Control statements

FLOW OF CONTROL

The flow of control jumps from one part of the
program to another,depending on calculations
performed in the program.

Program statements that cause such jumps are
called control statements. There are two major
categories: loops and decisions.

FLOW OF CONTROL

STATEMENTS

Compound Simple

Selection/Decision Iteration/Loop

FL else Switch care For While Do-While
Jump statements

Break Continue Go to Return Exit

 Statements are the instructions given to the computer to perform
any kind of action , be it data movements, be it making decisions or
be it repeating actions.
 Statements are the smallest executing unit of a C++ program.
COMPOUND STATEMENT (BLOCK )
A compound statement in C++ is a sequence of statements enclosed by
a pair of branches { }.

{
statements 1 ;
statements 2 ;
:
}
represents a compound statement

Sequence = The sequence construct means the statements are
being executed sequentially. This represents the default flow of
statement.
STATEMENT 1

STATEMENT 2

STATEMENT 3
The sequence construct

Selection = The selection construct means the execution of
statements depending upon a condition evaluates true, a set of
statements is followed.

A set of statements
true
Condition Statement 1 Statement 2
?

Statement 1
Another
set of
statement Statement 2

The selection construction

if-else Statement Syntax

• 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

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;
}

Nested Statements

• 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!

The switch Statement
 A new statement for controlling multiple branches
 Uses controlling expression which returns bool data type (true or false)

The switch: multiple case
labels

Execution "falls through" 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"

Switch Pitfalls/Tip

Forgetting the break;
No compiler error
Execution simply "falls through" other cases until break;
Biggest use: MENUs
Provides clearer "big-picture" view
Shows menu structure effectively
Each branch is one menu choice

LOOP

Loops cause a section of your program to be repeated
a certain number of times. The repetition continues while
a condition is true. When the condition becomes false,
the loop ends and control passes to the statements
following the loop.

Iteration = The iteration construct means repetition of a set
of statements depending upon a condition - test. Till the
time a condition true { or false depending upon the loop },
a set-of-statements are repeated again and again. As soon
as the condition becomes false { or true },

False
Condition
?
The exit condition

True

Statement 1

The loop body
Statement 2

The interaction construct

3 Types of loops in C++
 for
Natural "counting" loop
 do-while
Least flexible
Always executes loop body
at least on
 while
Most flexible
No “restrictions”

For Loop Syntax

for (Init_Action; Bool_Exp; Update_Action)
Body_Statement

Like if-else, Body_Statement can be
a block statement

For Loop Example

for (count=0;count<3;count++)
{
cout << "Hi "; // Loop Body
}

How many times does loop body execute?
Initialization, loop condition and update all
"built into" the for-loop structure!
A natural "counting" loop

While Loop Example

Consider:
count = 0; // Initialization
while (count < 3) // Loop Condition
{
count++; // Update expression
}
Loop body executes how many times?

Do while Loop Example
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!

Loop Issues
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
}

Loop Pitfalls: Misplaced ;

Watch the misplaced ; (semicolon)
Example:
while (response != 0) ;
{
cout << "Enter val: ";
cin >> response;
}
Notice the ";" after the while condition!
Result here: INFINITE LOOP!

Loop Pitfalls: Infinite Loops

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!

The break and continue Statement

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!

Nested Loops

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

Control statements

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Control statements