Computer Science 111 Fundamentals of Computer Programming I

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Computer Science 111Fundamentals of Computer
Programming I

• Control Statements
• The While Statement

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The While-Statement

• The syntax for the While-Statement is
  while (condition) statement
• The statement may be compound - more than one
  statement enclosed in a pair of

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While-Statement example

• writer.println(Enter 50 numbers)count0
  total0while (count lt 50)
  writer.print(Enter number ) num
  reader.readDouble() total total
  num count count1 writer.print(The
  total is )writer.println(total)

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Common While Structure

• initialize accumulatorinitialize counterwhile
  (cond about counter) get some input
  do computation update accumulator
  increment counter output results

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Common Mistakes

• Initialization
• Forget to initialize accumulator has old value,
  etc.
• Initialize inside loop accumulator doesnt
  accumulate
• Terminating condition
• Use lt instead of lt, etc. Off by one errors.
• Loop body
• Forget to increment, etc. Infinite loop.
• Output
• Inside loop outputs every time through loop

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Example

• We want a program that takes as input a positive
  number, limit, and outputs how many consecutive
  integers, starting at 1 that we can add together
  without exceeding limit.
• For example, if limit is 25 the output should be
  6 since 12345621 and 123456728.

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Example cont.

• We will need variables
• limit
• sum for the running total
• current for the number we are currently adding to
  sum
• We should initialize sum to 0, current to 1
• We get the value for limit from the user before
  the loop.

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Example cont.

• int limit, current1,sum0limit
  reader.readInt(Enter upper limit )while (sum
  lt limit) sum sum current
  current current 1writer.print(Answer is
  )writer.println(current)

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Example cont.

• limit 25iteration current sum0
  1 01 2
  12 3
  33 4 64
  5 105
  6 156 7
  217 8
  28
• Output is 8 - Wrong

• int limit, current1,sum0limit
  reader.readInt(Enter upper limit )while
  (sum lt limit) sum sum current
  current current 1writer.print(Answer is
  )writer.println(current)

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Example cont.

• int limit, current1,sum0limit
  reader.readInt(Enter upper limit )while
  (sum lt limit) sum sum current
  current current 1writer.print(Answer is
  )writer.println(current)

• writer.println(current-1)
• We still have a problem try limit 15 for
  example.
• lt should be lt

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Ok boys, there's a bunch of those furry
creatures, see if you can lasso them...