Computing primes with CSS

Thursday 29 September 2016

I've been working on a redesign of this site, so doing more CSS, finally internalizing Sass, etc. During my reading, the nth-child pseudo-class caught my eye. It's oddly specific, providing syntax like "p:nth-child(4n+3)" to select every fourth paragraph, starting with the third. It isn't an arbitrary expression, it has to be of the form An+B, where A and B are integers, possibly negative. An element is selected if it is the An+B'th child of its parent, for some value of n ≥ 0.

It struck me that this is just enough computational power to compute primes with a Sieve of Eratosthenes, so I whipped up an demonstration (see it live here):

/* A stupid pet trick by Ned Batchelder @nedbat */
html { max-width: 40rem; }
span { display: inline-block; width: 2em; text-align: right; }
span:first-child { display: none; }

The code has only linear sequences of numbers. There are spans for 1 through 999, the candidate numbers. These are arranged so that the number N is the Nth child of their containing div. The CSS has nth-child styles for 2 through 32, the possible factors.

The Sieve will hide numbers that are determined not to be primes with a "display: none" rule. A first-child selector hides 1, which is typical, seems like you always have to treat 1 specially when looking for primes. The other selectors for the display:none rule select the multiples of each number in turn. "nth-child(2n+4)" will hide elements 4, 6, 8, and so on. "nth-child(3n+6)" will hide 6, 9, 12, and so on.

So CSS has two features that together are just enough to implement the Sieve. The nth-child selector accomplishes the marking of factors. The overlapped application of separate rules implements the multiple passes, one for each factor.

Of course, I didn't write this file by hand, I wrote a Python program to do it. It's pretty simple, I won't clog up this post with the whole thing. But, it was my first use of a new feature in Python 3.6: f-strings. The loop that writes the nth-child selectors looks like this:

for i in range(2, 33):

The f"" string has curly-bracketed expressions in it which are evaluated in the current scope. This string in Python 3.6:


is equivalent to this in previous Pythons:

"span:nth-child({i}n+{i2})".format(i=i, i2=2*i)

It felt really natural to use this new feature, and really convenient.

» 1 reaction


Vedran Čačić 6:27 PM on 30 Sep 2016

It's also interesting to note that print of f-string looks very similar to printf call. :-)

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