I was just writing an example for a Sonic Pi demonstration, and thought I’d show looping. But I got it wrong; and when I heard the results, it was totally obvious. This struck me as an example of where visualising code as sound can be a powerful learning tool.
To demonstrate loops, I thought I’d bounce between two notes, with an interval which got less by one each time round the loop, and so I wrote this…
# Welcome to Sonic Pi v2.0 use_synth :beep use_bpm 120 an_octave = 12 interval = an_octave loop do play 60 sleep 1 play 60 - interval sleep 1 if (interval >= 0) then interval -= 1 end end
The loop test checks for
interval >= 0 before decrementing the interval. So when I ran it, this was the result.
Something’s wrong: instead of ending with the second tone the same pitch as the first, it’s higher—a classic ‛off-by-one’ error, of the sort which has been the cause of many computing woes, including security breaches. And it’s an easy fix to change the test to
interval > 0 , but the immediacy of the way in which the sound highlighted the problem was very surprising. (And I wished I’d had such a guide in the innumerable times I’ve been caught out by such errors in the past :-)