LECTURE # 7 :
STRUCTURED PROGRAMMING
Selection Statements
Tr.Hadeel
Content
2

Control structures

Types of selection statements

if single-selection statement

if..else double-selection statement

Nested if..else statements

Dangling else problem

switch multiple-selection statement

Common errors
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Control Structure (Logic Structure)
3

Used to design data flow in modules and program as a whole

Basic structures
1.
Sequential structure
 Processes one instruction after another
2.
Selection structures
3.

Decision structure
 Make choices by using relational or logical operators

Case structure
 Enable to pick up one of a set of tasks
Loop structure
 Enable to repeat tasks
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Selection Statements
4

Three types

Single-selection statement


Double-selection statement


Select or ignore a single group of actions
Select between two groups of actions
Multiple-selection statement

Select among many group of actions
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if single-selection Statement
5

Syntax



Single action
Multiple actions
if (condition)
action;
if (condition)
{
action1;
action2;
..
actionN;
}
If condition is true, action is performed

Otherwise, action is ignored
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if(grade >= 60)
Cout << “Passed”;
if..else double-selection Statement
6

Begin with if followed by condition; then action or group of actions
are listed

End with else then action or group of actions are listed
if (condition)
action1;
else
action2;

if(grade >= 60)
cout << “Passed”;
else
cout << “failed ”;
If condition is true, action that followed by if is performed
Otherwise, action that followed by else is performed
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Question ?
7
Which is the operator that provide the similar
result of if..else double-selection statement?
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Nested if..else Statements
8

One inside another, test for multiple cases

Once condition met, other statements skipped
if (condition1)
action1;
else
if (condition2)
action2;
else
if (condition3)
action3;
...
else
actionN;
if (condition1)
{
if (condition2)
action1;
else
{
if (condition3)
action2;
else
action3;
}
}
else
action4;
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Example
9
if ( grade >= 90 )
cout << "A";
else
if ( grade >= 80 )
cout << "B";
else
if ( grade >= 70 )
cout << "C";
else
if ( grade >= 60 )
cout << "D";
else
cout << "F";
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Dangling-else Problem
10

Each else associated with immediately preceding if

There is exception when placing braces { }
int x = 10,
Have logic error
y = 2;
if ( x > 5)
if( y > 5)
cout << "x and y are > 5"<< endl;
else
cout<<"x is <=5“;
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int x = 10,
y = 2;
Correctness
if ( x > 5)
{
if( y > 5)
cout << "x and y are > 5"<< endl;
}
else
cout<<"x is <=5";
Example:Determine the output for each of the following
when x is 9 and y is 11 and when x is 11 and y
11
a) if ( x < 10 )
b) if ( x < 10 ) {
if ( y > 10 )
if ( y > 10 )
cout << "*****" << endl;
cout << "*****" << endl;
else
}
cout << "#####" << endl;
else {
cout << "$$$$$" << endl;
cout << "#####" << endl;
ANS:
cout << "$$$$$" << endl;
}
ANS:
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Using bool Variables
12
bool
flag1 = true, flag2 = false;
if
( flag1 )
… …
else
… …
if
( flag1 || flag2
… … …
else
… … …
)
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Implicit Typecasting
13
int
if
x1 = -15, x2 = 0;
( x1 )
… …
else
… …
if
( x1 || x2
)
… … …
else
… … …
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Confusing ==
14

Confusing the equality operator == with the assignment operator =
results in logic errors
if
( x==2 )
cout<<“x is equal to 2”;
else
cout<<“x is not equal to 2”;
This message will always be printed !!!
if
( x=2 )
cout<<“x is equal to 2”;
else
cout<<“x is not equal to 2”;
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switch Multiple-selection
15
Statement

Perform actions based on possible values of
variable or expression

Begin with switch followed by controlling
expression

Value of expression compared to case labels
then execute action for that case

No matching, the execution go to the optional
default statement

if (condition1)
switch (expression)
{
case value1:
action1;
break;
case value2:
action2;
break;
...
break causes immediate exit from switch
case valueN:
actionN;
break;
default:
statement
action;
}
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Example
16
switch (number)
{
case 0: cout << "Too small, sorry!";
break;
case 5: cout << "Good job! " << endl;
case 4: cout << "Nice Pick!" << endl;
case 3: cout << "Excellent!" << endl;
case 2: cout << "Masterful!" << endl;
case 1: cout << "Incredible!" << endl;
break;
default:cout << "Too large!" << endl;
break;
}
//
//
//
//
fall
fall
fall
fall
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through
through
through
through
Printing Values of Enumerated Type
17
enum Day { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY };
Day today = FRIDAY ;
switch (today)
{
case 0: cout << “MONDAY”;
break;
case 1: cout << “TUESDAY”;
break;
case 2: cout << “WEDNESDAY”;
break;
case 3: cout << “THURSDAY”;
break;
case 4: cout << “FRIDAY”;
break;
}
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Common Compilation Errors
18

Placing semicolon (;) after if condition or else keyword

Omitting spaces between case keyword and value

Specifying expression including variables (a + b) in case label of
switch statement


Providing identical case labels
Forgetting a break statement when one is needed in a switch
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Exercise - 1
19

Write a program that asks for an integer and reports
whether the number is odd or even. Use if .. else
statement.

Write another version of program using switch
statement.
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20
End
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