MIS410 logic

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Computing with Business Applications

1: Programming Logic and Design (comprehensive); by Joyce Farrell, Ninth Edition, Cengage Learning.

2: VBA for Modelers: Developing Decision Support Systems with Microsoft Office Excel; by Albright 5th edition, South Western Cengage Learning.

Chapter 5

Creating Loops

1: Programming Logic and Design (comprehensive); by Joyce Farrell, Ninth Edition, Cengage Learning.

pages: 177-217

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

3© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Understanding 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

4© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Understanding the Advantages of Looping (continued)

Figure 5-1 The loop structure

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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 variable (have a separate variable) • 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

6© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

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

7© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Figure 5-3 A counted while loop that outputs Hello four times

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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

9© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Figure 5-4 An indefinite while loop that displays Hello as long as the user wants to continue

the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Nested Loops

• Nested loops: loops within loops

• Outer loop contains the Inner loop

• Needed when values of two (or more) variables repeat to produce every combination of values

11© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Dealing with 2D ranges

• While row_num < 5 • Print row number

• While column_num < 6 • Print column number

• Increase column number

• Increase row number

• What it will print:

1, 1,2,3,4,5, 2, 1,2,3,4,5, 3, 1,2,3,4,5, 4, 1,2,3,4,5

Programming Logic & Design, Seventh Edition 12

Is Anything Wrong?

Programming Logic & Design, Seventh Edition 13

Figure 5-10 Incorrect logic for greeting program because the loop control variable initialization is missing

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Is Anything Wrong?

Programming Logic & Design, Seventh Edition 15

Figure 5- 11

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Programming Logic & Design, Seventh Edition 17

Is Anything Wrong?

Figure 5-12

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Programming Logic & Design, Seventh Edition 19

Is Anything Wrong?

Programming Logic & Design, Seventh Edition 20

Figure 5-14 Improved discount sticker-making

program

Using a for Loop

• for statement or for loop is a definite loop

• Provides three actions in one structure • Initializes

• Evaluates

• Alters

21© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

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 3

• If evaluation is true, print “Hello”

• Increases count by 1

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Using a for Loop

• 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

23© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

In VBA

• For loops

• For Each Loops

• Do loops

Programming Logic & Design, Seventh Edition 24

For Loops

• For counter = first to last [Steps increment] • Statements

• Next [counter]

For i = 1 To 10 Steps 2

MsgBox (i)

Programming Logic & Design, Seventh Edition 25

For Each Loops

• For Each Item in collection • Statements

• Next

Dim cell as Range

For Each cell in Range(“A1:A20”)

cell.value = 1

Programming Logic & Design, Seventh Edition 26

Do Loops

• Next week…

Programming Logic & Design, Seventh Edition 27

• 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

Common Loop Applications

the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Common Loop Applications

• Validating data input • Validating data requires a variety of methods

• isNumeric() or similar method • Provided with the language translator you use to write your programs

• 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

30© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

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

31© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

Control Logic - Condition If … Then … Else … Endif

If some condition Then a statement to be done if the

condition is True

If some condition Then

a bunch of statements to be done if the

condition is True

Else

a bunch of statements to be done if the

condition is False

End If

Single line

Block style (preferred)

Sec. 7.3

Else block is optional

Mandatory

Control Logic - Condition If … Then … Else … Endif

If dblValue < 0 Then MsgBox “Must enter non-negative #”

If dblValue < 0 Then

MsgBox “Must enter non-negative #”

Else

dblSummation = dblSummation + dblValue

intSampleSize = intSampleSize + 1

End If

Go back and look at FirstProgramFinished-InClass.xls See CountHighSales_TitleBar() for if then else and for enhancing MsgBox

Control Logic - Looping For…Next

For counter = start To stop

bunch of statements to be

repeated

Next counter

For intMonth = 1 To 12

TotalCost = TotalCost + Range(“Costs”).Cells(intMonth)

Next intMonth

Sec. 7.5-7.7

When intMonth=13, the program will “break out of the loop” and execute the statement immediately AFTER the Next statement.

Control Logic - Looping For Each…Next

For Each object In collection

bunch of statements to be

repeated

Next object

Sec. 7.6

This is the example from HW1-Rubric.xlsm that we looked at when we started learning VBA.

Control Logic - Looping Do While…Loop

Do While condition

bunch of statements to be

repeated

Loop

Do While blnStillEnteringNumbers

intNumEntered = intNumEntered + 1

BLAH BLAH BLAH .....

Loop

Sec. 7.5-7.7

If condition is true

Execute these statements

When condition is False, the program will “break out of the loop” and execute the statement immediately AFTER the Loop statement.

For-Next and Do-While Do While condition

bunch of statements to

be repeated

Loop

For i = start to stop

bunch of statements to

be repeated

Next i

vs.

For i = 1 to numEmployees

For j = 1 to numRegions

bunch of statements that use i and j

Next j

Next i

 How might we modify one of the For-Next loops in FirstProgram to use Do-While instead?

 Loops are one of the most useful constructs in programming  Allow us to repeat a block of similar code many times, very quickly

Iteration

Conditional

Sequential Flow

More on Control Logic (Ch7) • There are additional variants of conditional and looping

constructs • If-Then-ElseIf-ElseIf…-EndIf

• Very general form of If

• Select-Case

• Avoids complicated If

• Do Until … Loop

• Do something until something is true

• Do … Loop While, Do … Loop Until • Doesn’t check looping continuation until after deed is done

• Nesting of conditional logic and looping logic is very common • Use indenting and comments to avoid going crazy

• See Chapter 7 of VBA for Modelers for all the details

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

40© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.

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

41© 2013 Cengage Learning. All Rights Reserved. This edition is intended for use outside of the U.S. only, with content that may be different from the U.S. Edition. May not be scanned, copied, duplicated, or posted to a publicly accessible website, in whole or in part.