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4.1 Introduction to Decision Structures
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4.2 Dual Alternative Decision Structures
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4.3 Comparing Strings
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4.4 Nested Decision Structures
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4.5 The Case Structure
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4.6 Logical Operators
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4.7 Boolean Variables
Remember: sequence structure – goes in order, top to bottom p. 121 pseudocode
A decision (aka selection) structure allows a program to perform actions only under certain conditions
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EXAMPLE: >40 hours for payroll
Different types of decisions include
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If, also called single alternative (see coat example below)
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If then else, also called dual alternative
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Case structure for multiple alternative decisions
The if statement
Evaluate condition action only occurs if the decision is true
If condition Then
Statement
Statement
End If
A diamond symbol is used in flowcharts
Align IF and END IF; indent other statements as needed (page 128)
Combine sequence structures and decision structures (Figure 4-2)
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Get temp Wear coat? Go outside
Sequence structures can be nested in decision structures (Figure 4-3)
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Coat Hat Gloves
Build Single Decision (Cold / Wear Jacket) Raptor file.
Relational Operators
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Determines whether a specific relationship exists between two values >40 = Pay overtime
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Used within the condition, a Boolean expression true or false IF statements
• x > y x<y x >=y x<y x <=y x==y x!=y (Table 4-1 p. 125)
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== is a comparison operator = is an assignment operator set bonus = 500
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!= is written as <> in VisualBasic
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Figures 4-4 through 4-6
Program 4-1 on page 129 is available in pseudocode as in the book AND as QBasic code at the following link: http://www.3mit.com/rcc/cis115/ch04qb.pdf
These examples will help you learn how to convert the pseudocode to syntax specifically for QBasic.
Here is a list of QBasic commands: http://www.freewebs.com/computerghost/QBasic/Commands.htm
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If then else statement Figure 4-8 p. 131 If, Else, End If must align on left
Executes one group of statements if its Boolean expression is true, or another group if its Boolean expression is false
If condition Then statement
If temperature < 40 Then
Display “A little cold” statement CONDITION IS TRUE
Else statement statement CONDITION IS FALSE
End if Don’t forget END IF
Program 4-2 on p. 133 illustrates IF, THEN, ELSE for payroll.
Else
End if
Display “Get a coat!”
Display “Nice weather”
Display “And sunny!”
Build Dual Alternative (Password) Raptor file.
Most languages allow you to compare strings
Other String Concerns
Strings can be compared name1 == name 2 test the value of a string
String and string literals can be compared
Month != “October”
String comparisons are generally case sensitive
You can also determine whether one string is greater than or less than another string (allows for sorting strings)
Letters are stored by ASCII characters A through Z = 69 through 90 p. 138 Blank space = 32
Codes for Mary and Mark on p. 139 Mary is “greater than” Mark because y has higher code than k
Program 4-3 on p. 137 illustrates IF, THEN, ELSE for testing a password entered by the user.
Program 4-4 on p. 140 illustrates IF, THEN, ELSE for alphabetizing names. NOTE: In QBasic, the name must be surrounded by double quotes if you want to separate last name and first name with a comma. “Smith, Jack”
Build Comparing Strings (Two Letters) Raptor file.
Decisions are nested in order to test more than one condition
The if then else if statement can make nested logic simpler to write
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If score < 60 Then
Display “Grade is F.”
Else If score < 70 Then
Display “Grade is D.”
Else If score < 80 Then
Display “Grade is C.”
Else If score < 90 Then
Display “Grade is B.”
Else
Display “Grade is A.”
End If
From Program 4-7 p. 147 (easier to read than Program 4-6)
Condition = true…ignores rest of conditions
Flowchart for Program 4-5 on p. 142
IMPORTANT: Look at proper indentation on lines 14 through 24 on p. 142 see p. 144 Also End Ifs on p. 146
Build Nested Decision (Loan Qualification) Raptor file.
Build Nested Decision 2 (Grades) Raptor file.
Build Nested Problem 2 (Area of Two Rectangles) Raptor file. See QBasic code below.
10 PRINT "What is the length of Rectangle 1?"
20 INPUT length1
30 PRINT "What is the width of Rectangle 1?"
40 INPUT width1
50 PRINT "What is the length of Rectangle 2?"
60 INPUT length2
70 PRINT "What is the width of Rectangle 2?"
80 INPUT width2
90 area1 = length1 * width1
100 area2 = length2 * width2
110 IF area1 > area2 THEN
120 PRINT "Rectangle 1 has a greater area."
130 ELSE
140 IF area1 < area2 THEN
150 PRINT "Rectangle 2 has a greater area."
160 ELSE
170 IF area1 = area2 THEN
180 PRINT "Both rectangles have equal area."
190 END IF
200 END IF
210 END IF
Notice the line numbers. Some instructors require those. Keep those IF’s aligned properly.
The case structure lets the value of a variable or an expression determine which path of execution the program will take
Can be used as an alternative to nested decisions Case structure is known as “switch statement” in some languages.
Case structure is easier to read than Figure 4-19 on p. 150.
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SHIPPING CALCULATOR (IF-THEN-ELSE)
5 CLS
10 PRINT "What is the weight of the package?"
20 INPUT weight
30 IF weight <= 2 THEN
40 charge = weight * 1.1
50 PRINT "The rate to ship is $"; charge
60 ELSE
70 IF weight <= 6 THEN
80 charge = weight * 2.2
90 PRINT "The rate to ship is $"; charge
100 ELSE
110 IF weight <= 10 THEN
120 charge = weight * 3.7
125 PRINT "The rate to ship is $"; charge
130 ELSE
140 charge = weight * 3.8
150 PRINT "The rate to ship is $"; charge
160 END IF
170 END IF
180 END IF
See pages 149-150.
For QBasic, use CASE IS for comparison operators.
CASE ELSE is the default statement.
PRINT "Enter a number."
INPUT num
SELECT CASE num
CASE IS < 10
PRINT "The number is less than 10."
CASE 10 TO 20
PRINT "The number is between 10 and 20."
CASE ELSE
PRINT "The number you gave is greater than 20."
END SELECT
SHIPPING CALCULATOR (SELECT CASE)
PRINT "Welcome to UPS RCC."
PRINT "What is the weight of the package?"
INPUT num
SELECT CASE num
CASE IS <= 2
PRINT "Weight charge is $1.10 per pound."
charge = num * 1.1
CASE IS <= 6
PRINT "Weight charge is $2.20 per pound."
charge = num * 2.2
CASE IS <= 10
PRINT "Weight charge is $3.70 per pound."
charge = num * 3.7
CASE IS > 10
PRINT "Weight charge is $3.80 per pound."
charge = num * 3.8
END SELECT
PRINT " Your total shipping charge is $"; charge
Build Nested Problem 9 (Shipping Calculator: IF-THEN-ELSE) Raptor file. See QBasic code above.
Logical Operators are used between conditions to create complex (compound) Boolean expressions
AND – Both conditions must be true
Truth table on p. 155
OR – Either condition must be true
Truth table on p. 155
NOT – Reverses the truth of an expression
Truth table on p. 156
If temperature < 20 AND minutes > 12 Then
Display “The temperature is in the danger zone.”
End If
If temperature < 20 OR temperature > 100 Then
Display “The temperature is in the danger zone.”
End If
If NOT (temperature > 100) Then
Display “This is below the maximum temperature.”
End If
Enclosed in ( ) to force correct order of operations
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Tables 4-3 and 4-4 on page 154
Often used for range checking
When checking for a number inside a range, use AND
If x >=20 AND x <=40 Then
Display “The value is in the acceptable range.”
When checking for a number outside a range, use OR
If x < 20 OR x >40 Then
Display “The value is outside the acceptable range.”
End If End If
Short circuit evaluation: AND statement If left side is false, condition will be false, skips 2nd part of condition
Short circuit evaluation: OR statement If left side is true, condition will be true, skips 2nd part of condition
Don’t get logic of logical operators confused!! ERROR: If x < 20 AND x > 40 (Mathematically impossible)
Problem 4-9 is written simpler than Problem 4-5 because of the AND logical operator.
A variable of the Boolean data type can hold one or two values: true or false
See sales quota example on page 161.
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