Lecture 10 Repetition Structures

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Lecture 10Repetition Structures

• ENGR17 Engineering Programming
• Section 1
• Fall 2001

10/12/01
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Outline

• Three Repetition Structures
• While Loop
• Write pseudocode for the while repetition
  structure
• Create a flowchart for the while repetition
  structure
• Code the while repetition structure
• Initialize and update counters and accumulators
• Nested Repetition Structures
• For Loop
• for statement to implement a counter-controlled
  loop
• Do-While Loop
• Write the pseudocode for the do-while loop
• Create a flowchart for the do-while loop
• Code the do-while loop

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The Repetition Structure

• Directs the computer to repeat one or more
  instructions until a condition (loop condition)
  is met
• Also called looping or iteration
• Three forms
• while
• for
• do-while

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The Repetition Structure
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while Repetition Structure

• Often begins with repeat while and ends with end
  while
• Loop body instruction(s) between repeat while
  and end while
• Referred to as a pretest loop or top-driven loop
• Evaluates loop condition before processing
  statements within the loop
• Each loop condition results in either a True or a
  False answer only

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while Loop in Pseudocode
Loop Body
While Loop
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Repetition Structure Flow Chart

• Symbol is the Diamond
• Diamond contains a question that has a True or
  False answer only
• Question represents the loop condition that the
  repetition structure must evaluate
• Has two flowlines leaving the symbol
• Has two flowlines leading into the diamond,
  rather than just one

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Flow Chart for while Loop
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Flow Chart for while Loop
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while Loop in C

• Use the C while statement

not needed after while
Loop Body
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while Loop in a Program
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while Loop in a Function
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Using Counters and Accumulators

• Numeric variables used within a repetition
  structure to calculate subtotals, totals, and
  averages
• Counter
• Used for counting something
• Accumulator
• Used for accumulating (adding together) something
• Initializing
• Assigning a beginning value to the counter or
  accumulator
• Typically done outside the repetition structure
• Updating (incrementing)
• Adding a number to the value stored in the
  counter or accumulator
• Counter incremented by a constant value
• Accumulator incremented by a value that varies
• Placed within the repetition structure

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Accumulator Example
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The Sales Express Inc. Program
Problem?
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The Sales Express Inc. Program
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Sentinel Values

• Values used to end loops
• Should be easily distinguishable from the valid
  data used by the program
• Also called trip values or trailer values

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Nested Repetition Structures

• One loop (inner loop), is placed entirely within
  another loop (outer loop)

Outer
Inner
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while Loop Example
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Counter-controlled while Loop

• Counter-controlled loop
• Uses a counter to control the number of times the
  loop is processed
• Uses the value in the counter variable to
  determine if the required number of loop
  repetitions have been performed

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Using the while Loop
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Counter-controlled for Loop

• Allows coding of the loop in a more convenient
  and compact manner

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for Loop
Use count
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Tasks Performed by the for Loop

• Creates and initializes a numeric variable (the
  counter variable) to a beginning value
• Done only once at the beginning of the loop
• Evaluates a loop condition to determine if loop
  instructions should be processed
• If loop condition evaluates to True,loop
  instructions are processed and third task is
  performed otherwise, the loop stops
• Updates the value in the counter variable

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for Statement Demo Program
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for Example
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The do-while Loop

• Used to repeat one or more statements either a
  specified number of times or while some loop
  condition is True
• Often begins with do and ends with end do
• Is referred to as a posttest loop or
  bottom-driven loop
• Evaluates the loop condition after first
  processing the loop instructions

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The do Loop
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do-while Loop Example
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Summary

• Three Repetition Structures
• How to use the while form of the repetition
  structure
• How to use counters and accumulators
• How to use nested loops
• How to use the for form of the repetition
  structure
• How to use the do-while form of the repetition
  structure

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Example

• Write 4 functions that print the numbers n to 1
  using
• While Loop
• For Loop
• Do-While Loop
• Recursion