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Programming Logic and Design Seventh Edition Chapter 5 Looping

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Objectives In this chapter, you will learn about: The advantages of looping Using a loop control variable Nested loops Avoiding common loop mistakes Using a for loop Common loop applications

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Understanding the Advantages of Looping Looping makes computer programming efficient and worthwhile Write one set of instructions to operate on multiple, separate sets of data Loop: a structure that repeats actions while some condition continues

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Understanding the Advantages of Looping (continued)

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Using a Loop Control Variable As long as a condition remains true, the statements in a while loop’s body execute Control number of repetitions Loop control variable initialized before entering loop Loop control variable tested Body of loop must alter value of loop control variable Repetitions controlled by: Counter Sentinel value

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Using a Definite Loop with a Counter Definite loop Executes a predetermined number of times Counter-controlled loop Program counts loop repetitions Loop control variables altered by: Incrementing Decrementing

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Using an Indefinite Loop with a Sentinel Value Indefinite loop Performed a different number of times each time the program executes The user decides how many times the loop executes

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Understanding the Loop in a Program’s Mainline Logic Three steps should occur in every properly functioning loop Provide a starting value for the variable that will control the loop Test the loop control variable to determine whether the loop body executes Alter the loop control variable

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Nested Loops Nested loops: loops within loops Outer loop: the loop that contains the other loop Inner loop: the loop that is contained Needed when values of two (or more) variables repeat to produce every combination of values

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Nested Loops (continued)

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Avoiding Common Loop Mistakes Mistake: neglecting to initialize the loop control variable Example: get name statement removed Value of name unknown or garbage Program may end before any labels printed 100 labels printed with an invalid name

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Avoiding Common Loop Mistakes (continued) Mistake: neglecting to alter the loop control variable Remove get name instruction from outer loop User never enters a name after the first one Inner loop executes infinitely Always incorrect to create a loop that cannot terminate

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Avoiding Common Loop Mistakes (continued) Mistake: using the wrong comparison with the loop control variable Programmers must use correct comparison Seriousness depends on actions performed within a loop Overcharge insurance customer by one month Overbook a flight on airline application Dispense extra medication to patients in pharmacy

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Figure 5-12 Incorrect logic for greeting program because the wrong test is made with the loop control variable Figure 5-12 Incorrect logic for greeting program because the wrong test is made with the loop control variable

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Avoiding Common Loop Mistakes (continued) Mistake: including statements inside the loop that belong outside the loop Example: discount every item by 30 percent Inefficient because the same value is calculated 100 separate times for each price that is entered Move outside the loop for efficiency

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Using a for Loop for statement or for loop is a definite loop Provides three actions in one structure Initializes Evaluates Alters Takes the form: for loopControlVariable = initialValue to finalValue step stepValue do something endfor

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Using a for Loop (continued) Example for count = 0 to 3 step 1 output "Hello" endfor Initializes count variable to 0 Checks count variable against the limit value 3 If evaluation is true, for statement body prints the word “Hello” Increases count by 1

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Using a for Loop (continued) while statement could be used in place of for statement Step value: the amount by which a loop control variable changes Can be positive or negative (incrementing or decrementing the loop control variable) Default step value is 1 Programmer specifies a step value when each pass through the loop changes the loop control variable by a value other than 1

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Using a for Loop (continued) Pretest loop: the loop control variable is tested before each iteration for loops and while loops are pretest loops Posttest loop: the loop control variable is tested after each iteration do…while is a posttest loop

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Common Loop Applications Using a loop to accumulate totals Examples Business reports often include totals List of real estate sold and total value Accumulator: variable that gathers values Similar to a counter Counter increments by 1 Accumulator increments by some value

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Common Loop Applications (continued) Accumulators require three actions Initialize the accumulator to 0 Accumulators are altered: once for every data set processed At the end of processing, accumulators are output Summary reports Contain only totals with no detail data Loops are processed but detail information is not printed

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Common Loop Applications (continued)

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Using a loop to validate data Using a loop to validate data Defensive programming: preparing for all possible errors before they occur When prompting a user for data, no guarantee that data is valid Validate data: make sure data falls in acceptable ranges (month values between 1 and 12) GIGO: Garbage in, garbage out Unvalidated input will result in erroneous output

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Common Loop Applications (continued) Limiting a reprompting loop Reprompting can be frustrating to a user if it continues indefinitely Maintain a count of the number of reprompts Forcing a data item means: Override incorrect data by setting the variable to a specific value

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Common Loop Applications (continued) Validating a data type Validating data requires a variety of methods isNumeric() or similar method Provided with the language translator you use to write your programs Black box isChar() or isWhitespace() Accept user data as strings Use built-in methods to convert to correct data types

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Common Loop Applications (continued) Figure 5-21 Checking data for correct type

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Common Loop Applications (continued) Validating reasonableness and consistency of data Many data items can be checked for reasonableness Good defensive programs try to foresee all possible inconsistencies and errors

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Summary Loops write one set of instructions that operate on multiple, separate sets of data Three steps must occur in every loop Initialize the loop control variable Compare the variable to some value Alter the variable that controls the loop Nested loops: loops within loops Nested loops maintain two individual loop control variables Alter each at the appropriate time

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Summary (continued) Common mistakes made by programmers Neglecting to initialize the loop control variable Neglecting to alter the loop control variable Using the wrong comparison with the loop control variable Including statements inside the loop that belong outside the loop Most computer languages support a for statement for loop used when the number of iterations is known Loops are used to accumulate totals in business reports and to reprompt users for valid data