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  • After breaking Ethic out of prison,

  • Hedge flies them both towards a frontier settlement

  • in the shadow of the Bradbarrier, the great wall that encircles the nation.

  • All the settlers there will soon gather for the monthly feeding.

  • The people of the wall spend their days gathering up works of art and literature,

  • from all across the land.

  • On feeding day, the furnace-bots arrive, ravenous.

  • If they eat, the lights stay on, and the food gets delivered.

  • If they starve, the people do too.

  • Hedge's fuel supply runs out just as he and Ethic reach the outskirts of town,

  • and they come in for a crash landing.

  • Luckily, everyone is too busy preparing for the feeding to notice.

  • Today's feeding is where Ethic can find the leader

  • of an underground resistance movement.

  • This person knows the location of the first of three powerful artifacts.

  • The problem is, Hedge and Ethic don't know

  • the resistance leader's name or appearance.

  • But Hedge has gathered the following information:

  • The leader has green eyes.

  • If the leader has red hair, their name has at least one consecutive double letter.

  • If the leader wear glasses, their name has exactly 2 vowels.

  • Otherwise, their name has exactly 3 vowels.

  • There is exactly one person for whom these are all true.

  • As a fugitive,

  • Ethic can't sneak into the crowd without drawing attention to herself.

  • But she can give instructions to Hedge.

  • And one tool she has is what programmers call a conditional.

  • That's a statement of the formIf A, then B.”

  • Flowcharts are great illustrations of how those work.

  • This conditional translates to: if A is true, carry out instruction B.

  • There are also conditionals that account for different possibilities.

  • This says, “If A is true, perform instruction B.

  • Otherwise, carry out instruction C.”

  • So what instructions does she give Hedge so he can find the resistance leader?

  • Pause now to figure it out for yourself.

  • With a problem like this, it can help to simplify first.

  • What if Hedge just has to examine this one person?

  • What information does he need to collect about her?

  • He might ask, “Does she have green eyes?”

  • What other questions should Hedge ask to find the resistance leader,

  • and how can he track those answers?

  • Pause now to figure it out for yourself.

  • It may seem intuitive how you'd approach this problem as a human.

  • But Hedge isn't a human,

  • and so the challenge comes from needing to give him systematic instructions

  • that will work in any scenario.

  • Hedge needs to examine the settlers, one at a time,

  • until he discovers the right person.

  • In other words, like with the lock on the prison cell,

  • this is a loop that repeats the same instructions.

  • Only this time the loop will involve a series of questions

  • in the form of conditionals,

  • and will end as soon as Hedge finds his target.

  • But first, you'll want to organize your information.

  • Each person has a set of characteristics: Eye color, hair color, glasses, and name.

  • Does this person have green eyes?

  • If so, mark a check next toeye color." If not, mark an X there.

  • If they have red hair, does their name contain a double letter?

  • If so, mark a check next tohair color.”

  • If they don't have a double letter, mark an X next tohair color.”

  • Anyone with red hair and no double letter can't be the resistance leader.

  • But notice that if they have blue hair,

  • Hedge will skip this question and go on to the next one.

  • For the last question, we can say,

  • If they wear glasses, does their name have exactly 2 vowels?

  • If they don't have glasses, does their name have exactly 3 vowels?”

  • There will be people in the crowd with glasses and 1 vowel,

  • or no glasses and 2 vowels.

  • But they're not who we're looking for, so they'll get X's.

  • The resistance leader must be someone with either check marks

  • or blanks next to every question.

  • Blanks are ok, because if someone has blue hair,

  • the rule about red hair doesn't apply to them.

  • You could have Hedge ask every question about every person,

  • and then choose the person with only checks and blanks.

  • But there's a way to save yourself lots of time: as soon as Hedge marks an X,

  • have him move on to the next person.

  • You don't need to know the answer to every question;

  • just one X means they're not the target of your search.

  • Hedge buzzes through the crowd,

  • and within minutes finds Adila, the resistance leader,

  • and brings her back to Ethic.

  • Adila agrees to help them steal the first artifactthe node of power

  • but under one condition:

  • that Ethic and Hedge jump-start the revolution

  • by reprogramming the furnace-bots that terrorize the town.

  • And right on cue, the robots descend.

After breaking Ethic out of prison,

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