the value of season is, I want to execute a specific

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code.

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So let's say season is one, then I want to execute the code, which just says it's spring then of season

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is two.

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I want to execute the code that is going to be summer.

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So you can see the structure is the value, then an empty space and then minus greater sign and afterwards

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to code that you want to execute.

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Now, of course, you can also go ahead and execute a multi-line code so it doesn't have to be a single

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line code.

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And here I'm just going to say print and fall or print L.N. or in this case, both print L.N..

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Autumn, and then, of course, we have our final season, which is winter.

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All right, so pendelton winter.

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So what is the default case?

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So the case that should be executed if none of the above is true.

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So if season is neither one nor two or three No.

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Four, well, then we can execute the else block with the keyword.

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All right.

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So here I'm just going to say invalid season.

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So at least on Earth, we only have four seasons.

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So let's say we were on another planet.

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Maybe there were more seasons, but these are all the seasons that we have.

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So this is in general how we go about using when expression's.

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So you have a variable that you look at.

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It doesn't have to be an integer.

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It could be a string as well.

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But we are looking at a variable and then depending on the value that we have, we execute some code.

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Now, the cool thing is we can go ahead and do a slightly different approach to this as well.

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So I'm just going to go ahead and say I have a month instead of a season.

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So here the month will also be three.

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But now when the month is one, then we, for example, have winter to use.

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Also winter three, spring for spring five for spring and so forth.

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So each season has three months.

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So now we can go ahead and do as we used to when keyword again in the brackets we say which variable

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we want to look at and then in the curly brackets, that's where our code, our execution code will

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be, where we check the different states.

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So let's say I want to.

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Executes three dot dot five.

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So in those months of months of three to five, I want to print.

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Spring, then in six.

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Until 8:00, I want to print.

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Summer.

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So you see a very similar approach, only that now we're not looking at just one value, but we're looking

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at a range of values here.

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The ranges from six to eight.

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And then, of course, we can also go ahead from nine to 11.

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So Brent Allen fall.

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All right, but now what do we do with winter?

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All right.

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We cannot do something like in 12 dot dot to.

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Parental and winter, because if you hover over it, it says, did you mean to use down to because it

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says this range is empty?

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Well, we could try down two, but now what it will do is it will go from 12 down to two.

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And we had those cases here already.

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You can see that if you want to go arrange down, you can use a keyword down, too.

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But what I want to do is actually have the values to one and two so I can do it like that.

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I could just say, OK, if those three values are true or if the month is one of those three values,

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then please go ahead and just print the long winter.

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And then, of course, our case, we can just use the same as we did before, so else PRINDLE An invalid

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season and now if we execute that.

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Then we will see its fall autumn, and then finally we have spring because month three is March and

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March will be spring.

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Already now a little challenge for you, please rebuild.

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What we had here.

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But do it with a one statement or one expression, but don't just use the autocomplete feature here,

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OK?

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All right.

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So I hope you tried it.

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So what we need to do is, of course, go ahead and use one page.

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And now we have two different cases, so let's say somebody is 21 and above, so we can use 21 dot dots,

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let's say 150, because no one is older than this age.

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Of course, we need to use that in key words.

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So let's say in that range, if the values in the range don't please just go ahead and print.

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You now may drink in the US and by drinking I mean water, of course.

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Then in 18 to 20, we can say something like, you may vote now.

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And 16 and 17.

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And here we don't need to in key word, actually, so let's get rid of those.

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We can just say you may drive now and then finally, we have an LST blog which says you're too young.

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So this is something that we can, of course, do, especially the one here at the top.

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But what if I don't want to write it like that?

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Because I just want to say if somebody is none of the below, so none of the less than 20 or 20 included?

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Well, what you can do is you can use this exclamation mark before the ink word and you just say, OK,

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if it's none or not within zero to 20, then run this expression.

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So what that will do is it says, all right, if the person is less than zero, so younger than zero

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or older than 20, then execute this code.

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All righty, so now you saw to use when expressions.

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Now I want to show you one more thing about One Expression's, because we also can use a keyword, which

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is called is so.

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We have this variable X, let's say, and it's of type any so I specifically say that it's of type any,

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which means that it's of type object and other programming languages.

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So if you know any other programming, language just means that this is going to be of any type of type.

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All right.

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It could be any type.

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That's pretty much what I'm saying here.

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And I'm just assigning a value or thirteen point three seven and now I can check.

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X for its type, so I can just say if it is an integer, then please print something like X is an end.

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But if it is a double, then please print, it's a double and then of course, of course, if it is

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a string print and is a string and finally, we also have this plot here as well where we say, OK,

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print and X.

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Is none of the above.

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OK, so this is something that you can also do using this is key word to check if it is of a certain

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type, that can be very useful because sometimes you get data and you don't know of what type it is

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and then you want to execute code based on the type that the data is so that the variable is that you

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are looking at.

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All right.

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So if we execute this.

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We can see that it's this thirteen point three, seven is a double.

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Of course, it is a double because there is a point or a dot there, so let's say we would add an F

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here, which makes this a float value.

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So instead of double now, it's going to be float.

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Let's see what we get because it should say is none of the above.

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So thirteen point thirty seven is none of the above, so it's neither a double nor a string nor an end

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while looking at one expression's, you can, of course also use not is so you can say something like

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not is double, then is not a double.

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All right, let's run this again just to see if this or else will be executed and there it is, thirteen

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point three seven is not a double because it is not of type double.

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It's of another type.

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But you can also see this is never printed because.

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The statement here is true, it's now could be of any type except for Int and double for this whole

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thing to be true.

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So let's just say we want to make this a string and see if the string here at the bottom will be executed.

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Let's see.

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If it says X or X is a string and it says X is not a double.

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So you see we have the same order here.

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It goes from top to bottom because this statement here is true, actually.

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This one here, this one and that one.

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So those following ones will never be executed, even though this is in fact a string, because we added

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two exclamation marks around it.

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This is a string, in fact.

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But this code will never be executed because one of the above is already executed or it follows the

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same structure as we had when looking at if and if.

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All right.

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So it always goes from top to bottom.

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So you see, when it's just an alternative for if, but sometimes it's more convenient and more precise

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and dense.

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And it just reads, when X is of certain value or type, then do something, so do whatever is after

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the arrow.