A couple of friends and I were at a hackathon last year, building this cool tool that could play the piano along with a user in real-time. Basically, it would play out notes on the piano which would complement any tune the user was attempting.
We were terribly stuck and we had no idea why. The tool was playing a diarrhea of notes all at the same time, and it was “unpleasant” to put it politely.
It was about 3:30 AM, we’d been awake for about 18 hours, and we were exhausted. Like zombie level exhausted. No amount of coffee was keeping us fresh and awake anymore.
I slapped myself, sat in front of my screen and squinted long and hard. Mentally going through every line, every expression, every calculation. We’d spent close to 3 hours in this pit, and I wanted to dig us out of it. (Perhaps “dig” isn’t a good verb, as that would only put one deeper into the pit. But yeah, you get the idea)
I’m looking at this one snippet, and then it hits me. I wanted to stab myself. Repeatedly. I look up to the heavens in despair and motion my friends to come over. I very slowly demonstrate what the bug is, and their reactions are quite poetic. One walks away in disgust and stares at the wall for a good 5 minutes, while the other hurled an expletive and went to bed. I fix the error, and everything from there worked like a charm.
You see, Python 2.x does this funky thing where an expression like “1/2” evaluates to 0, and not 0.5 as humans would expect. Integer division, for those who know their jargon.
- >>> x = 1/2 # what we were doing
- >>> y = 1/2.0 # what we should have done
- >>> x,y
- (0, 0.5)
All along, the value for the time delay between playing consecutive notes was being calculated this way resulting in a delay of zero seconds. In other words, we were essentially telling the tool to go batshit crazy and spit out every note at the exact same instant, thus explaining the aforementioned diarrhea.
Three hours, my friends. Three hours. That’s how long it took to figure this one out. I know that I come off very heroic in this tale I tell, but perhaps that image of me will be altered when you learn that the bug was introduced by yours truly.