Friday, 23 March 2012

Doing murky arithmetic

More features that resemble (somewhat) conventional programming. I gave a talk in the Auckland CS department on Wednesday, describing the extended audience for end-user programming, beyond those who "think like engineers" (the EUSES agenda) to sloppy thinkers, as I've described them in this blog. Sam Aaron has sent an encouraging email, saying he likes this novel emphasis, so I'd better credit it properly - Thomas and other PPIG folk used to distinguish "neat" and "scruffy" programming styles, but both those styles described practices within the spectrum of professional programming. My recent emphasis comes more from a comment made at a EUSES AGM by Mary Shaw, when she told me that the Computational Thinking education campaign was intended to discourage "murky thinking". However from my perspective (and as I said on Wednesday), many typical artistic practices are unavoidably "murky" - relying on creative ambiguity, social interpretation, emergent behaviour, and other things that you don't really want in standard programming.

So how do you do arithmetic within a framework of murky thinking? I haven't yet found any need for standard (integer/real) number representation, with count and proportion being better suited to the operation parameters I need. Counting is counting, with little need for arithmetic, but it's getting a bit boring having proportions that are either the same as each other or inverse. I therefore set out to make a four function calculator for proportions. This isn't going to work the same way as a standard four function calculator, because all inputs and outputs have to be in the range 0-1 for compatibility with the rest of the system. This means that "multiply" is actually scaling up, and "division" scaling down, relative to a log slider. Addition and subtraction combine and compare proportions. All results are clipped to the floor and ceiling values.

A solution to the problem of how to visualise this non-standard arithmetic model was inspired by a hint from Helen, that she explains multiplication to children in terms of the area of a rectangle. The input value is therefore visualised as a precisely scaled version of the original proportion layer, with the output value visualised as a rectangle whose area can be larger or smaller than the original layer boundary. All four functions are controlled by sliders - vertical add and subtract change the relative height of the rectangle, while horizontal multiply and divide modify the width with log-scaling. Hopefully, direct manipulation of these sliders makes their behaviour sufficiently familiar, that the effects of subsequent parameterisation (by dragging and dropping another value layer onto a slider) can be easily anticipated.

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