## Egyptian multiplication with Haskell

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Progress is great and new things are always exciting... but that doesn't mean old things don't have any value! Photo by Crissy Jarvis on Unsplash

Very recently I watched a Youtube video posted by my friend MathGurl, in which she explained the ancient Egyptian multiplication method. At the same time, and for no particular reason, I remembered Haskell, so I decided to implement the method. It was not a big feat of programming, but I did enjoy relearning the basics of Haskell I once knew. You can find the file with the implementation in this GH file or right here:

(The implementation only works for non-negative integers)

The method is quite simple and works because of the binary expansion of a number. Basically, if you want to calculate $$a \times b$$, either $$b$$ is even or odd. If $$b$$ is even, just cut $$b$$ in half and duplicate $$a$$ to compute $$(2a)\times(\frac{b}2)$$. If $$b$$ is odd, then $$ab = a + (2a)\times\frac{b-1}2$$. Another way of thinking about this is by writing $$b$$ in the form

$b = 2^{k_1} + 2^{k_2} + \cdots + 2^{k_n}$

and then having

$a\times b = a(2^{k_1} + 2^{k_2} + \cdots + 2^{k_n}) = a2^{k_1} + a2^{k_2} + \cdots + a2^{k_n}.$

If I muster the courage to do it, I might also redo the functions in the post about Kleene recursive functions, in Haskell...

If you liked this article and would like to support the mathspp project, then you may want to buy me a slice of pizza 🍕.

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