Loops

Loops & Its Logic
 What is a Loop?
 An iterative statement is called loop. A type of control structure that
repeats a statement or set of statements is known as looping
structures. It is also known as iterative or repetitive structure. There
are two elements of the loop: the body of loop which is to be executed
number of times and a loop condition which terminates the loop when a
particular condition is met.
Repetition
A single statement
A set of statements
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Types of Loop
 Types of Loop
 Counter loop (for loop)
 Conditional loops (while, do while)
Types of Loop
For Loop While Loop Do While Loop
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For Loop Operation
Initialization Expression
False
Test Exit
Expression
True
Body of Loop
Increment Expression
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For Loop Structure
Test Expression
Initialization Expression Increment Expression
for ( j=0 ; j < 5 ; j++ )
statement;
Single Statement Loop Body
keyword
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For Loop Structure
Initialization Expression
Test Expression
for ( j=0 ; j < 5 ; j++ )
{ statement;
statement;
statement;
}
Increment Expression
Multiple Statement Loop Body
keyword
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For Loop
// demonstrate simple for loop
# include <iostream>
#include<conio.h>
void main()
{
int j;
for (j=0 ; j < 5 ; j++)
cout<< j * j;
getch();
}
Output: 0 1 4 9 16
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For Loop Variations
 Multiple initialization and increment Expressions
for (x=0 , j=0 ; j < 5 ; j++ , x++)
controlling
 Increment or Decrement
for ( j=5 ; j > 0 ; j-- )
for ( j=0 ; j <10 ; j+=2 )
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The Infinite loop
 A loop that does not terminate is called
Infinite loop.
A loop who's condition always remain true is
called infinite loop.
e.g.
for(int i=0;i>0;i++)
Cout<<i;
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While Loop
while (test expression )
statement;
Multiple Statement Loop Body
while ( test expression )
{
Statement;
Statement;
Statement;
}
False
Test Exit
Expression
True
Body of Loop
While Operation
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The body of the loop is executed until the condition becomes false. Here is a
program which illustrates the working of the while loop.
void main ()
{
int c=0;
int i=0;
char str=‘y’;
while(str==’y’)
{
cout<<”Enter the number you want to add”<<endl;
Cin>>i;
c=c+1;
cout<<”Do you want to enter another number y/n:”<<endl;
Cin>>str;
}
Cout<<“The sum of the numbers are : <<c;
getch();
}
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 // program that inputs a number from the user and displays a
table of that number using while loop.
 void main()
 {
 int n,c;
 C=1;
 Cout<<“Enter a number”;
 Cin>>n;
 while(c<=10)
 {
 Cout<<n<<“ * ”<<c” = ”<<n*c<<endl;
 C=c+1;
 }
 Getch();
 }
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Do While Loop
Do
while (test expression ); Body of Loop
Do
statement;
MMuullttiippllee Multiple SSttaatteemmeenntt Statement LLoooopp Loop BBooddyy
Body
{
Statement;
Statement;
Statement;
}
while ( test expression );
False
Test Exit
Expression
True
Do While Operation
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The body of the loop is executed until the condition becomes false. Here is a
program which illustrates the working of the do while loop.
void main ()
{
int c=0;
int i=0;
char str=‘y’;
do
{
cout<<”Enter the number you want to add”<<endl;
Cin>>i;
c=c+1;
cout<<”Do you want to enter another number y/n:”<<endl;
Cin>>str;
} while(str==’y’);
Cout<<”The sum of the numbers are : <<c;
getch();
}
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//Program that gets starting and ending point from the user and
displays all odd numbers in the given range using do-while loop
void main()
{
int c,s,e;
Cout<<“Enter Starting Number: ”;
Cin>>s;
Cout<<“Enter Ending Number: ”;
Cin>>e;
C=s;
do
{
If(c%2!=0)
Cout<<c<<endl;
C=c+1;
} while(c<=e);
}
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When to use Which Loop?
 Advance knowledge
 For Loop
 No Prior Knowledge
 While Loop
 Do While (executes at least once)
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Break & Continue Statement
 To exit a loop you can use the
break statement at any time.
This can be very useful if you
want to stop running a loop
because a condition has been
met other than the loop end
condition.
 With “continue;” it is possible
to skip the rest of the
commands in the current loop
and start from the top again.
(the loop variable must still be
incremented).
EXAMPLE
int main()
{
int j, total;
for (int i=0; i<20;i++)
{
cin>>j;
if ( j == 10)
continue;
total = i + j;
cout<<“total is=“<<total;
}
return 0;
}
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Continue statement
 With “continue;” it is possible to skip the rest of the commands in
the current loop and start from the top again. (the loop variable
must still be incremented).
EXAMPLE
int main()
{
int i;
i = 0;
while ( i < 20 )
{
i++;
continue;
printf(“hey wait! you are skipping men”);
}
return 0;
}
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Nested Loop
 A loop within an other loop
is called nested loop. e.g.
 for(int i=1;i<=5;i++)
 {
 for(int j=1;j<=i;j++)
 {
 Cout<<j;
 }
 Cout<<“n”;
}
Output
1 12
123
1234
12345
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Nested Loop
 for(int i=5;i>=1;i--)
 {
 for(int j=i;j>=1;j--)
 {
 Cout<<j;
 }
 Cout<<“n”;
}
Output
54321
4321
321
21
1
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Nested Loop
 for(int i=5;i>=1;i--)
 {
 {
 Cout<<j;
 }
 Cout<<“n”;
}
Output
12345
1234
123
12
1
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Nested Loop
 for(int i=5;i>=1;i--)
 {
 {
 Cout<<“*”;
 }
 Cout<<“n”;
}
Output
*****
****
***
**
*
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Nested Loop
 int i=5;
 while(i>=1)
 {
 int j=1;
 while(j<=i)
 {
 Cout<<j;
 j++;
 }
 i--;
 Cout<<“n”;
}
Output
12345
1234
123
12
1
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What makes a bad program?
 Repeating trial and error without
understanding the problem
 Writing Code without detailed
analysis and design (Abstraction,
Algo)
 Writing tricky and dirty programs

Programms:
 Write a program using for, while and do while loops to:
 Display numbers from 0 to 100
 Display numbers from 100 to 0
 Display even numbers from 0 to 100
 Display odd numbers from 0 to 100
 Display even numbers from 100 to 0
 Display odd numbers from 100 to 0
 Display Square of numbers from 0 to 100
 Display Cube of numbers from 0 to 100
 Display Average of even numbers from 1000 to 1200
 Display Average of odd numbers from 1200 to 1000
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Programs:
 Write a program using for, while and do while and nested loops to display the
following outputs. You are also required to submit the dry runs of all these
programs on paper.
12345678910
123456789
12345678
1234567
123456
12345
1234
123
12
1
===========
****************
********
****
**
*
===========
* ***
*****
*******
*******
*****
***
*
1 12
123
1234
12345
123456
1234567
12345678
123456789
12345678910
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Programs:
 Write a program using for, while and do while and nested loops to display the
following outputs. You are also required to submit the dry runs of all these
programs on paper.
13579
13579
1357
1357
135
135
13
1
*
***
*****
*******
*****
***
*
& &&
&&&
&&&&
&&&&&
&&&&&
&&&&
&&&
&&
&
0 02
024
0246
02468
0246810
024681012
02468101214
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Loops

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Loops