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    <p>The ultimate reason is that if you do both general-purpose programming and numerical computations, it is useful to have a large complement of binary operators available. For example, if you store numbers in two-dimensional arrays, you may want to multiply the arrays elementwise, or you may want to compute the matrix product of two arrays. In Matlab these two operators are <code>.*</code> and <code>*</code>; in R they are <code>*</code> and <code>%*%</code>. Python has <a href="http://www.python.org/dev/peps/pep-0211/" rel="noreferrer">resisted</a> <a href="http://www.python.org/dev/peps/pep-0225/" rel="noreferrer">attempts</a> to add new operators, and so <a href="http://numpy.scipy.org/" rel="noreferrer">numpy</a> differentiates between the two kinds of product by having two classes: the array class is multiplied elementwise, the matrix class is multiplied in the linear-algebra sense.</p> <p>Another example from Python is that for lists, plus means concatenation: <code>[1,2,3]+[4,5] == [1,2,3,4,5]</code>. But for numpy arrays, plus means elementwise addition: <code>array([1,2]) + array([4,5]) == array([5,7])</code>. If your code needs to do both, you have to convert between classes or use function notation, which can lead to cumbersome-looking code, especially where mathematics is involved.</p> <p>So it would sometimes be convenient to have more operators available for use, and you might not know in advance what sorts of operators a particular application calls for. Therefore, the implementors of R have chosen to treat as operators anything named like <code>%foo%</code>, and several examples exist: <code>%in%</code> is set membership, <code>%x%</code> is Kronecker product, <code>%o%</code> is outer product. For an example of a language that has taken this to the extreme, see <a href="https://web.archive.org/web/20110716162714/http://projectfortress.sun.com/" rel="noreferrer">Fortress</a> (section 16 of the specification starts with the rules for operator names).</p> <p>In the blog post you mentioned, the author is using the <a href="http://had.co.nz/ggplot2/" rel="noreferrer">ggplot2</a> graphing package, which defines <code>%+%</code> to mean some kind of combination of two plot elements. Really it seems to add a method to the bare <code>+</code> (which is a generic function so you can define what it means for user-defined objects), but it also defines <code>%+%</code> so that you can use the ggplot2 meaning of <code>+</code> (whatever it is) for other objects. If you install ggplot2, type <code>require(ggplot2)</code> and <code>?&#x60;%+%&#x60;</code> to see the documentation of that operator, and <code>methods(&#x60;+&#x60;)</code> to see that a new definition has been added to <code>+</code>.</p>
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    1. POWhat does eg %+% do? in R
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    1. USJouni K. Seppänen
    1. USJouni K. Seppänen
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    1. COwauw - thank you so much Jouni! What an excellent answer - it is all beginning to make sense now :-)
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