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Sober come back again in this class and we're going to talk about a feature of programming for Iowas

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which is called category's and answer specific to Objective C categories are a way for us to add the

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functionalities and add new methods to existing classes.

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Let's see how we could do that before I actually go to school them you know try to make an app that

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explains category's.

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I want to show you that you somehow have been seeing categories all along the way.

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I'm going to take a nap.

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There's going to be a temporary hour.

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You don't have to make this it's going to be a single view application in Objective C and I'm going

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to call it tamp up.

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And if you remember whenever we make one of these applications you know objective Objective-C immediately

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end up seeing something like this at interface view controller a known name in ASH and something in

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the past I probably told you this is called a private category.

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And that's exactly what it is.

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It's a private category that helps us put private stuff in here things that we don't want our data h

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to have it's interface to have it so it cannot be exposed to other classes.

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As you can imagine any class that imports this one view controller will have access to whatever is here

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which is in our data h.

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So we use this one as some sort of a private place for declaring different stuff that we don't want

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anyone else to essentially having access to them or we don't want them to be exposed outside.

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So that's kind of how you declare and use a category.

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Now let's go ahead and see how we will do it in a much simpler way.

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I'm going to go to school it's say fine new project is going to be a command line tool in my class and

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I'm going to call this one.

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Categories are total categories tool.

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The only thing it has to be objective c of course when added in here is what they want to do.

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I want to go to my dad and my main fine.

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I want to have a string.

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Let's say that is this thing.

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Hello world.

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All right.

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It is a.

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It is called the hello as we are and that is this one.

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I want to have a way to reverse it.

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Essentially say r t s or l e h but I don't want to have a function here and I don't want to have it

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as a separate class simply because I want the string to be able to do that very much like the way that

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you could say hello.

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Or a string by appending a string which is a function built into a string.

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I want to make it like that simply because this is a function that I'm going to be using on and on.

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Of course reversing it is not necessarily that case but you understand the meaning of it you could do

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something that you can always use from that particular class.

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So here's how we're going to do that.

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I'm going to go up here say it's interface just like all of you have always been doing are declaring

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classes but this interface is actually going to be a string type and it doesn't have a superclass.

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It doesn't have a call of course but it does have two pounds.

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Is that a stand for the name of the category.

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And I'm going to call this category reversal.

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It's going to be my reverse third category of any system.

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And of course I have implemented its implementation.

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What do you want to implement.

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I want to implement this guy and then it does whatever it does and then we'll have to deal with that.

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So the first thing I should do I should say I have a method in here that returns a string that's called

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reverse.

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So this method whenever we send something to it it has to reverse it.

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So I'm going to say this thing is within the implementation as well.

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And it has an end as a string let's call it the raise and return address.

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Now obviously between these two we have to actually reverse it.

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I'm not talking about that real soon.

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But before I do that I want to put my interface and my implementation in separate files.

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So I'm going to go ahead and say file mewl file my new file is going to be a cocotte class of that type

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and it's a string because that's what we're trying to make.

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And there's a very particular way for naming categories.

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You don't have to be it's a very very common practice and that is called and that's a string and then

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plus whatever it does for instance in this case it becomes an sustaining Plus the reverse.

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This is minus things category that reverses.

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And I think so if you find the file outside that somebody else has written and it has this particular

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format it has a plus by then and then it has some sort of label.

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It means that it does that thing.

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So I'm going to say next I have it in here over there the house.

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And then I'm going to copy this to.

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So that's quite unnecessary.

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Reverse the implicit interface of it somewhere pasted in here.

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And then again I'm going to call this one implementation Olduvai.

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I'm going to put my that in fact.

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So that's about that.

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This is my dad H and this is my dad.

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And of course I can and my dad use it but we don't really know how it is going to work just yet.

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So the next thing is write the body of here to do reverse smy a string.

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So let's see how we going to do that.

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Here is the logic behind it.

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Let's say I have this very long set of characters I'm going to say first of all find out how many are

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there.

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I can count this but let's say 25 and then we're going to say well if they're 25 get this started from

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24 which is that one.

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Then add the 2010 which is that one and then add the 20th second which is that one so on and so forth

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until you reach to the very beginning of it.

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And that would be essentially the technique I'm going to be using to reverse a string.

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So let's go ahead and implement that since I'm going to be changing this text.

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I have to change this into.

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And this mutable string.

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It has to be an immutable string.

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And then I'm going to go ahead and say without initialises of course and US mutable string new then

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I have to find out the how how long is my time.

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I'm going to go ahead and say it is an integer and that integer is called.

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Let's say there are lengths for instance or a string length string length is going to be self simply

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because V or of it in the end it's a string as a cell.

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So how long is it.

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This is for instance the one that returns 25 for us.

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Now I could simply put this inside a 42.

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I could go ahead and say.

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Or better yet unaffordable for a situation like this is a while.

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So I'm going to go ahead and say while the condition is this while this guy is still larger than zero.

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All right.

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So as we go through it Dad dad dad dad we go there as long as we're here at least we are zero.

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You'd do this.

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You say first of all subtract from yourself.

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So it becomes a one less imagine indice 25.

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We are Cranley landing.

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I don't know somebody like 17.

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So we are here on this.

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So we have to go and say here right before W go for one candidate and that we do it with something called

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arr..

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So here is how we introduce a range and then I say.

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A syringe and in my honest range I'm going to call it substring range.

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That's a very good name usually substring range is and this may range and the range we want to make

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is from here for one CACs.

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So as you can imagine it says Major range from 17 12:56.

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So only this one.

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So what you want to do with that.

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I want to find that.

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There's something to say and it's a string of let's call it next block that actually makes more sense

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to quite the previous character.

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So previous character is going to be self wherever I am substring by range and this range.

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So I'm going to say here if we wanted to find 17 and only 17.

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Just one of them.

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So now go to self find that place.

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I'm putting something called previous sky that I'm going to say.

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RAZ This one resolved append a string.

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What do you want to append to it.

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This one I went up with that one and that's about nuts.

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Now you manage to go through every one in here and get it.

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And at the end return that's value.

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So let make clear all of these and just quickly talk about once again.

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So what's happening in here is that we have a set of characters we're going to find out how many they

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are.

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Once we found out how many they are we're going to go from the top and the using of white and up we're

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going to stop trying to 1 at the very beginning simply because remember if testing thinking is 25.

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Their starting point of them is 24.

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So we first subtract one from them and then we find the range based on that number and of one more character

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we find that particular character.

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We added two hours just so that things are going to become a little bit easier.

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I'm going to go ahead and print this value.

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You're going to have to do this.

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I'm going to say print.

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What do you have at every given time for us.

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So as we go through this process print it for me.

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Now I have managed to add in your category to a string called to reverse it.

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If I go to Maine and say import import then is a in reverse it I can go in here and say well and it's

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a string.

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Reverse the string is.

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Hello Esti are reversed it's now reverse it is actually working just like any function from the strings

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because it has been added to the string at the implementation level of the string.

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There is now a new function.

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And now I can go in here and say well show me what is reverse it's so I'm going to say and Aslaug slog

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reverse these.

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And what is it going to be is going to be this.

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And since I know this guy is printing some random stuff I'm actually going to push this down a few lines

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so I'm going to sing a few lines down.

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So let's run it.

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Now it should do what you expected it to do.

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Hopefully should say OK.

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So first of all it says reverse it is the reverse of not the exclamation mark the L or O.W. so on and

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so forth.

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And this is the process that went through.

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It said The first one then the next one then the next one then the next one all the way to the end.

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It found all of them so that worked the way we expected it.

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This is how you add categories to objective c which is essentially a way of marginalizing or code.

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If that class belongs to loss or a way of adding new functions to our existing classes if we can not

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change them.

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So and as a string and as array I don't know a text field.

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They're all kind of machine classes and if you want to add functionality to them we add them using categories.

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So let's keep it up to here and we'll move on to our next lesson.