Selection Structures
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Making Decisions
• Sample assignment statements to figure
worker pay with possible overtime
PayAmount = Hours * Rate
PayAmount = 40*Rate + (Hours – 40)*Rate*1.5
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Selection Structure
• Use to make a decision or comparison and then,
based on the result of that decision or
comparison, to select one of the paths.
• The condition must result in either a true (yes) or
false (no) answer.
• If the condition is true, the program performs one
set of tasks. If the condition is false, there may or
may not be a different set of tasks to perform.
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Selection Structure
IF THEN Flowchart
False/No
Condition True/Yes
Statements
Note: The arms branch right and left
Also, notice how the main sequence continues down the middle
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The IF Statement
• The most common decision structure is
the IF statement.
• A condition is a Boolean expression that
evaluates to either true or false.
• Conditions typically involve one of the six
relational operators.
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Relational Operators
=
>
>=
<
<=
<>
Equal to
Greater than
Greater than or equal to
Less than
Less than or equal to
Not equal to
These operators are evaluated from left to right, and are
evaluated after any mathematical operators.
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Expressions Containing Relational
Operators
10 + 3 < 5 * 2
• 5 * 2 is evaluated first,
giving 10
• 10 + 3 is evaluated
second, giving 13
• 13 < 10 is evaluated
last, giving false
7>3*4/2
• 3 * 4 is evaluated first,
giving 12
• 12 / 2 is evaluated
second, giving 6
• 7 > 6 is evaluated last,
giving true
All expressions containing a relational operator will result in
either a true or false answer only.
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Solving the Overtime Problem
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Nested Selection Structure
• A nested selection structure is one in which
either the true path or the false path includes
yet another selection structure.
• Any of the statements within either the true
or false path of one selection structure may be
another selection structure.
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Nested If Flowchart
False/No
False/No
Condition
Condition
True/Yes
True/Yes False/No
Condition True/Yes
Notice how the main sequence continues down the middle
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Nested IF Statements
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Long Distance Billing Problem
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Logical Operators
• NOT Used to reverse the condition that follows it.
• AND Used to combine two relational conditions
together. Both conditions must be true in
order for the combined condition to be true.
• OR Used to combine two relational conditions
together. If either of the conditions is true
the combined condition is true.
• XOR Used to combine two relational conditions
together. If conditions are opposite then
combined condition is true.
Note: logical operators are evaluated after any mathematical and relational operators and
have the precedence as listed here which can be altered by ( )
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NOT Operator Truth Table
Condition NOT Condition
True
False
False
True
NOT 1=1
NOT 5<2
NOT “a”=“b”
is FALSE
is TRUE
is TRUE
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Second Condition
AND Operator Truth Table
AND
First Condition
True
False
True
True
False
False
False
False
1=1 AND 2=2
1=1 AND 2=3
“a”=“a” AND 2<4
5<3 AND 6<2
is TRUE
is FALSE
is TRUE
is FALSE
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Second Condition
OR Operator Truth Table
OR
First Condition
True
False
True
True
True
False
True
False
1=1 OR 2=2
is TRUE
1=1 OR 2=3 is TRUE
“a”=“a” OR 2<4 is TRUE
5<3 OR 6<2
is FALSE
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Second Condition
XOR Operator Truth Table
XOR
First Condition
True
False
True
False
True
False
True
False
1=1 XOR 2=2
1=1 XOR 2=3
“a”=“a” XOR 2<4
5<3 XOR 6<2
is FALSE
is TRUE
is FALSE
is FALSE
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Compound Conditions
• In Visual Logic, a compound condition consists
of two conditions within parentheses joined
by a logical operator.
• (condition) logical operator (condition)
• Requiring the parentheses around the
conditions is syntax requirement of Visual
Logic. If you don’t do this you will receive an
error or an incorrect result.
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Compound Conditions
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Expressions Containing the And Logical
Operator
3 > 2 And 6 > 5
• 3 > 2 is evaluated
first, giving true
• 6 > 5 is evaluated
second, giving true
• true And true is
evaluated last, giving
true
•
•
•
•
10 < 25 And 6 > 5 + 1
5 + 1 is evaluated first, giving 6
10 < 25 is evaluated second,
giving true
6 > 6 is evaluated third, giving
false
true And false is evaluated last,
giving false
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Expression Containing the Or Logical
Operator
•
•
•
•
8 = 4 * 2 Or 7 < 5
4 * 2 is evaluated first, giving 8
8 = 8 is evaluated second, giving true
7 < 5 is evaluated third, giving false
true Or false is evaluated last, giving true
All expressions containing a relational operator will result
in either a true or false answer only.
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Example of Logical Operators used in
the condition
• To pass a course, a student must have an
average test score of at least 75 and an
average project score of at least 35. Write
the compound condition using the
variables AvgTest and AvgProject.
AvgTest >= 75 And AvgProject >= 35
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Example of Logical Operators used in
the condition
• Only employees with job codes of 34 and
67 will receive a raise. Write the compound
condition using the variable Code.
Code = 34 Or Code = 67
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Nested If Example 1
Show a selection structure that assigns a sales
tax rate to the Tax variable. The tax rate is
determined by the state code stored in the
Code variable. Codes of 1 and 3 represent a
4% rate; a code of 2 represents a 5% rate. All
other codes represent a 2% rate.
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Nested If Example 1
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Select Case Form of the Selection
Structure
• Referred to as the extended selection
structure
• Easier than the nested If to write and
understand
• Typically used when a selection structure has
several paths from which to choose one
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Select Case Flowchart
Select
Expression
A
B
DoA
DoB
…
N
DoN
Each case can contain multiple instructions.
The main sequence continues down the middle.
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Select Case Example 1
Write a selection structure that assigns a sales
tax rate to the Tax variable. The tax rate is
determined by the state code stored in the
Code variable. Codes of 1 and 3 represent a
4% rate; a code of 2 represents a 5% rate. All
other codes represent a 2% rate.
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Select Case Example 1
Select Case
Code
1, 3
Tax = .04
2
Tax = .05
else
Tax = .02
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Write a program that inputs a number between 1 and
10 and displays the number with the appropriate twoletter ending (e.g., 1st, 2nd, 3rd, 4th, 5th, ...).
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