B1 Intermediate US 18 Folder Collection
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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.
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The Train Heist | Think Like A Coder, Ep 4

18 Folder Collection
ally.chang published on February 26, 2020
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