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# Guelph Coffee and Code + Project Euler

Posted 2011-03-24 Tags: None

I'm talking about Project Euler at tonight's Guelph Coffee and Code. My talk is going to be short, but here's the main talking points.

### Lenhard Euler: Mathematician

Euler (pronounced "Canada's Worst Hockey Team that's not The Senators") was a Mathematician in the 1700s. He was a genius, and he helped shape the world of mathematics that we know and love today. Check out Euler on Wikipedia for more in depth info about the fellow.

Project Euler is gets you to combine mathematical insights with computer programming in an effort to find answers to a series of over 300 problems. It's not tied to a particular language, though there are some languages that will serve you better than others. It's all about finding the answer in an elegant fashion.

### Why should I care?

Project Euler will help you to identify areas of interest for you, and to make you a better all around programmer (and mathematician). There's a ton of different subjects that are covered, and it's an opportunity to push your self in new directions.

### Let's do an example!

Sure thing! Look at Question #1.

Add all the natural numbers below one thousand that are multiples of 3 or 5.

Not hard right? Make a loop that goes from 1 to 1000, and for each number check if it's divisible by 3 or divisible by 5. If it is, add it to a running sum. Pseudocode looks something like this:

x=1
sum=0
while x is under 1001
if x is a multiple of 3 then sum = sum + x
if x is a multiple of 5 then sum = sum + x

And at the end return sum. Does that give the right answer? Are we forgetting something?

Of course, if x is a multiple of both 3 and 5, then we've counted it twice. We can fix that in a few ways, like subtracting off x one time if it's a multiple of 3 and 5.

if x is a multiple of 3 and 5
then sum = sum - x

Or we could just solve it with an else

if x is a multiple of 3 then sum = sum + xM
else if x is a multiple of 3 then sum = sum + x

Now a lot of people will understand that the "trick" is not really tricky in this case. Remember not to count multiples of 3 and 5 twice is pretty simple. But the curve is pretty steep - after 20 or 30 questions, it's much more hidden.

And that's the gist of what I'm saying tonight.

By the way, here's that pseudocode in python:

f=0
x=1
while x < 1000:
if x%3==0:
f=f+x
elif x%5==0:
f=f+x
x=x+1
print f