Stephen Royle

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This deserved a bit of further explanation, due to the stupidity
involved.

> "Debugging is like being the detective in a crime movie where you are
> also the murderer." -- Filipe Fortes

My code was giving an unexpected result and I was having a hard time
figuring out the problem. The unexpected result was that a resampled set
of 2D coordinates were not being rotated randomly. I was fortunate to be
able to see this otherwise I would have never found this bug and
probably would've propagated the error to other code projects.

I narrowed down the cause but ended up having to write some short code
to check that it really did cause the error.

I was making a rotation matrix and then using it to rotate a 2D
coordinate set by matrix multiplication. The angle was randomised in the
loop. What could go wrong? I looked at that this:

``` wp-block-preformatted
theta = pi*enoise(1)
rotMat = {{cos(theta),-sin(theta)},{sin(theta),cos(theta)}}
```

and thought "two lines -- pah -- it can be done in one!". Since the
rotation matrix is four numbers \[-1,1\], I thought "I'll just pick
those numbers at random, I just want a random angle don't I?"

``` wp-block-preformatted
rotMat = enoise(1)
```

Why doesn't an alarm go off when this happens? A flashing sign saying
"are you sure about that?"...

My checks showed that a single point at 1,0 after matrix multiplication
with this method gives.

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:::

When it should give

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:::

And it's so obvious when you've seen why. The four numbers in the
rotation matrix are, of course, not independent.

I won't make that mistake again and I'm going to try to think twice when
trying to save a line of code like in the future!

---

Part of a series on computers and coding.


This deserved a bit of further explanation, due to the stupidity involved. My code was giving an unexpected result and I was having a hard time figuring out the problem. The unexpected result was that a resampled set of 2D coordinates were not being rotated randomly. I was fortunate to be able to see this otherwise I would have never found this bug and probably would’ve propagated the error to other code projects.

Messaggi di Rogue Scholar

Adventures in Code VI: debugging and silly mistakes