## Calculating Pi (when you’re ill)

So, I’ve not been well lately. And because a few days lying on my back with insufficient mental stimulation is a quick route to insanity for me, I’ve been trying to spend my most-conscious moment doing things that keep my brain ticking over. And that’s how I ended up calculating pi.

Pi (or π) is, of course, the ratio of the circumference of a circle to its diameter, for every circle. You’ll probably have learned it in school as 3.14, 3.142, or 3.14159, unless you were one of those creepy kids who tried to memorise a lot more digits. Over the years, we’ve been able to calculate it to increasing precision, and although there’s no practical or theoretical reason that we need to know it beyond the 32 digits worked out by Ludolph van Ceulen in the 16th Century, it’s still a fascinating topic that attracts research and debate.

Most of the computer-based systems we use today are hard to explain, but there’s a really fun computer-based experimental method that can be used to estimate the value of pi that I’m going to share with you. As I’ve been stuck in bed (and often asleep) for the last few days, I’ve not been able to do much productive work, but I have found myself able to implement an example of how to calculate pi. Recovery like a nerd, am I right?

Remember in school, when you’ll have learned that the formula to describe a circle (of radius 1) on a cartesian coordinate system is x2 + y2 = 1? Well you can work this backwards, too: if you have a point on a grid, (x,y), then you can tell whether it’s inside or outside that circle. If x2 + y2 < 1, it’s inside, and if x2 + y2 > 1, it’s outside. Meanwhile, the difference between the area of a circle and the area of a square that exactly contains it is π/4.

Take those two facts together and you can develop an experimental way to determine pi, called a Monte Carlo method. Take a circle of radius 1 inside a square that exactly contains it. Then randomly choose points within the square. Statistically speaking, these random points have a π/4 chance of occurring within the circle (rather than outside it). So if we take the number of points that lie within the circle, divide that by the total number of points, and then multiply by 4, we should get something that approaches the value of pi. You could even do it by hand!

The software illustration I’ve written is raw JavaScript, HTML, and SVG, and should work in any modern web browser (though it can get a little slow once it’s drawn a few thousand points!). Give it a go, here! When you go to that page, your browser will start drawing dots at random points, colouring them red if the sum of the squares of their coordinates is less than 1, which is the radius of the circle (and the width of the square that encompasses it). As it goes along, it uses the formula I described above to approximate the value of pi. You’ll probably get as far as 3.14 before you get bored, but there’s no reason that this method couldn’t be used to go as far as you like: it’s not the best tool for the job, but it’s super-easy to understand and explain.

Oh, and it’s all completely open-source, so you’re welcome to take it and do with it what you wish. Turn off the graphical output to make it run faster, and see if you can get an accurate approximation to 5 digits of pi! Or slow it down so you can see how the appearance of each and every point affects the calculation. Or adapt it into a teaching tool and show your maths students one way that pi can be derived experimentally. It’s all yours: have fun.

And I’ll update you on my health at some other point.

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## One of my favourite hosting companies is recruiting using anonymous online interviews, in an effort to combat industry sexism

This link was originally posted to /r/girlsgonewired. See more things from Dan's Reddit account.

So, in May 2015, we made the huge decision to start an anonymous recruitment process. The biggest change compared to tradition recuiting is this – your first two interviews are truly anonymous. We conduct them over instant messaging and run our skills tests remotely too.

You won’t even have to give your real name or a CV in the initial stages. We don’t know anything about you that you don’t choose to present in the interview.

That makes us work hard for explicit goals. We want to know about your:

Most valuable skills
Ability to learn
Ability to work effectively in a team

We make decisions based on those factors. Avoiding the “X factor” of cultural fit, which we’ve seen as an excuse for all kinds of implicit and explicit bias across industries.

We also want to be respectful of your time, your enthusiasm and your interests – we’ll test not just what you know but what you can learn. Our focus is on letting you put your abilities to the fore, without fear that you’ll be judged on irrelevant things. We define the job, we define the skills, and we want to test those without bias.

Our culture comes from you, the best person for the job at the end of the process. Of course we still need to meet you, we want to meet you. But we will start our interviews on the solid foundation of anonymity. Only at the final stages will you be asked to come in for a face-to-face interview.

## “MarI/O” – how an evolutionary neural network learned to play Mario

This is a repost promoting content originally published elsewhere. See more things Dan's reposted.