The Train Heist Think Like A Coder Ep 4
Ethic, Hedge, and Adila, the leader
of the revolution,
plot out how they can steal an artifact
called the Node of Power.
It’s being used to run a heavily fortified
train that runs all around the country,
providing supplies
to settlements and facilities.
This armored behemoth
undergoes a complex and unpredictable
unloading procedure—
a procedure which is displayed, in detail,
on a screen within the engine car.
Right means the train will go one
car length forward,
and left means the train will go the
same distance backwards.
While unloading, the train frequently
moves back and forth,
so a typical sequence
might look like this.
Also within the engine car is a button
that can only be pressed once.
When pressed, it lets down the force
field over the artifact for 10 seconds.
The engine car is tiny
and designed for a robot.
Of your team, only Hedge can fit.
The members of the resistance have
positioned a crane over the train tracks
that can pluck the artifact
once it’s exposed.
They’ll know when to lower
the crane by sight.
But the only way Hedge can determine
the train’s position
and know when to lower
the force field
is by analyzing the unloading procedure,
because he’ll be inside
the windowless engine car.
Hedge can’t program himself though, so
it’s up to Ethic to tell him what to do.
The artifact is in the car
10 positions behind
the one that’s directly under the
crane at the start.
What instructions can Ethic give Hedge
so that he hits the button
at just the right moment?
Here’s a hint to get you started.
The key to this problem, as with many
programming challenges,
is to reframe the information in a way
that a computer can work with.
A computer doesn’t know
what a train is, nor does it need to.
It can, however, work with variables.
Try making a variable that tracks
the train’s position.
How will it change as the train moves?
Let’s start by breaking this problem
into two objectives.
The first is to know where the train will
be as it carries out its instructions.
The second is to hit the button when
the train is in just the right position.
For the first objective it’ll help to
think of the train as a big number line.
Let’s make 0 the car with the node,
1 the car in front of it, and so on.
That means car 10 is under
the crane at the start.
When the train moves one car right,
car 9 is under the crane.
So a right arrow can be
thought of as “subtract 1.”
And when the train moves left
from there,
10 is back under the crane,
making a left arrow the same as “add 1."
Let’s set our train position variable
to 10, since that’s where we start.
We can now use a loop
to read the instructions one at a time,
adding or subtracting as we go,
to track which car is under the crane.
The nice thing about setting up
the variable this way
is that it tells us how far the node
is from the crane.
So as soon as the variable hits 0,
Hedge should hit the button.
And here’s what happens.
Ethic gets into position on the crane
while Hedge rushes off
and slips into the engine car unnoticed,
just before the train lurches to life.
It rolls 3 cars back. 1 forward,
another 4 back.
Then so far forward Ethic loses track
before it reverses once more.
When the artifact
finally rolls into position,
Adila lowers the crane,
hoping Ethic and Hedge got it right.
At the last possible moment,
the force field sputters and falls.
Ethic swoops in, and lifts the
Node of Power to freedom.
When Ethic gives the node
to Hedge for safe keeping,
something incredible happens.
The artifact shimmers to life
with a vision of the past:
When the crystal was unearthed,
no one could make the console inside work.
The government put out a call
for people to try their luck with it,
one at a time.
Ethic loved to figure out what made
things tick, so she signed up.
Within moments at the console,
something clicked into place,
and she created her first robot.
The government hired Ethic as
chief robotics engineer on the spot.
Within a year her creations ran almost
every aspect of society,
and the nation and its people thrived,
no longer needing to toil
in the fields and factories.
The vision ends, and Hedge detects
the second artifact in the 198forest,
to the southeast.
Luckily, the train is going there next,
and has just enough reserve
fuel for the trip.
Ethic and Hedge smuggle themselves aboard
and find a hiding spot
for the long journey ahead.