Chapter 5 Decision Making

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Chapter 5 Decision Making
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Simple if Statement

• Capable of making decisions
• Relational expression evaluated
• Evaluated as true or false

if (relational expression)
statement(s)
Lesson 5.1
3
Relational Expressions

• Type of logical expression
• Produces result of true or false
• Compares values of two arithmetic expressions

left_operand relational_operator right_operand
Lesson 5.1
4
Relational Operators

• lt less than
• lt less than or equal to
• equal to
• gt greater than
• gt greater than or equal to
• ! not equal

Lesson 5.1
5
Simple If/Else Statement
if (expression) statement1a
statement1b else
statement1a statement1b

• Provides block of statements to be executed for
  either true OR false
• Braces optional if only one statement in block

Lesson 5.2
6
if Examples

• x 3
• if (x lt 2)
• cout ltlt x is smaller than 2

Simple if statements only done on true,
but when false, skips to end of if
Full if code for when true and also when false

• if (x lt 12)
• cout ltlt smaller than twelve
• else
• cout ltlt twelve or larger

7
Ternary Operator

• Three operands needed for proper usage
• Conditional ? operator
• Question mark separates relational expression
  from rest of statement
• Colon separates two operands

expression1? expression2 expression3
x (y lt z) ? y z
Lesson 5.2
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Example
if (a gt b) ans 10 else ans
25
a gt b ? (ans 10) (ans 25)
ans (a gt b) ? 10 25
Lesson 5.2
9
Nested if-else Statements
if (outer) if (inner)
else
else
Lesson 5.3
10
Three Logical Operators

• Exclamation mark !
• NOT (negation)
• unary
• Two ampersands
• AND (conjunction)
• binary
• Two vertical pipes
• OR (inclusive disjunction)
• binary

Lesson 5.4
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Logical NOT Operator

• Reverses result of relational expression
• Example ! (x y)

Evaluate relational expression, does 3 equal 7?
! ( false ) so negates to true
! ( true ) so negates to false
Lesson 5.4
12
Logical AND Operator

• Combines two relational expressions
• Combined expression true only if BOTH expressions
  are true

true
false
false
expression1 expression2
false
false true
false
false false
true true
true
Lesson 5.4
13
Logical OR Operator

• Combines two relational expressions
• Combined expression false only if BOTH
  expressions are false

true
false
true
expression1 expression2
true
false true
false false
false
true true
true
Lesson 5.4
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Values of Relational Expressions

• Result of relational expression
• False, C compiler gives zero
• True, C compiler gives one
• Value of relational expression
• Zero, result is false
• Nonzero, result is true
• any number other than zero, including negatives

Lesson 5.5
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Precedence of Operators
( ) Parentheses L to R 1
, -- Postincrement L to R 2 ,
-- Preincrement R to L 3
! Logical NOT L to R 3 ,
- Positive, negative L to R 3 , /,
Multiplication, division L to R 4
, - Addition, subtraction L to R 5 lt,
gt, gt, lt Relational operator L to R 6
, ! Relational operator L to R 7
Logical AND L to R 8
Logical OR L to R 9 , -, , /,
Compound assignment R to L 10
Assignment R to L 10
Lesson 5.5
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Example
Assume a 4, b -2, and c 0 x (a gt b
b gt c a b)
x ((a gt b) (b gt c) (a b))
Group
x ((4 gt -2) (-2 gt 0) (4 -2))
x (TRUE (FALSE FALSE))
Group
x (TRUE FALSE)
x (TRUE)
x 1
Lesson 5.5
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Example of Single Variable

• logical value
• False if value is 0
• True if value is nonzero

if (c) statement1 else statement2
Statement2 executed if c has value of zero,
statement1 is executed if c has any other value
but zero.
Lesson 5.5
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if-else-if

• Shifts program control, step by step, through
  series of statement blocks
• Control stops at relational expression that tests
  true

Lesson 5.6
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if-else-if Form
if (relational_expression_1)
statement_block_1 else if
(relational_expression_2)
statement_block_2 . .
. else if (relational_expression_n-1)
statement_block_n-1 else
statement_block_n
false
else
false
else
true
Lesson 5.6
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switch Control Structure

• Constructed like if-else-if
• Used to transfer control
• Can nest switch control structures
• Keyword switch followed by expression

switch (expression)
statement block
Lesson 5.6
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switch Statement Block Syntax
switch (expression) case constant1
statement1a
statement1b
case constant2 statement2a
statement2b
default
statements

• Keyword case only used in switch statement
• case used to form label
• case label is constant followed by colon
• Constant1, constant2, etc must be integer
  expressions
• Constant expressions must be unique
• default optional, used when no match is found

Lesson 5.6
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break Statement

• Terminates execution of switch statement
• Sends control to point of closing brace
• Usually last statement for each case
• If no break, then statements in next case executed

switch (expression) case constant1
statement1a
break case
constant2 statement2a
break
default statements

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bool Data Type

• Named after mathematician George Boole
• Can contain one of only two values (0 or 1)
• true or false, fail or pass, off or on
• Declare by using keyword bool
• Example bool salty, hard, acidic
• Assign value
• Example acidic (pH lt 7)

Lesson 5.7
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bool Variables

• Assign keywords true or false
• good_taste true
• Assign bools to int variables
• int i
• i good_taste
• Print bool variables values using cout
• Prints 1 for true or 0 for false
• boolalpha manipulator gives words true or false

Lesson 5.7
25
Summary
Learned how to

• Create simple if and if-else statements
• Create relational expressions
• Utilize logical operators
• Interpret the order of precedence for all
  operators (arithmetic, relational, and logical)
• Create multiple decision statements (if-else-if
  or switch statements)
• Utilize bool data type