Code style query: multiple assignments in if/elif tree

[Chris Angelico]

On Tue, Apr 1, 2014 at 6:20 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Tue, 01 Apr 2014 16:01:40 +1100, Chris Angelico wrote:
>
> [...]
>>> The scenario you describe has (effectively) infinite rate-of-change-of-
>>> acceleration, often called "jerk". (A jerk is a rapid change in
>>> acceleration.) Human comfort is (within reasonable limits) more
>>> affected by jerk than acceleration. The passengers will feel three
>>> quite distinctive jerks, one when the brakes are first applied (which
>>> is probably reasonable), then one at 1s, then again at 2s. That's not
>>> comfortable by any stretch of the imagination.
>>
>> It actually is a smooth increase in deceleration, but I'm operating the
>> simulator on a 1s period, so it's actually an average across the first
>> second, and an average across the next second...
>
> Hmmm. A 1-second resolution doesn't really sound too great to me.
>
> Suppose the deceleration increases linearly from 0 to 0.85 m/s over two
> seconds. Averaging it in the way you suggested, we get a total distance
> of:
>
>     2*u - 0.5125
>
> (see my previous post for details) where u is measured in metres per
> second. Multiply by seconds to get the units right. For a Japanese bullet
> train where u = 320 km/hr that corresponds to
>
>     py> 2*88.888889 - 0.5125
>     177.26527800000002
>
> metres.
>
> Now let's do it properly! Given our assumption that the deceleration is
> linear, the jerk will be constant:
>
>     j = Δa/Δt
>       = (0.85 - 0)/2
>       = 0.425 m/s^3
>
> Let's start integrating!
>
> py> from sympy import integrate
> py> from sympy.abc import t
> py> j = '0.425'
> py> a = integrate(j, t)
> py> v = 88.888889 - integrate(a, t)
> py> s = integrate(v, (t, 0, 2))
> py> s
> 177.211111333333
>
> compared to 177.26527800000002 calculated the rough way. That's not bad,
> only about 5cm off! Effectively, your rough calculation was accurate to
> one decimal place.

5cm when we're dealing with track distances measured in meters and
usually hundreds of them? Sounds fine to me!

> Of course, if the equation for acceleration was more complex, the
> approximation may not be anywhere near as good.

And that's the bit I can't know. I've added a comment to the code and
will toss it back to my brother - hopefully he'll either know the
answer or know where to look it up / who to ask.

> [...]
>>> This becomes a simple question for the four standard equations of
>>> motion:
>>>
>>> (1) v = u + at
>>> (2) s = 1/2(u + v)t
>>> (3) s = ut + 1/2(at^2)
>>> (4) v^2 = u^2 + 2as
>>>
>>> Only (1) and (3) are needed.
>>
>> Okay, what's u here? Heh.
>
> They're *standard* equations of motion. Weren't you paying attention
> through the, oh, three years of high school where they teach this? :-P

They're certainly the standard equations, but I learned that:

d = v₀t + at²/2

which is the same as you had, except for the names: d for distance, v₀
(V zero, that's a subscript zero if encodings or fonts kill you here)
for initial velocity (ie velocity at time zero - vₜ (that's a
subscript t, U+209C, but it isn't working in this font) is velocity at
time t, which is v₀+at), and a and t are the same as you have.

> s = displacement (distance)
> t = time
> u = initial velocity
> v = final velocity
> a = acceleration

I have to say, the one-letter variables are a lot easier to work with.
Guess I learned them the hard way, heh. Although remembering that v is
velocity is easier than remembering which of u and v is initial and
which is final.

>> Fair enough. :) That's why I asked. Is it naughty enough to break into
>> two statements, or is it better to combine it into a single multiple
>> assignment?
>
> Since they are unrelated assignments, of two different variables, they
> should be written as two separate assignments, not one using sequence
> unpacking.

Change made. Yeah, that looks a bit cleaner. Although I would prefer a
simple formula to what I have there, and I'm still not perfectly sure
it's accurate.

Advice appreciated. :)

ChrisA