C++ Basics - More Flow of Control

Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
3.1
Using Boolean Expressions
Slide 3- 6
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Using Boolean Expressions

A Boolean Expression is an expression that is
either true or false

Boolean expressions are evaluated using
relational operations such as

= = , < , and >= which produce a boolean value

and boolean operations such as

&&, | |, and ! which also produce a boolean value

Type bool allows declaration of variables that
carry the value true or false
Slide 3- 7
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Boolean expressions are evaluated using values
from the Truth Tables in

For example, if y is 8, the expression
!( ( y < 3) | | ( y > 7) )
is evaluated in the following sequence
Display 3.1
! ( false | | true )
! ( true )
false
Evaluating Boolean Expressions
Slide 3- 8
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Order of Precedence

If parenthesis are omitted from boolean
expressions, the default precedence of
operations is:

Perform ! operations first

Perform relational operations such as < next

Perform && operations next

Perform | | operations last
Slide 3- 9
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Display 3.2
Precedence Rules

Items in expressions are grouped by precedence
rules for arithmetic and boolean operators

Operators with higher precedence are
performed first

Binary operators with equal precedence are
performed left to right

Unary operators of equal precedence are
performed right to left
Slide 3- 10
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Precedence Rule Example

The expression
(x+1) > 2 | | (x + 1) < -3
is equivalent to
( (x + 1) > 2) | | ( ( x + 1) < -3)

Because > and < have higher precedence than | |

and is also equivalent to
x + 1 > 2 | | x + 1 < - 3
Slide 3- 11
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Evaluating x + 1 > 2 | | x + 1 < - 3

Using the precedence rules of Display 3.2

First apply the unary –

Next apply the +'s

Now apply the > and <

Finally do the | |
Slide 3- 12
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Short-Circuit Evaluation

Some boolean expressions do not need to be
completely evaluated

if x is negative, the value of the expression
(x >= 0) && ( y > 1)
can be determined by evaluating only (x >= 0)

C++ uses short-circuit evaluation

If the value of the leftmost sub-expression
determines the final value of the expression, the rest
of the expression is not evaluated
Slide 3- 13
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Using Short-Circuit Evaluation

Short-circuit evaluation can be used to prevent
run time errors

Consider this if-statement
if ((kids != 0) && (pieces / kids >= 2) )
cout << "Each child may have two pieces!";

If the value of kids is zero, short-circuit evaluation
prevents evaluation of (pieces / 0 >= 2)

Division by zero causes a run-time error
Slide 3- 14
Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley
Type bool and Type int

C++ can use integers as if they were Boolean
values

Any non-zero number (typically 1) is true

0 (zero) is false

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