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Algorithms; don't you love them?

About once a month, I walk with a small walking group.  Earlier this year, while enjoying the wonderful landscape of Dartmoor, one of my fellow walkers turned and asked, "David, what is an algorithm?" 

He knew that algorithms affected his daily life, and his experience of the internet, but nobody had ever explained what an algorithm really was.  My starting point was cookery recipes, as an example of a sequence of instructions to be followed one after the other, in order to transform an "input" into an "output".  Then we moved on to other kinds of instructions, including the apocryphal "hair shampoo infinite algorithm" ((1) Wet hair thoroughly; (2) Apply shampoo; (3) Rinse hair and repeat) which has no stopping rule. 

I gave him some other examples, which hopefully allowed him to recognise that when he searches online for some item, then the search engine takes his key word(s) and matches them to pages which fit.  Input: key word(s); output: a l…

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