chap2cpp4th.pdf

Chapter 2
Flow of Control
Copyright © 2010 Pearson Addison-
Wesley. All rights reserved

Learning Objectives
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Boolean Expressions
Building, Evaluating & Precedence Rules
Branching Mechanisms
if-else
switch
Nesting if-else
Loops
While, do-while, for
Nesting loops
2-2
Copyright © 2010 Pearson Addison-Wesley. All rights
reserved.

Boolean Expressions:
Display 2.1 Comparison Operators
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Logical Operators
Logical AND (&&)
Logical OR (||)
2-3
reserved.

Evaluating Boolean Expressions
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Data type bool
Returns true or false
true, false are predeﬁned library consts
Truth tables
Display 2.2 next slide
2-4
reserved.

Evaluating Boolean Expressions: Display 2.2
Truth Tables
2-5
reserved.

Display 2.3
Precedence of Operators (1 of 4)
2-6
reserved.

Display 2.3
2-7
reserved.

Display 2.3
2-8
reserved.

Display 2.3
2-9
reserved.

Precedence Examples
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Arithmetic before logical
x + 1 > 2 || x + 1 < -3 means:
(x + 1) > 2 || (x + 1) < -3
Short-circuit evaluation
(x >= 0) && (y > 1)
Be careful with increment operators!
(x > 1) && (y++)
Integers as boolean values
All non-zero values  true
Zero value  false
2-10
reserved.

Branching Mechanisms
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if-else statements
Choice of two alternate statements based
on condition expression
Example:
if (hrs > 40)
grossPay = rate*40 + 1.5*rate*(hrs-40);
else
grossPay = rate*hrs;
2-11
reserved.

if-else Statement Syntax
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Formal syntax:
if (<boolean_expression>)
<yes_statement>
else
<no_statement>
Note each alternative is only
ONE statement!
To have multiple statements execute in
either branch  use compound statement
2-12
reserved.

Compound/Block Statement
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Only "get" one statement per branch
Must use compound statement { }
for multiples
Also called a "block" stmt
Each block should have block statement
Even if just one statement
Enhances readability
2-13
reserved.

Compound Statement in Action
• Note indenting in this example:
if (myScore > yourScore)
{
cout << "I win!n";
wager = wager + 100;
}
else
{
cout << "I wish these were golf scores.n";
wager = 0;
}
2-14
reserved.

Common Pitfalls
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Operator "=" vs. operator "=="
One means "assignment" (=)
One means "equality" (==)
VERY different in C++!
Example:
if (x = 12) Note operator used!
Do_Something
else
Do_Something_Else
2-15
reserved.

The Optional else
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else clause is optional
If, in the false branch (else), you want "nothing" to
happen, leave it out
Example:
if (sales >= minimum)
salary = salary + bonus;
cout << "Salary = %" << salary;
Note: nothing to do for false condition, so there is no
else clause!
Execution continues with cout statement
2-16
reserved.

Nested Statements
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if-else statements contain smaller statements
Compound or simple statements (we’ve seen)
Can also contain any statement at all, including
another if-else stmt!
Example:
if (speed > 55)
if (speed > 80)
cout << "You’re really speeding!";
else
cout << "You’re speeding.";
Note proper indenting!
2-17
reserved.

Multiway if-else
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Not new, just different indenting
Avoids "excessive" indenting
Syntax:
2-18
reserved.

Multiway if-else Example
2-19
reserved.

The switch Statement
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A new stmt for controlling
multiple branches
Uses controlling expression which returns
bool data type (true or false)
Syntax:
Next slide
2-20
reserved.

switch Statement Syntax
2-21
reserved.

The switch Statement in Action
2-22
reserved.

The switch: multiple case labels
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Execution "falls thru" until break
switch provides a "point of entry"
Example:
case "A":
case "a":
cout << "Excellent: you got an "A"!n";
break;
case "B":
case "b":
cout << "Good: you got a "B"!n";
break;
Note multiple labels provide same "entry"
2-23
reserved.

switch Pitfalls/Tip
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Forgetting the break;
No compiler error
Execution simply "falls thru" other cases until
break;
Biggest use: MENUs
Provides clearer "big-picture" view
Shows menu structure effectively
Each branch is one menu choice
2-24
reserved.

switch Menu Example
• Switch stmt "perfect" for menus:
switch (response)
{
case "1":
// Execute menu option 1
break;
case "2":
break;
case 3":
break;
default:
cout << "Please enter valid response.";
}
2-25
reserved.

Conditional Operator
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Also called "ternary operator"
Allows embedded conditional in expression
Essentially "shorthand if-else" operator
Example:
if (n1 > n2)
max = n1;
else
max = n2;
Can be written:
max = (n1 > n2) ? N1 : n2;
"?" and ":" form this "ternary" operator
2-26
reserved.

Loops
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3 Types of loops in C++
while
Most ﬂexible
No "restrictions"
do-while
Least ﬂexible
Always executes loop body at least once
for
Natural "counting" loop
2-27
reserved.

while Loops Syntax
2-28
reserved.

while Loop Example
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Consider:
count = 0; // Initialization
while (count < 3) // Loop Condition
{
cout << "Hi "; // Loop Body
count++; // Update expression
}
Loop body executes how many times?
2-29
reserved.

do-while Loop Syntax
2-30
reserved.

do-while Loop Example
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count = 0; // Initialization
do
{
count++; // Update expression
} while (count < 3); // Loop Condition
Loop body executes how many times?
do-while loops always execute body at least once!
2-31
reserved.

while vs. do-while
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Very similar, but…
One important difference
Issue is "WHEN" boolean expression is checked
while: checks BEFORE body is executed
do-while: checked AFTER body is executed
After this difference, they’re
essentially identical!
while is more common, due to it’s
ultimate "ﬂexibility"
2-32
reserved.

Comma Operator
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Evaluate list of expressions, returning
value of the last expression
Most often used in a for-loop
Example:
first = (first = 2, second = first + 1);
first gets assigned the value 3
second gets assigned the value 3
No guarantee what order expressions will
be evaluated.
2-33
reserved.

for Loop Syntax
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for (Init_Action; Bool_Exp; Update_Action)
Body_Statement
Like if-else, Body_Statement can be
a block statement
Much more typical
2-34
reserved.

for Loop Example
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for (count=0;count<3;count++)
{
}
How many times does loop body execute?
Initialization, loop condition and update all
"built into" the for-loop structure!
A natural "counting" loop
2-35
reserved.

Loop Issues
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Loop’s condition expression can be ANY boolean
expression
Examples:
while (count<3 && done!=0)
{
// Do something
}
for (index=0;index<10 && entry!=-99)
{
// Do something
}
2-36
reserved.

Loop Pitfalls: Misplaced ;
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Watch the misplaced ; (semicolon)
Example:
while (response != 0) ;
{
cout << "Enter val: ";
cin >> response;
}
Notice the ";" after the while condition!
Result here: INFINITE LOOP!
2-37
reserved.

Loop Pitfalls: Infinite Loops
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Loop condition must evaluate to false at
some iteration through loop
If not  infinite loop.
Example:
while (1)
{
cout << "Hello ";
}
A perfectly legal C++ loop  always infinite!
Infinite loops can be desirable
e.g., "Embedded Systems"
2-38
reserved.

The break and continue Statements
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Flow of Control
Recall how loops provide "graceful" and clear flow of
control in and out
In RARE instances, can alter natural flow
break;
Forces loop to exit immediately.
continue;
Skips rest of loop body
These statements violate natural flow
Only used when absolutely necessary!
2-39
reserved.

Nested Loops
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Recall: ANY valid C++ statements can be
inside body of loop
This includes additional loop statements!
Called "nested loops"
Requires careful indenting:
for (outer=0; outer<5; outer++)
for (inner=7; inner>2; inner--)
cout << outer << inner;
Notice no { } since each body is one statement
Good style dictates we use { } anyway
2-40
reserved.

Summary 1
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Boolean expressions
Similar to arithmetic  results in true or false
C++ branching statements
if-else, switch
switch statement great for menus
C++ loop statements
while
do-while
for
2-41
reserved.

Summary 2
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do-while loops
Always execute their loop body at least once
for-loop
A natural "counting" loop
Loops can be exited early
break statement
continue statement
Usage restricted for style purposes
2-42
reserved.

chap2cpp4th.pdf

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