An if-else
statement in C/C++ is needed whenever the programmer needs to make a decision. This
decision is fed into the if-else statement and the flow of control is
regulated depending on the result of the decision. All decisions in programming
language result in only two possibilities, either the condition is true
or it is false. If-else statement deals with this scenario.
The if-else statement in C/C++
An if statement
tests a particular condition; if the condition is true, a
course-of-action is followed, i.e. a particular statement or set of statements
is executed. Otherwise (if the condition evaluates to false), the
course-of-action is ignored.
General syntax for an if
statement is :
if(expression)
statement;
where a statement may
consist of a single statement, a compound statement, or even an empty (or NULL)
statement. The expression must be enclosed within parenthesis(). If the
expression evaluates to true, the statement is executed, otherwise ignored. For instance,
The code fragment
if(ch== ‘ ’)
spaces ++;
checks whether variable
ch stores
a space or not. If it does, the value of spaces is incremented by 1.
Let’s look at one example :
int
A,B,C;
cin>>A>>B>>C;
if(A
> 10 && B < 15)
{
C= A+B;
cout<< “The result is ”<<C;
}
The above code uses a
block of statements. Also, look at the expression used. Here two relational
conditions are joined by logical AND (&&).
The expression part is an important aspect of if-else statement. We know all conditions result in either true or false. The compiler takes 0 (zero) as false and 1 (or any other integer other than 0) as true. So now our expression may take the following forms :
1. if(n)
here n can be any
number entered by the user or simply a constant. If n is not zero, the expression is taken as true.
For example ,
if(23)
statement;
This expression is true
and statement
will be executed.
2. if(n>10)
Here, expression is a
single relational expression which is true if n is greater than 10 and
false if n is less than or equal to 10.
3. if(n>10
&& n<20)
Here, expression is a
combination of two relational expressions joined by logical AND. So, if n is greater
than 10 and it is less than 20 simultaneously, then only the expression will be
true, otherwise it will be false.
4. if(n>10,
n>20, n<15)
Here, expression is
combination of relational expressions separated by comma operator. As we know
expressions separated by comma are evaluated from left to right and the value
of entire expression is the value of rightmost expression. Thus
(n>10,n>20,n<15) is actually equivalent to only (n<15). So, if n is less
than 15 then the condition as a whole will be true.
What if there is
another course of action to be followed if the expression evaluates to false?
There is another form of if that allows for this kind of either-or condition by
providing an else clause. The syntax of the if-else statement is
the following :
if
(expression)
statement 1;
else
statement 2;
If the expression evaluates to true, the statement 1 is executed, otherwise, statement 2 is executed. statement 1 and statement 2 can be single statements, block of statements or empty statements.
The above code can be represented in a flowchart as :
Now look at a program
in C++ to find greater number among the two.
Input : 10 20
Output : Greater number
is 20
Nested if
A nested if is
an if that has another if in its if’s body or in its else’s
body. The nested ifs can have 3 different forms :
Form 1 : if
nested inside if-part
if
(expression 1)
{
if(expression 2)
statement 1;
else
statement 2;
}
else
statement 3;
//rest
of program
Explanation : If expression 1 is true, the control
moves into the if block and checks expression 2. If it is true then statement 1
is executed and control moves to rest of program. If expression 2 is false,
statement 2 is executed and control moves to rest of program. If the expression
1 is false, the control doesn’t enter the if and executes statement 3 and then
moves to rest of program.
Form 2 : if
nested inside else-part
if(expression
1)
statement 1;
else
{
if(expression 2)
statement 2;
else
statement 3;
}
Form 3 : if
nested inside both if-part and else-part
if(expression
1)
{
if(expression 2)
statement 1;
else
statement 2;
}
else
{
if(expression 3)
statement 3;
else
statement 4;
}
Note:
All else statements are always optional.
Well let us take a
test.
#include<iostream>
using namespace std;
int main()
{
int n; cin>>n;
if(n % 2==0)
{
if (n
% 5==0)
cout<< “Yes”;
else
cout<< “Part yes”;
}
else
cout<< “No”;
return 0;
}
Assume inputs from your
side and comment down the output.
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.
General form :
if
(expression 1) statement 1;
else
if(expression 2) statement 2;
else
if(expression 3) statement 3;
……
else statement 4;
Although it is
technically correct to have indentation in the if-else-if ladder as
shown above. But it generally leads to overly deep indentation. For this
reason, the if-else-if ladder is generally indented like :
if(expression
1)
statement 1;
else
if (expression 2)
statement 2;
else
if (expression 3)
statement 3;
……..
else
statement 4;
Few
important points about if-else-if ladder :
1.
if
statement must be present; and it should be before all else if and else
statements.
2.
Any number
of else-ifs can be used but they should all be placed between the if and the
else statement (if present).
3.
else is not
always needed but if present, it should be only one in number and after all
else-ifs.
4.
Once the
control enters any of the statements (be it if or any of the else-ifs), it
ignores all the following statements and control comes out of if-else-if
ladder.
A common
Mistake :
While comparing for
equality, make sure to use the relational equality operator (==) and not the
assignment operator (=).
if(p=4)
is always TRUE statement. To check if
value in p is equal to 4, we need to use if(p==4).
p=4 actually assigns 4
to p and all assignments other than 0 (zero) are regarded as true. So, if(p=0) will
be always false and any other assignment to p will be always true. So, always
keep in mind to use the relational operator (==) and not assignment operator
(=).
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