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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 assubtract 1.”

  • And when the train moves left from there,

  • 10 is back under the crane,

  • making a left arrow the same asadd 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.

Ethic, Hedge, and Adila, the leader of the revolution,

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B1 US TED-Ed train ethic crane hedge artifact

The Train Heist | Think Like A Coder, Ep 4

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    ally.chang posted on 2020/02/26
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