CHAPTER 3: Using C++ constructs : Selection control statements and looping statements
Control Statements in C++
Objectives:
• to analyze
syntaxes of various programming constructs available in C++.
• to draw
comparison between various programming constructs.
• to apply the
syntax of various programming constructs in problem solving
Categories
of available constructs
After exploring
into various types of flow of control / logic in different programming
situations let us go through the detailed syntax/format of each of the
programming constructs available in C++, using which we can monitor flow of
control in our program. Here is one diagram which categorizes C++ constructs in
detail :
Programming Constructs
Conditional Constructs :
points
to remember :
i) a logical
statement always evaluates as true / false.
ii) any value in
C++ other than zero ( positive / negative) is considered to be true whereas a
zero (0 ) is
considered to be false.
iii) If there are multiple statements to be executed in an if we must enclose in braces { }.
iv) if there
exists only one line of program statement under if( ) scope then we may ommit
curly braces { }
The statement
kept under simple if ( ) gets executed only when the conditional
expression/logical statement under it is evaluated as true.
Example :
int x = 1 , y =
3;
x += y;
if ( x > y )
{
cout<<”x
is greater”;}
In the above example the conditional statement under if ( ) will be always evaluated as true
because the
value of x will become 4 before the comparison thus the expression 4 > 3
yields a true letting the statement under if( ) to execute i.e. the output of
the above code snippet would be:
“ x is greater ”
2. if ____ else
syntax :
if ( <
conditional statement > )
{
Executable statement(s) if condition is True
}
else
{
Executable statement(s) if condition is False
}
Example :
int x = 0 , y =
3;
x += y;
if ( x > y )
{
cout<<”x
is greater”;
}
else
{
cout<<”we
are in else part because x and y both became equal.”;
}
The above code
snippet has two different paths of execution. If the conditional
statement under if() is evaluated to be true then the statement under if ( )
block will be executed otherwise the statements under else block would be
executed.
The above code produces an output as “we are in else part because x and y both became equal.” because the conditional statement under if ( ) evaluates as false as x is not greater than y, it is same as that of y.
The above code produces an output as “we are in else part because x and y both became equal.” because the conditional statement under if ( ) evaluates as false as x is not greater than y, it is same as that of y.
3. If ( ) : if – else ladder
syntax :
if (
<condition -1> )
{
// statement 1 if condition-1 is satisfied
}
else if (
<condition – 2 >)
{
// statement 2 if
condition -2 is satisfied
}
else if
(<condition – 3 >)
{
// statement 3 if condition- 3 is satisfied
}
:
: // many more
n-1 else - if ladder may come
:
else if( <
condition – n >)
{
// statement if condition – n is satisfied
}
else
{
// when none of
the above else-if( )
//conditions
gets satisfied.
}
In the above
syntax there are multiple conditions presented by each if( )
All of these conditions are mutually exclusive that is only one of them would get satisfied and all the conditions below it would not be evaluated and is discarded.
All of these conditions are mutually exclusive that is only one of them would get satisfied and all the conditions below it would not be evaluated and is discarded.
Say suppose if
condition-3 gets satisfy i.e. it yields a true value for the condition, the
statement under the block of third if( ) gets executed and all other n number
of if( ) conditions below it would be discarded.
If none of the n
if ( ) conditions gets satisfied then the last else part always gets executed.
It is not compulsory to add an else at the last of the ladder.
Example :
char ch = getch( );
if ( ch >=
'a' && ch <= 'z' )
{
cout<<”you
have inputted a lowercase alphabet”;
}
else if ( ch
>= 'A' && ch <= 'Z' )
{
cout<<”you
have inputted an uppercase alphabet”;
}
else if ( ch
> '0' && ch <= '9' )
{
cout<<”you
have inputted a digit”;
}
else
{
cout<<”you
have inputted any special character or symbol”;
}
In above code snippet a character is being inputted from user and is being checked upon by various if ( ) condition as alphabets, digit or any other special symbol. If the first condition gets satisfied then the character inputted is a lower case alphabet, if not then the second if ( ) is evaluated , if the second if( ) gets satisfied then the character is an upper case alphabet, if not then the third if ( ) is being evaluated , if it is satisfied then the character is a digit if not then finally it is inferred as any other symbol in the last else( ) .
The benefit of
this type of conditioning statement is that we can have multiple conditions
instead of just having one or two as seen in case of earlier if( ) constructs.
4. Nested if-else
We are now
considering nesting of an if ( ) construct within another if ( ) construct i.e
one if( ) is enclosed within
the scope of another if( ). The construct thus formed is called nested if( ). A
few of the syntax forms of nested if( )
:
Syntaxes:
Simple if ( ) nested within scope of another simple if ( )
if ( <outer- condition > )
{
if ( <inner-condition>
)
{
//some
statements to be executed
// on
satisfaction of inner if ( ) condition.
} // end of
scope of inner if( )
//some
statements to be executed
// on
satisfaction of outer if ( ) condition.
} // end of the
scope of outer if( )
Compound if ( ) nested within scope of another compound if ( )
if ( <outer- condition > )
{
if (
<inner-condition> )
{
//some
statements to be executed
// on
satisfaction of inner if ( ) condition.
}
else
{
// statements on
failure of inner if( )
}
//some statements
to be executed
// on
satisfaction of outer if ( ) condition.
}
else
{
// statements on
failure of outer if( )
}
Ladder if-else-if ( ) nested within scope of another ladder if-else-if ( )
if ( <outer- condition-1 > )
{
if (
<inner-condition - 1> )
{
//some
statements to be executed
// on
satisfaction of inner if ( ) condition.
}
else if (
<inner-condition – 2> )
{
// statements on
failure of inner if( )
}
else
{
// last
statement of the inner ladder if-else-if
}
//some
statements to be executed
// on
satisfaction of outer if ( ) condition.
}
else if (
<outer-condition-2 >)
{
// statements on
failure of outer if( )
}
Example:
program 2.1
// program to
compare three integer values to find highest out of them
#include<iostream.h>
void main( )
{
int n1= n2 = n3
= 0;
cout<<”Input
three integers”;
if( n1 > n2
&& n1 > n3)
cout<<n1
<< “ is the highest value”;
else if( n2
>n1 && n2 > n3)
cout<< n2
<< “ is the highest value”;
else if ( n3
> n1 && n3 > n2 )
cout<< n3
<< “is the highest value”;
else
cout<<
“all values are equal”;
}
Explanation :
Dry Run :
Case – I
let us assume
that the value of n1 = 1 , n2 = 5 and n3 = -7 evaluation of
the first if( ) in the ladder :
if( n1 > n2
&& n1 > n3 )
==> if( 1
> 5 && 1 > -7 )
==> if (
false && true )
==> if( false
) // as per truth table of logical and ( && ) operator
since the first
condition is evaluated as false so the next condition in the ladder would be
evaluated
if( n2 > n1
&& n2 > n3 )
==> if ( 5
> 1 && 5 > -7 )
==> if ( true
&& true )
==> if(true) // as per truth table of logical and ( && ) operator
since the second
logic is evaluated as true, it open the block of second if( ) and the
statements under the second
else-if ( ) block gets executed and an output is printed on the console as : “5 is highest
value”
Workout yourself
:
i) Try to
execute the above program 2.1 using dry run method with values n1 = 1 , n2 = 1
and
n3 = 1
ii) Try to
implement the above program using any other type of if( ) construct
program 2.2
// program to
find whether a 4 digit inputted year is a leap year
#include<iostream.h>
void main( )
{
int year = 0 ;
cout<<”Input
a 4 digit year”
cin>>
year;
if ( year % 4 ==
0 )
{
if ( year % 100
== 0 )
{
cout<<
“Year : “ << year <<” is a leap year”;
}
else
{
cout<<”Year
: “<< year << “ is not a leap year”;
}
else
{
cout<<“Year
: “ << year <<” is a leap year”;
}
}
else
{
cout<<”Year
: “<< year << “ is not a leap year”;
}
} // end of main
( )
The above program is a very good example showing the use of nesting of if( ).
Explanation:
Let us first
understand which year would be called leap year , a leap is a year which :
i ) is divisible by 4 but not divisible by 100
ii) is divisible
by 4 as well as divisible by 100 and at the same time divisible by 400
Any other criteria will make the year as a non-leap year candidate.
Dry Run -1
Let us parse the
gray area code of the above program with a dry run having year = 1994
if( 1994 % 4 ==
0)
==> if ( 2 ==
0 )
==> if (
false )
Since the first
if condition is not satisfied the flow of the program proceeds to its else
block and prints the output as “
Year : 1994 is not a leap year”
Dry Run – 2 :
Let us parse the
gray area code of the above program with a dry run having year = 2000
if( 2000 % 4 ==
0)
==> if ( 0 ==
0 )
==> if ( true
)
Since the first condition evaluates out to be true it opens up its block and the next statement in the inner block gets executed as :
==> if (2000 % 100 == 0)
==> if ( 0 ==
0 )
==> if ( true
)
Since the first nested if ( ) condition evaluates out to be true it opens up its block and the next statement in the inner block is continues as :
==> if( 2000 % 400 == 0)
==> if ( 0 ==
0 )
==> if ( true
)
Since the second nested if ( ) condition evaluates out to be true it opens up its block and the next statement in the inner block is continues to print output on console as :
“Year : 2000 is a leap year”
Check your progress:
1. Find error in code below and explain.
#include<iostream.h>
void main()
{
int x = 5 ;
if( x > 5 )
{
x += 5;
int y = 8;
}
y += x;
}
2. Find the output of the code below :
void main( )
{
int NoOfGirls =
4;
int NoOfBoys =
10 ;
if ( NoOfBoys =
8 && NoOfGirls <= NoOfBoys )
cout<<”Great
achievement”;
else
cout<<”Greater
achievement”;
}
3. Find the output of the code below :
void main( )
{
int circle = 5 ,
rectangle = 0 , square = 4 , triangle = 0 ;
if( circle)
{
if( rectangle ||
square )
{
cout<<”Draw
diagram”;
}
else if( !
rectangle && ! square )
{
cout<<”Invalid
diagram”;
}
else
{
if( circle ==
rectangle || square == triangle )
{
cout<<”Canvas
Available”;
}
}
}
cout<<”Invisible
diagram”;
}
4. Find the output of code below:
void main( )
{
int x = 3 , y =
5;
if( x <= 5 );
cout<<”Hurray”;
else
cout<<”
Trapped”;
}
5. Write a program which inputs day(dd) , month(mm) and year(yyyy) and checks whether
it is a valid
date or not. [ A valid date must lie on the calendar ]
6. Dipu Jwellers gives discount on a fixed purchase total based on following criteria :
Offer Offer Months Discount % Purchase total
Winter Bonanza Oct to Feb 30 Between
3000 to 5000
Summer Bonanza Mar to June
20 Between 10000 to 12000
Monsoon Bonanza July to Sep
10 Between 2000 to
10000
If the purchase amount of a customer does not lies between the given purchase total then no discount should be given.
Write a program
to calculate the Bill amount of a purchase after giving proper discount.
5. switch-case statement
switch-case construct is a type of conditional construct which resembles the same flow of logic as that of ladder if-else-if with few exceptions.
Syntax :
switch ( <switching variable | switch expression >)
{
case
<value-1> : Statement(s) 1;
break;// this is optional
break;// this is optional
//do something
if switching variable value matches
// with that of
value-1
case
<value-2> :
break;// this is optional
//do something if switching variable value matches
//do something if switching variable value matches
// with that of
value-2
case <
value-3> :
break;// this is optional
//do something if switching variable value matches
//do something if switching variable value matches
// with that of
value-3
:
:
:
case
<value-n> :
//do something
if switching variable value matches
// with that of
value-n
default :
//do something
if switching variable value does not matches
// with any
value between value-1 to value-n
}
A switch-case
control matches a particular switching value in a switch variable or a value
generated after
evaluating an expression with several values, when it finds an exact match
between switch
variable value and case value it enters into that particular case to execute
the
statements under
that case, and then on finding a break statement it jumps out of the case
and then comes
out of the scope of switch-case without considering other cases value.
Example:
int x;
cin>>x ;
switch
(x)
{
case 1 :
cout<<
“press key 's' to start the game”;
break;
case 2 :
cout<<”press
key 'n' to navigate through game plan”;
break;
case 3 :
cout<<”press
key 'c' to change the level of the game”;
break;
case 4 :
cout<<”press
key 'f' to fast your speed”;
break;
case 5 :
cout<<”press
key 'x' to exit from game”;
break;
default :
cout<<”you
have to choose between 1 to 5” ;
}
The above code
implements a game menu design where inputted integer value in the
switching
variable x is being matched with values 1 to 5 , each value
representing
one game action.
You may observe
that the flow of the switch-case construct is same as ladder if-else-if
construct,
because it also deals with multiple paths of execution out of which a selected
path
gets executed.
If none of the conditions gets satisfied then the statements under the default
case gets
executed.
Points to
remember while using a switch-case construct in a program :
a) a break statement always ends a case, if break is not placed then all the cases
after the
current case gets executed till a break statement is met.
b) a switch-case variable can be of type integer or character only. The case
value, must always be a single integer value or a single character.
Floats and
double values of switch variable are not valid. i.e. the code below is
invalid code :
float v = 0.0
cin>> v;
switch( v)
{
case 1.1 :
cout<<"Good";
break;
…
}
c) the cases in
the switch cannot provide a range of values to be matched with the
Invalid switch variable declaration
We can't write statements like >= 4 &&
<= 9 with cases. The following code
is invalid :
char ch = '*' ;
cin >> ch;
switch(ch)
{
case >='a'
&& <='z'
cout<<”You
have entered lowercase alphabet”;
break;
case >= 'A'
&& <= 'Z'
cout<<”You
have entered uppercase alphabet”;
break;
}
The above code is an example where ladder if-else-if has an advantage or switchcase,
because we can
have a logical statement having a range of values to be
compared.
d) The sequence
of case value does not matters i.e. It is not compulsory to keep a lower case
value as first
case and the highest case value being the last case. They can exist in any
order.
e) The default
case is optional and should be always kept at the last place in switch-case
construct
f) A switch-case
construct can also be nested within another switch-case operator.
Workout yourself :
i) Write a program which inputs a month number from user and then finds the total days
present in that
month. Use only switch-case construct. For example if user inputs month
number as 2 then
the program displays “February has 28 or 29 days”.
ii) Compare and contrast switch-case construct with ladder if-else-if construct in a tabular
form.
Difference between Switch case and if else:
Difference between Switch case and if else:
6. Conditional operator ( < >
? <> : <>)
Conditional
Operator is a small short hand operator in C++ which helps to implement flow of
logic based on some
condition like if-else construct. The syntax of the conditional operator is :
Syntax :
<logical expression> ? < true part > : < false part > ;
The logical expression in the operator gets evaluated either as true or false , if true then statement after symbol ( ? ) gets executed otherwise statement after symbol ( : ) gets executed. It acts much like an if ( ) - else construct but can only execute single statement. Like if-else the conditional operator cannot have a block of code to execute in its true or false part.
We often use
conditional operator to implement a short one line conditional expression.
Example:
int x = 5 , y = 7;
int result = ( x
> y ) ? 1 : 0;
cout<<
result;
The output of the above example would be 0 because the condition x > y is not being satisfied , hence the value 0 is being assigned to result. A conditional operator can be also nested within another conditional operator.
7. Nesting of all conditional
constructs
In real life programming situations often all the conditional constructs are being used in a single program. Any of the conditional construct can be nested within any of the other construct. For example a switch-case construct can nest a ladder if-else-if within its scope, or each ladder condition of a ladder if-else-if may nest a switch-case construct within its scope.
The following code snippet justifies this idea :
switch( < an expression> )
{
case
<value-1> :
if( <
condition -1 >)
{
// some code
}
else if (<
condition-2>)
{
// some code
}
else
{
// some code
}
break;
case <value-2
>
if( < condition
-1 >)
{
// some code
}
else if (<
condition-2>)
{
// some code
}
else
{
// some code
}
break;
…
} // end of
switch-case
Similarly we can
have :
if ( <condition -1 >)
{
switch(var1)
{
case
<value1> :
…
break;
case
<value2>
…
break;
case<value-3>
…
break;
}
else if (
<condition-2>)
{
switch(var1)
{
case
<value1> :
…
break;
case
<value2>
…
break;
case<value-3>
…
break;
}
}
...
Check Your
Progress :
1. Find the
output of the code given below :
#include<iostream.h>
void main( )
{
int x = 3;
switch(x)
{
case 1 :
cout<<
“One”;
break;
case 2 :
cout<<
“Two”;
break;
case 3 :
cout<<
“Three”
case 4 :
cout<<
“Four”;
case 5:
cout<<
“Five”;
break;
}
}
2. Find error in
following code :
#include<isotream.h>
void main()
{
int a = 10;
int b = 10;
int c = 20;
switch ( a ) {
case b:
cout<<”Hurray
B”;
break;
case c:
cout<<”Hurray
C”;
break;
default:
cout<<”Wrong
code”;
break;
}
}
Iterative Constructs (Looping):
Iterative Constructs (Looping):
Iterative
constructs or Loops enables a program with a cyclic flow of logic. Each
statement which is written under the scope of a looping statement gets executed
the number of times the iteration/looping continues for.
In previous
chapter we have seen few real life scenarios where a looping construct is
needed for performing a particular set of tasks repeatedly for a number of
times. Now we will go into details of all looping constructs available in C++.
In C++ there are
three basic types of looping constructs available, they are :
- while( ) loop // keyword while is used to loop
- do-while( )
loop // keywords do and while are used to loop
- for loop //
keyword for is used to loop
It is very important to understand that a looping construct enables repetition of tasks under its scope based on a particular condition called as loop-condition. This loop-condition evaluates as true or false. A loop may be continued till a loop-condition is evaluated as true or false, based on a particular program situation.
All the above mentioned three loops have three parts common in them i.e.
- the looping variable ( iterator )
- the
loop-condition
- logic to
change the value of iterator with each cycle/iteration
Iterator
loop-condition logic to modify iterator value
for( int i = 0 ;
i <= 7 ; i = i * 4 ) // some code
Similarly ,
while and do-while also have these three significant parts as shown below :
int i = 0 int i
= 0;
while ( i<= 7
) do
{ {
// some code
//some code
i = i * 4 ; i =
i *4;
} } while(i
<= 7 ) ;
1.while( )
loop construct
A while loop tests for its ending condition before performing its contents - even the first time. So if the ending condition is met when the while loop begins, the lines of instructions it contains will never be carried out.
Syntax :
while
(<loop-condition>)
{
...;
...;
}
A while continues iteration-cycle till its loop condition is evaluated as false. If the loop-condition is false for the first time iteration then loop will not execute even once.
Example :
int x = 0, sum =
0 ;
cout<<”Input
a natural number”;
cin>>x;
while(x > 0 )
{
sum = sum + x;
x - - ;
}
cout<<”The
sum is :” << sum;
The above code snippet finds the sum of n natural number using while loop. The loop is executed till the x is greater than 0 , as soon x becomes 0 the loop is terminated. We observe that within the scope of the loop the value of x is decremented so that it approaches to its next previous value. Thus with each iteration the value of x is added to a variable sum and is decremented by 1.
Lets understand the above program with a Dry Run. Let us assume that user inputs a value 4 for x then at :
1st Iteration
start
x is 4 , sum = 0 i.e. the loop-condition 4 > 0 is evaluated as true hence it opens
up the while
block
==> sum = 0 +
4 = 4
x will be
decremented by 1 , thus x = 3
2nd Iteration
start
x is 3 , sum = 4
i.e. the loop-condition 3 > 0 is evaluated as true hence it opens
up the while
block
==> sum = 4 +
3 = 7
x will be
decremented by 1 , thus x = 2
3rd Iteration start
x is 2 , sum = 7
i.e. the loop-condition 2 > 0 is evaluated as true hence it opens
up the while
block
==> sum = 7 + 2 = 9
x will be
decremented by 1 , thus x = 1
4th Iteration start
x is 1 , sum = 9
i.e. the loop-condition 1 > 0 is evaluated as true hence it opens
up the while
block
==> sum = 9 + 1 = 10
x will be
decremented by 1 , thus x = 0
5th Iteration start
x is 0 , sum =
10 i.e. the loop-condition 0 > 0 is evaluated as false because
0 == 0 hence it
locks up the while block and while() loop is terminated.
Thus coming out
of while loop block the value of sum is printed as 10
loop executes
for 4 times.
Workout yourself
:
Consider the
program given below :
void main( )
{
int x = 5, m = 1
, p = 0;
while( p <=
50 )
{
p = x * m ;
m++;
}
}
It is very
important to understand here that in while loop the looping condition is
evaluated at the beginning of the loop's scope i.e. prior to the entering into
scope of loop. This type of checking is called “Entry Control Checking”, if the
condition fails this checking it will not be allowed into the scope of the
loop. That is why while( ) loops are often called as “Entry Control Loop”.
Similarly if the
loop-condition fails at the first time itself the statements under the scope of
loop will not run
even once. Observe such a code scenario below :
noOfPersons = 4;
noOfTickets = 3
;
while(
noOfTickets >= noOfPersons )
{
cout<<”Welcome
to Gangs of C++ pure”
}
The above loop
will not execute at all because the loop-condition is failing at first time.
2. do-while( ) loop construct
A do-while loop
is identical to a while loop in every sense except that it is guaranteed to
perform the instructions inside once before testing for the ending condition.
Syntax:
do
{
// do something
;
// do something
;
} while(
<loop-condition> ) ;
Example:
Consider the following code snippet to find factorial of a given number n :
int f = 1;
int n = 0;
cin>> n;
do
{
f = f * n ;
- - n ;
} while( n >
0 ) ;
cout <<
“The factorial of : “ << n <<” is “ <<f ;
Lets us analyze
the execution of the above program : the factorial of any number n is evaluated
as fact = n * (n-1) * (n-2) * ….. * 1
This means that with each iteration the value of n is decremented by 1 and is multiplied with the previous value of n is stored cumulatively. The iteration-cycle stops when n is decremented upto a value equal to 1.
Let us conduct a Dry run on the above code snippet to understand the flow of logic for n = 4:
Initially
f =1, n = 4
1st
Iteration-cycle:
f = 1 * 4 = 4 ,
n = 3, condition n > 0 evaluates as true hence loop is continued
2nd
Iteration-cycle :
f = 4 * 3 = 12 ,
n = 2 , condition n > 0 evaluates as true hence loop is continued
3rd
Iteration-cycle :
f = 12 * 2 = 24
, n = 1 , condition n > 0 evaluates as true hence loop is continued
4th
Iteration-cycle :
f = 24 * 1 = 24
, n = 0 , condition n > 0 evaluates as false , as 0 == 0 hence loop is
terminated and
program flow comes out of the scope of the loop into program scope.
The output is printed on console as:
“The factorial of : 0 is 24”
Workout yourself :
Though the above
factorial code snippet has executed nicely and produced correct and valid
result of factorial of
4 i.e. 24 but while prompting output to user it wrongly says that “factorial of
:0 is 24” .
It must show that “factorial of 4 is 24”.
It is very important to understand here that in do-while loop the looping condition is evaluated at the end of the loop's scope i.e. after the last statement in the scope of loop. This type of checking is called “Exit Control Checking”, if the condition fails this checking it will not be allowed into the scope of the loop on next iteration-cycle. In the whole affair you are observing that the loop had executed at least once even when condition fails in 1st Iteration-cycle.
That is why do-while( ) loops are often called as “Exit Control Loop”. You can imagine a similar checking situation that your shopping bag items and purchase bill are being checked on your exit out of a shopping mall where you have been for shopping.
It must show that “factorial of 4 is 24”.
It is very important to understand here that in do-while loop the looping condition is evaluated at the end of the loop's scope i.e. after the last statement in the scope of loop. This type of checking is called “Exit Control Checking”, if the condition fails this checking it will not be allowed into the scope of the loop on next iteration-cycle. In the whole affair you are observing that the loop had executed at least once even when condition fails in 1st Iteration-cycle.
That is why do-while( ) loops are often called as “Exit Control Loop”. You can imagine a similar checking situation that your shopping bag items and purchase bill are being checked on your exit out of a shopping mall where you have been for shopping.
For example consider the program 2.3 ahead.
Often a looping
construct executes loop for a fixed number of times based on certain looping
condition placed
on the looping variable or the iterator, but sometimes the termination of loop
is not fixed i.e. the termination of loop depends on some outside input. These
types of loops where termination of
loop is not fixed and depends on outside input are called sentinels.
For example :
Consider the program given below :
program 2.3
// program to
count number of adults and minors
#include<iostream.h>
void main( )
{
int ad = 0 , m =
0;
int age = 0;
cout<<
“Input age :”;
cin>> age;
while(age >
0)
{
if( age >= 18
)
ad++ ;
else
m++;
cout<<
“Input age :”;
cin>> age;
}
cout<<
“There are total “ << ad <<” adults and” << m << “minors”;
}
Can you tell how
many times the loop in above question runs ? Your answer would be probably “No”
, or better you
will tell that since the termination of the loop depends upon the value of the
variable age, it is not fixed that how many times the loop may run.
Yes this is the feature a sentinel has , here the value of variable age controls the execution cycle of the while () loop. If the value of the age inputted by the used before the start of the loop is inputted as 0 then the loop will not be executed at all because the condition fails on the start of the first cycle itself.
Moreover it also
not fixed that after how many cycles the user may input a 0 value to stop the
loop , he/she may input after 3 cycles or 5 or may be 200 !
3 : for( ; ; ) loop
A for loop has
all of its three parts i,e the iterator variable , loop-condition and logic to
continue loop kept intact at one place separated by semi colon ( ; ) . This
loop is also an Entry Control Loop, as condition is checked before entering
into the scope of the loop. Let us analyze the syntax of a for looping:
Syntax:
for (
<variable(s) initialization> ; <looping-condition> ;
<incrementing / decrementing> )
{
// do something
…
}
The first part of a for loop is a place where all looping variables are declared and initialized in the same way we declare multiple variables of a single data type : e.g. int i = 0 , j = 2 , k = 9 … ;
This part of the loop is executed only once at the start up of the loop and never it is executed again (how can you re-declare variables with the same name! in same scope).
The second part of a for loop defines the looping condition using set of relational and logical operators and governs the number of times the loop would execute. This looping condition is checked before entering into loop, for e.g. i <= j+k etc.
The third part of the for loop defines how to change the value of iterative / looping variables with each cycle. This is very important as to execute the loop for a fixed number of times , for e.g. i++ , j = j+k etc
Let us now integrate all the above three parts to observe how a for loop works :
Example :
for(int i = 0 ;
i<=3 ; i++)
{
cout<<”Arun
Kumar\n”;
cout<<”Kamal
Kant Gupta\n”;
cout<<”Anil
Kumar\n”;
}
The above loop starts with declaring variable i = 0 , then it proceeds to the looping-condition part to check whether condition is satisfied (evaluates as true) so that the flow could enter into the scope of the loop. i.e. 0 <= 3 which is true , the control flow is allowed to enter into the scope of the loop and all the statements ( the 3 lines ) gets executed one after another. After executing the last statement within the scope of for() the control proceeds to the third part of the loop where it increments the value of looping variable i by 1 , so that the variable is getting the next consecutive higher value i.e. i = 1.
Hence the first Iteration of the for( ) loop is finished.
Now at the start of the next iteration the control flow directly proceeds to the second part of the for loop to evaluate looping-condition i.e. 1 <= 3 which is true , the flow of control then proceeds in the similar way as it did in the first iteration and this is repeated till the looping-condition becomes false i.e. when I will become equal to 4 .
Let us put all the things discussed above in the form of an iteration tracking table :
Iteration i
Looping condition Output
Starting of loop
0 0<=3 → true Arun Kumar
Kamal Kant Gupta
Anil Kumar
End of 1st
Iteration i++ → 1 1<= 3 → true Same as above
End of 2nd
Iteration i++ → 2 2<= 3 → true Same as above
End of 3rd
Iteration i++ → 3 3<=3 → true Same as above
End of 4th
Iteration i++ → 4 4<=3 → false No entry into for block ,
loop is
terminated
Observe the
above table carefully to find that the loop has executed for 4 times, and the
last value of the iterative
variable on the termination of loop is 4.
Variations in for( ) loop :
All the three parts of a for loop are optional part i.e. they may or may not be present.
Observe the valid variation in syntax of for( ) loop :
variation-1 :
int i = 0;
for( ; i<=5 ;
i++ )
cout<<”Hello
to all”;
In the above
for( ) loop the first part has been left i.e. we have not declared and
initialized the iterator in for(
) , though it has been declared outside the scope of for( ).
variation-2 :
int i = 0 ;
for( ; ; i++)
{
if( i<=5 )
cout<<”Hello
to all”;
else
break;
}
In the above variation-2 the loop is not having first and second part defined for it, though the execution of the program remains same as version 1 since iterator has been declared and the condition for terminating the loop is implemented through a if-else construct inside the loop.
You may observe
that to terminate the loop we have used the break statement.
Whenever a break
statement is found in a particular construct 's scope it immediately comes
out of the
current scope.
Variation-3 :
int i = 0 ;
for ( ; ; )
{
if( i<=5 )
{
cout<<”Hello
to all”;
i++;
}
else
break;
}
The above
variation-3 is implemented in a similar to that of the variation-3 but it has
its
incrementation
statement defined within if( ) construct.
In all of the
above variations you might have observed that it is compulsory to put a
semicolon ( ;)
within for( )
even when statements before and after it are absent.
While loop vs do while loop
.
While loop vs do while loop
.
Nested Loops
Students in
section 2.1.7 we have seen that any conditional construct can be nested within
any otherconditional
construct. Similarly any looping construct can also be nested within any other
looping construct and
conditional constructs.
Let us look at the following example showing the nesting of a for( ) loop within the scope of another for( ) loop :
for(int i = 1 ;
i<=2 ; i++) Outer for( ) loop
{
for( int j = 1 ;
j<=3 ; j++) Inner for( ) loop
{
cout<< i *
j <<endl ;
}
}
For each
iteration of the outer for loop the inner for loop will iterate fully up to the
last value of inner loop iterator.
The situation can be understood more clearly as :
1st Outer Iteration
i= 1
1st Inner
Iteration
j = 1 , output :
1 * 1 = 1
2nd Inner
Iteration
j = 2 , output :
1 * 2 = 2
3rd Inner
Iteration
j = 3 , output :
1 * 3 = 3
2nd Outer
Iteration
i= 2
1st Inner Iteration
j = 1 , output :
2 * 1 = 1
2nd Inner
Iteration
j = 2 , output :
2 * 2 = 4
3rd Inner
Iteration
j = 3 , output :
2 * 3 = 6
You can observe that j is iterated from 1 to 2 every time i is iterated once.
Jump Statements
1.The break statement
Using break we can leave a loop even if the condition for its termination is not fulfilled. It forces a loop to terminate as soon as we encounter it.
For eg:
#include <iostream.h>
Let us summarize
what we have learned till now :
i) There are two basic types of Conditional constructs : if() construct and switch-case
statement. The
if( ) has four different types of variations simple , compound ,
complex and
nested.
ii) There are three basic types of Iterative Constructs : while( ) , do-while( ) , and for( )
Out of these
while( ) and for() have similar flow of execution whereas do-while() is
bit different.
iii) for( ) and while( ) are called as Entry Control Loop , whereas do-while( ) is called as
exit-control
loop.
iv) Any of the Iterative construct could be nested within the scope of another iterative
construct. A
for( ) can be enclosed within scope of another for( ) , while( ) within a
for() , a for( )
within a do-while( ) etc.
v) while programming for real life situation we are often going to mix conditional and
iterative flow
of logic as and when required.
ADAPTED WITH CHANGES FROM CBSE STUDY MATERIAL
What was the need of putting a if case of 100 and 400 in leap year question? if it had been divisible by 4, the answer would come out.
ReplyDeleteThis is just for a double check. You may not want to count leap years before 11th century, so that's why, they provided modulus of 100 as well.
DeleteBut I think it is highly optional.
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