Selection statements

Made by:
Abhishek Kasana
Abhishek Sharma
Saurabh Aggarwal
Harsh Dabas

Flow Of Control
Flow of control is basically of three types:
 1) Sequential flow of
control
 2) Selection flow of
control
 3) Iteration flow of
control

Selection Flow Of Control
 Selection flow of
control is also known as
selective execution.
Here we may select one
of the two blocks to
execute based on a
certain condition.
Execution of one block
excludes the other .

Types Of Selection Flow Of Control
C++ provides two
Selection Statements:
 Single Branching
Statement [ If ()….else]
 Multiple Branching
Statement
[ Switch()..case]
if-else
Selection
switch

Single Branching Statement
*if()…else+
Single Branching statement is very versatile form of
selection statement. It offers following types of
selection.
 If
 If()…else
 Nested if
 Else if

The if Statement Of C++
 An if statement test a particular condition; if the condition evaluates to
true, a course-of-action is followed i.e., statement(s) following are
executed. Otherwise the course-of –action is ignored.
 Syntax:
if (expression )
{
statement(s);
}
Where if is the keyword, expression
is a booleon expression within a set
of parenthesis and statement can be
a simple or compound statement.

Some test expressions:
 (a) if(grade==‘A’)
cout<<“You Did Well”;
 (b) if(a>b)
cout<<“A has more than B has.”;
 (c) if(x)
{ cout<<“x is non-zeron”;
cout<<“Hence it results into “;
}
 (d) if((x>=2)&&(x<=10))
{
cout<<“A compound test condition resulted truen”;
}

Remember:
 A false value is 0 in C++ and a non–zero value is
considered true in C++.
 Please note that if(x) type of condition might not work
in newer compilers. Though it works in Turbo-C++, for
newer compilers like codeblocks, we change the
condition to if(x!=0) and change if(!x) into if(x==0).

Selection if( )…else …
 Also possible to make two way selection
 If the expression is
true, statement1 is
executed
 Otherwise statement2
is executed
 Syntax;
if (expression)
{
statement(s)1
}
else
{
statement(s)2
}

Always Remember:
 In an if-else statement, only the code associated with if
(i.e., statement 1) or the code associated with else (i.e.,
statement 2) executes, never both.

One or more if statements embedded within the if statement are
called nested ifs.
The following if-else statement is a nested if declaration nested to
level two:
Nested if

Nested if can have following the forms:
1)if nested inside if -
part
if(expresssion1)
{ :
if(expression2)
statement 1;
else
statement 2;
:
}
else
body-of-the-else;
2)If nested inside
else part
if (expression1)
body-of-if;
Else
{: if (expression 2)
statement-1;
else
statement-2;
:
}
3)If nested inside
both if part and else
part
If (exprssion1)
{ :
if (expression2)
statement 1;
else
statement 2;
:
}
Else
{ :
if(expression 3)
statement 3;
else statement 4;
:
}

The if else if ladder
 A common programming construct in C++ is the if-else-if ladder
,which is often also called the if-else-if staircase because of its
appearance . It takes the following general form:
if (expression1) statement 1;
else
if (expression 2) statement 2;
else
if (expression3)statement 3:
:
else statement n:
This can also be written as :
If (expression 1)
statement 2;
Else if (expression 2)
statement 2;
Else if (expression 3)
statement 3:
:
Else
statement n:

Graphical representation of if-else-if ladder:

The Dangling Else Problem
The nested if – else statement introduces a source of
potential ambiguity referred to as dangling else
problem. This problem arises when in a nested if
statement, number of ifs is more than the number of
else clause. This question then arise , with which if
does the additional else clause properly match up.

16
The Dangling else
 How to determine which if the else goes with
 Example:
if (abs (x - 7))
if (x < 7) cout << “x approaches 7 from left”;
else
cout << “x approaches 7 from the right”;
else
cout << “x not close to 7”;
Rule : An else goes with the closest unmatched if
?
?

17
The Dangling Else
 Rule : an else goes with the closest unmatched if
 Consider … how do you force an else to go with a
previous if?
if (x < y)
if (y > 3) cout << “message about y > 3”;
else cout << “message about x and y”;
if (x < y)
{ if (y > 3) cout << “message about y > 3”; }
else cout << “message about x and y”;
Use { curly brackets } to nest the
statements

This statement is used when we have to select one option out of many
alternatives.
It is a multi branch statement that makes the control to jump to one of
the several
statements based on the value of an integer variable or an expression.
The general
Form of the switch is:
switch(expression)
{
case constant 1: statement sequence 1;
break;
break;
break;
.
.
case constant n-1: statement sequence n-1;
break;
default: statement sequence;
}
Multiple Branching Statement
Switch statment

Nested Switch
Like if statement , a switch statement can also be nested .There can be a switch
as part of the statement sequence of another switch.
Example:
Switch(a)
{
Case 1 : switch(b)
{
case 0: cout<<“divide by Zero ---Error!!”;
break;
case 1: res=a/b;
}
break;
Case 2 :
:
:
}

Some important things to know about Switch
 A switch statement can work only for equality comparisons.
 No two case labels in the same switch can have identical values. But, in
case of nested switch statements the case constant of the inner and
outer switch can contain common values.
 If character constants are used in the switch statement , they are
automatically converted to their integers(i.e.. their equivalent ASCII
codes) .
 Switch Statement is more efficient than if in a situation that supports
the nature of switch operation.
 If a case statement does not include a break statement then fallthrough
occurs.
 Default statement gets executed when no match is found. The default
statement is optional and , if it is missing , no action takes place if all
matches fail.

Switch vs. If Else
S.no Switch If Else
1 The Expression is tested for equality only
.
The expression cam be tested for
inequality as well. (<,>)
2 Only one value is used to match against
all case labels.
Multiple expression can be tested for
branching.
3 Switch case is not effective when
checking for a range value.
If else is a better option to check ranges.
4 Switch case cannot handle floating point
values
If else can handle floating point values
5 The case label must be
constant(Characters of integers).
If else can use variables also for
conditions.
6 Switch statement provides a better way to
check a value against a number of
constants .
If else is not suitable for this porpose .

Conclusion
 C++ provides two kinds of selection statements: if and
switch.
 The if-else statement tests an expression and depending
upon its truth value, one of the two sets-of-action is
executed.
 The if-else can be nested also i.e., an if statement can
have another if statement .In a nested if-else statement, an
else goes to immediately preceding unmatched if .
 A switch is another selection statement in C++ that tests a
value against a set of integer or character constants.
 A switch statement can be nested also .
 An if-else is more flexible and versatile compared to
switch but switch is more efficient in a situation when the
same variable is compared against a set of values for
equality.

Selection statements

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