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So as you have seen, jQuery selectors are very powerful utilities for selecting content inside web pages. So now we are going to look at a feature of jQuery that make selectors even more powerful while retaining the CSS style simplicity that goes along with the selectors we have already seen. And these are called filters. Filters work in conjunction with selectors and they provide even more fine-grained control over how elements are selected in the document.
jQuery filters fall into six different categories. The first category is Basic and these provide basic filtering like getting the first or the last or the even or odd numbered items in the returned set from a selector. The next is content based filtering. Content filters take a set of elements and they filter out elements based on their content. Like for example, whether an element contains a particular string. Then there is Visibility filters and these filters act on elements using their visibility setting.
And they will filter out elements that are either hidden or visible. Next comes Attribute filters. Attribute filters will examine a given attribute on an element and they will use that attributes' value to determine if it should be filtered out or included in the selector's result set. Then there is Child filters, and Child filters select elements based upon their relationship with their parent element. Then finally, there is the special set of filters that work on form elements and you can use these as really convenient powerful ways of processing elements in forms based upon what kind of form fields they are, whether they are enabled or not, whether they are checked or not, and that kind of thing.
Okay, so let's take a look at the Basic jQuery filters. So Basic filters allow you to refine the results of a jQuery selector by only including elements that match certain conditions. If you're taking a look down this list and you are familiar with CSS, you will probably notice that again these match up very closely with CSS syntax. So for example, the :first filter selects only the first instance of the selector's returned set.
Similarly, last, it will select only the last instance. Then there is even and odd which select only the even numbered elements or the odd numbered elements in the returned set that the selector came back with. Then there is a few more down here that operate on position. So the eq filter will take an index and it will filter out elements that are not positioned at that index. So it will only include the element at that index in the result set. Then there is a greater than and less than, and they do what you might think. For example, the greater than operator will only include elements that are past this given index whereas the less than filter will only include elements that are before that index.
The header filter will select all header elements in the page, H1, H2 for example all the way up to H6. The animated filter will select all the elements that are currently being animated by a jQuery in some way, and you would use this in conjunction with the animation ability of jQuery, which we will get to later. Then finally, there is a negation filter and it's called the not filer. The not filter takes a selector and that will include elements in the result set that don't match the given selector.
So we have reached the point now where we can jump over to the code and see how some of these work. So let's fire up our editor and see how we can get even more power out of jQuery selectors by adding filters. If you look in the exercise_files folder, you can see there is a file in here named BasicFilters and there is a start and a finished version. Now I am going to use the start version to work towards the goal. If you want to follow along with me, you can or you can just go ahead and jump ahead to the finished version to see the results for yourself. So here we are in the code.
I am going to go back and show you the Design view. So this is the same document that we were using in the previous example, only now we are going to use filters as well. Okay, so let's jump right in and try some things out. So I am going to write a jQuery expression. Now I remember last time I started off by just getting p tags and then I added a little css trick. Again, I don't want you to focus too much on this css right now, because I am just doing this to make the results visible. We will get to jQuery's css handling later on in the course.
So for now I am just doing this to make it possible to see the results. So instead of just getting p tags, however, let's try some filters. So I am going to write p:first. So what this will do is find the first paragraph tag and put a border around it. So let's see if that works. I am going to save, and let's bring this up in the browser. And you can see that it worked. So the first paragraph now has a border around it. Okay, let's go back to the code. Let's try getting a little more fancy.
So I will copy that and paste. So let's try the last operator. And as you might expect the last operator should do pretty much the same thing. Let's go to the browser, let's refresh and now you can see that the last paragraph is selected. Let's get a little more fancy. Let's go back to the code, comment this guy out there. Now we are going to do the even numbered paragraphs. So let's save and go back to the browser and I am going to refresh.
And now you can see that the even numbered, and this may not look even numbered because that's the number one, but remember jQuery is starting off indexing at index 0. So this is 0, this is 1, this is 2, and this is 3, even though I have got them labeled starting at 1, 2, 3, and 4. In programming parlance the 0 based index is first and that's obviously an even number. Okay, let's go back to the code and let's just try odd just to show that it works. Okay, and let's refresh and now you can see the odd ones are highlighted.
So that works obviously with tags. Let's try it with some classes. Now I am going to comment that line. Let's try getting the first instance of whatever has the a class applied to it. You can see that it's going to be this list item right here. So let's try that out, okay, we refresh. And sure enough, that first time with the a class is selected. Let's go back to the code. Let's try every even numbered thing that has b assigned to it.
So let's look for the class b and even. So we save and let's refresh. And you can see that this is item number 0 for b, this is the second b item right there. So those are both being highlighted. Let's comment this. Now let's try looking for paragraph tags, but only paragraph tags that are greater than index 1. So this is going to be index 0; this is going to be index 1.
So we should get 2 and 3 here included. So let's save, go to the browser and refresh. And sure enough that's exactly what happened. Only indexes 2 and 3 got highlighted. All right, finally, let's try the not operator. Okay, so here we are back into the code. This time let's try looking for paragraphs that are paragraphs not equal to index 2. So it's going to find the paragraph at index number 2 and it's going to select all the paragraphs that are not that one.
So they should all have a nice red border around them. So what that means basically is-- let's 0, 1, 2. So this guy will be excluded, but the other three should have a border. So we will save and we will go back to the browser and let's refresh. And you can see results are exactly what we expected. This one here is not bordered and the other three are. Okay, so that pretty much wraps it up for basic jQuery filters. Now you know how to combine filters and selectors to do some pretty powerful selecting techniques.
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