add a class to every other three lines in jquery?

<ul id="开发者_Python百科test">
<li>test</li>
<li>test</li>
<li>test</li> // add a class to this li
<li>test</li>
<li>test</li>
<li>test</li>  // add a class to this li
<li>test</li>
<li>test</li>
<li>test</li> // add a class to this li
</ul>

how to use jquery to add a class to the above li lines which i add a comment. thank you.

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$('#test > li:nth-child(3n)').addClass('foo');

Demo: http://jsfiddle.net/mattball/32tKY/
API docs: http://api.jquery.com/nth-child-selector/

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Here is an excellent breakdown on how nth-child works at CSS Tricks.

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you can do it like this

$(document).ready(function(){
    $('ul li:nth-child(3n)').addClass('blah');
});

check out this jsfiddle