CS 177 Recitation

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CS 177 Recitation

• March 23rd, 2007 Week 10
• Conditional Repetition

2
Announcements

• Project 4 is due at 9pm on Wednesday, April 4th.
• Start now if you have not started yet.
• The Final Exam schedule is now available.
• The CS177 Final will be on Friday, May 4th.
• It will take place in 800-1000am in WTHR 200

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While Loops

• General form of while looks quite similar to
  the if statement, but they are completely
  different beasts!
• For if, depending on the result of
  BOOLEAN_TEST, how many times can the
  corresponding statements be executed?
• One or none
• For while, depending on the result of
  BOOLEAN_TEST, how many times can the
  corresponding statements be executed?
• Possibly many times (until the test fails)

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Example

• How many times will the statements in the body of
  the loop be executed?
• sum 0
• value 0
• while (value gt 0)
• value parseFloat (prompt ("Enter value",
  ""))
• sum sum value
• Answer 0 (the boolean test fails with the
  initial value)

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Priming the Loop

• Notice the redundancy before the loop. Why is
  it done?

• Can avoid it by just initializing the roll1 and
  roll2 variables to some dummy values (priming),
  so that the first boolean test succeeds

• Whenever you do priming, keep in mind that the
  correctness of the code for all cases must be
  preserved!

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Loop Tests

• A while loop is NOT a do-until loop
• Opposite to the way we usually think
• We think
• keep rolling dice until you get doubles (i.e.,
  roll1 roll2)
• What must the while loop be?
• keep rolling dice while you get different faces
  (i.e. while (roll1 ! roll2). . .)
• The boolean expression must be true to keep you
  in the loop and false to get you out

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Counter-Driven Loops

• What if you need to do a fixed number of
  iterations?
• The test needs to be based on a counter variable
• The counter is initially set to 0 and is
  incremented on each repetition of the loop body
• When the counter reaches our fixed number, the
  boolean test fails and the loop terminates

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Examples

• Does it have to count up from 0?
• No
• repCount 1
• while (repCount lt 101)
• document.write(HOWDY! ltbr /gt)
• repCount repCount 1
• Does it have to count up at all?
• No, it can count down
• count 10
• while (count gt 0)
• document.write(count ltbr /gt)
• count count - 1
• document.write(BLASTOFF!)

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More Examples

• Does it have to increment/decrement by 1?
• No
• count 0
• while (count lt 1000)
• document.write(HOWDY! ltbr /gt)
• count count 10
• count 100
• while (count gt 0)
• document.write(count/10 ltbr /gt)
• count count - 10
• document.write(BLASTOFF!)

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Last Example

• What is stored in the variable product at the
  conclusion of this loop?
• x parseFloat (prompt("Enter any number",""))
• n parseFloat (prompt("Enter any integer gt
  0",""))
• product 1
• while (n gt 0)
• product product x
• n n - 1
• Answer xn (variable product is the result of n
  multiplications by x)

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Conclusions from Examples

• You will usually have a lot of counting options
  (start from 0/start from something else,
  increment/decrement, by 1/by something else)
• Always choose the most intuitive way and make
  sure that correctness is preserved
• What is the most common error in counter-driven
  loops?
• "off by 1" error
• How can it be avoided?
• Carefully think about the initial counter value
  and the value at which the boolean test fails
• From these two values, figure out how many times
  your loop will iterate
• When processing arrays and strings, indexing is
  easiest if we begin the loop counter at 0

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Hailstone Sequence

• Remember the Hailstone Sequence? (Assume that
  isOdd(x) returns true if x is odd and false
  otherwise)
• while (number ! 4) if (isOdd(number))   
  number number 3 1  else    number
  number / 2  document.write(number )
• What will be the output if number 7?
• 22 11 34 17 52 26 13 40 20 10 5 16 8 4

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Questions?