Amrit Kaur Looping Constructs in C++

1

Chapter 3: Loop Constructs

A loop construct allow set of statements of a program to be executed certain number of

times. The statement will continue to execute until loop condition remains true. When the

condition becomes false, loop ends and control is passes to the statement following loop.

3.1 The while Loop

While loop continues until the evaluating condition becomes false. The condition

should be a logical expression.

Syntax initial value; while(logical_expression) { statement(s); increment; }

Example 1: A program to print even numbers beween 0 to n where value of n is specified by user. #include<iostream.h> #include<conio.h> void main() { int n, i; clrscr(); cout<<"\n *** Program to print EVEN series upto limit ***"; cout<<"\nEnter Limit "; cin>>n; cout<<"\nEven Number between 0 to "<< n <<endl; for(i=0;i<=n;i=i+2) //here i is loop variable { cout<<i<<" "; } }

3.2 The do...while Loop

The do..while loop is similar to while loop as both executes until the logical expression

becomes false.

Syntax initial value; do { statement(s); increment;

Amrit Kaur Looping Constructs in C++

2

} while(logical_expression);

Example 2 : A program to print even numbers beween 0 to n where value of n is specified by user.

#include<iostream.h> #include<conio.h> void main() { int n, i;

clrscr(); cout<<"\n *** Program to print EVEN series upto limit ***"; cout<<"\nEnter Limit "; cin>>n; cout<<"\nEven Number between 0 to "<< n <<endl; //initalise loop variable i i=0; do //loop execute atleast onec { cout<<i<<" "; i=i+2; // increment } while(i<=n); // loop execute second time if condition is true

}

3.3 Difference between while and do...while loop

1. In while loop, logical expression is evaluated first. If it evaluates to true the body

of while loop will execute otherwise not. On the other hand, in do...while loop,

the body of loop executes first and then the logical expression is evaluated. If

the expression is true, next time the

loop will execute.

2. In while loop, the body will execute

only when the logical expression is

true. On the other hand, in

do...while loop body of loop will

execute at least one time

irrespective of logical expression.

Amrit Kaur Looping Constructs in C++

3

3.4 The for Loop

The for loop is used to repeat the statement or set of statements known or specified

number of times. The for statement consists of the keyword for followed by

parenthesis containing three expressions, each

separated by semicolon. The expressions are

i. Initialisation expression: Used to initialise the

loop variable. It is executed only once.

ii. Test expression / condition: Used to evaluate the

loop condition. It is executed each time control

passes to beginning of loop. The loop will

execute only if condition is true.

iii. Increment or decrement expression: It is always

executed when control return to beginning of the

loop.

Syntax

for(initialisation; test_expression; increment) {

statement(s); }

Example 3: A program to print even numbers beween 0 to n where value of n is specified by user. #include<iostream.h> #include<conio.h> void main() { int n, i; clrscr(); cout<<"\n *** Program to print EVEN series upto limit ***"; cout<<"\nEnter Limit "; cin>>n; cout<<"\nEven Number between 0 to "<< n <<endl; for(i=0;i<=n;i=i+2) //here i is loop variable { cout<<i<<" ";

Amrit Kaur Looping Constructs in C++

4

} }

NOTE:

You can omit the Initialization Expression, but then the looping variable is

initialized before the for loop

You can omit the test_expression / condition, but then the condition is specified

inside the body of the for loop, generally using an if statement.

You can omit the increment, but then the increment expression is written inside

the body of the for loop.

You can omit all three, but then the loop is called Infinite loop

3.5 The break Statements

The break statement is used to exit the loop before the loop condition is re evaluated.

Example 4: A program to print even numbers between 0 to n where value of n is specified by

user and program terminates when i is greater than 10.

#include<iostream.h> #include<conio.h> void main() { int n, i; clrscr(); cout<<"\n *** Program to print EVEN series upto limit ***"; cout<<"\nEnter Limit "; cin>>n; cout<<"\nEven Number between 0 to "<< n <<endl; //initalise loop variable i i=0; do //loop execute atleast onec { cout<<i<<" "; i=i+2; // increment if (i==10) { break; } } while(i<=n); // loop execute second time if condition is true }

3.6 The continue statement

Amrit Kaur Looping Constructs in C++

5

The continue statements is used to skip all the subsequent / remaining instructions

and take the control back to the loop (next increment/decrement value).

Example 4: A program to print even numbers between 0 to n where value of n is specified by

user and increments value of n by 10.

#include<iostream.h> #include<conio.h> void main() { int n, i; clrscr(); cout<<"\n *** Program to print EVEN series upto limit ***"; cout<<"\nEnter Limit "; cin>>n; cout<<"\nEven Number between 0 to "<< n <<endl; //initalise loop variable i i=0; do //loop execute atleast onec {

cout<<i<<"\n"; i=i+2; // increment if(i>20) { n=n+10; cout<<"\nProgram continue to print even nos upto limit n="<<n<<endl; continue; } } while(i<=n); // loop execute second time if condition is true }