Archive for September, 2011

Programming Praxis – Tetrahedral Numbers

September 13, 2011

In today’s Programming Praxis, our goal is to find the base of the three-sided pyramid that has 169179692512835000 spheres in it. Let’s get started, shall we?

A quick import:

import Data.List

The tetrahedral numbers are based on the triangular numbers, so let’s start with those.

triangular :: [Integer]
triangular = scanl1 (+) [1..]

The tetrahedral numbers are formed in much the same way as the triangular ones.

tetrahedral :: [Integer]
tetrahedral = scanl1 (+) triangular

All that’s left to do is to is to find the base of the pyramid.

main :: IO ()
main = print . maybe 0 succ $
       findIndex (== 169179692512835000) tetrahedral
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