Iteration%20(Loops)

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Iteration (Loops)

• Loops repeat a set of instructions
• Two types of loops
• Definite loops ( for ) perform instructions
  explicit number of times
• Indefinite loops ( while ) perform instructions
  an indefinite number of times (until a certain
  test fails)

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Questions for You

• a new class youre writing has a method with the
  following signature
• move (int numMoves)
• should it be implemented with a definite loop or
  an indefinite loop? justify your answer
• could it be implemented with the other type of
  loop? if so, should we/could we/good idea/bad
  idea???

3
Questions for You

• take 5 minutes and draw a situation on paper that
  would necessitate that you use a while loop to
  solve the problem

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for Loops

• General formfor ( ltinitialize
  variablegt lttestgt ltincrementgt
  ) ltinstruction setgt

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for Loops (cont.)

• ltinitialize variablegt sets a variable/identifier
  to a certain value (variable is usually count, i,
  j)
• lttestgt is a test that is evaluated each time the
  body is about to be executed (when false, loop is
  exited)
• ltincrementgt changes the loop control variable
  (usually x or x--)

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for Loops (cont.)

• Examplefor (int x1 xlt5 x) karel.move()
  
• This causes karel to move 5 times

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for Loops (cont.)

• Flow of for loops
• Initialization statement executes
• If test is true, proceed to step 3 if test is
  false, go to step 6
• Instructions in body of loop are executed
• Increment statement executes
• Return to step 2
• Program proceeds to statements after loop

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while Loops

• General formwhile ( lttestgt ) ltinstruction
  listgt
• Loop continues until test is false

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Questions for You

• back on slide 3 you drew up a situation that
  would best be solved using a while loop - now
  write the code to solve your problem

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while Loops (cont.)

• Examplewhile (karel.frontIsClear
  ()) karel.move ()
• Causes karel to move continuously until there is
  a wall in front of it

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while Loops (cont.)

• Flow of while loops
• If test is true, proceed to step 2 if test is
  false, go to step 4
• Instructions in body of loop are executed
• Go to step 1
• Statements after loop are executed

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Building a While Loop Formalizing (making
easier) the ad-hoc way youve been writing while
loops to this point. This will be extremely
helpful to you at various times in the course and
on the AP exam.

• Step 1 Identify what is true when the loop is
  finished (this is always easier than determining
  what is false while it is still executing)
• Step 2 Use the opposite form of step 1s result
  as the lttestgt for the loop. You just need to
  negate the entire thing. (note Now and again you
  might find that DeMorgan-izing your result is
  helpful when answering AP-like questions.)

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Building a While Loop (contd)

• Step 3 make progress toward the goal
  (completion of the loop) within the loop
• Step 4 Do whatever is required before and/or
  after the loop to ensure that we solve the given
  problem
• Example Pick all beepers from a corner without
  knowing how many there are.

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Lets build a while loop

• Move a robot until it is not on a corner with any
  other robots and there is a wall on the left,
  then pick up the beeper that will be waiting
  there (the hard part is getting the condition
  correct without using a trial-and-error method
  you dont get to run any programs during our
  tests nor on the AP exam also, youll code your
  labs faster if you have a way to ensure correct
  conditions EVERY time without the crutch of
  running your code)
• You may use and for this exercise. Use the
  4-step process now
• while (???)
• ???

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Your try the while condition

• You may use and for this
• Write a loop that moves a robot forward until it
  is either next to a robot, next to a beeper, or
  there is no wall on its right
• while ( ??? )
• ???

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Infinite Loops

• In a for or while loop, it is possible for the
  test to never be false
• When this happens, the loop continues infinitely
• Depending on the compiler, application, and other
  circumstances, an error may occur or the app may
  crash