Looping (iteration )Statements while,do while and for ,sample programs using loops and programs to try

Loops are used in programming to repeat a specific block of statements until some end condition is met. 

 
Types of Loops in C

Depending upon the position of a control statement in a program, looping in C is classified into two types:

 

1. Entry controlled loop

2. Exit controlled loop

In an entry control loop in C, a condition is checked before executing the body of a loop. It is also called as a pre-checking loop.

In an exit controlled loop, a condition is checked after executing the body of a loop. It is also called as a post-checking loop.

The control conditions must be well defined and specified otherwise the loop will execute an infinite number of times. The loop that does not stop executing and processes the statements number of times is called as an infinite loop. An infinite loop is also called as an "Endless loop."

 

There are three looping statements in C programming:

while loop ( entry controlled)
do...while loop ( exit controlled)
for loop ( entry controlled)for Loop

 while loop

It is an entry controlled  looping statement
The syntax of a while loop is:
while (testExpression)
{
//codes
}

where, testExpression checks the condition is true or false before each loop.The while loop evaluates the test expression.If the test expression is true (nonzero), codes inside the body of while loop are executed. The test expression is evaluated again. The process goes on until the test expression is false.When the test expression is false, the while loop is terminated.

 

// Program to find factorial of a number 

// For a positive integer n, factorial = 1*2*3...n
#include <stdio.h>
int main()
{
int number;
long long factorial;
printf("Enter an integer: ");
scanf("%d",&number);
factorial= 1;
//loop terminates when number is less than or equal to 0
while(number > 0)
{
factorial*= number;
--number;
}
printf("Factorial=%lld", factorial);
return 0;
}

 

do...while loop

The do..while loop is similar to the while loop with one important difference. The body of do...while loop is executed once, before checking the test expression. Hence, the do...while loop is executed at least once.It is exit controlled.

Syntax: do...while loop

do
 {
// codes
}
while (testExpression);

The code block (loop body) inside the braces is executed once.Then, the test expression is evaluated. If the test expression is true, the loop body is executed again. This process goes on until the test expression is evaluated to 0 (false). When the test expression is false (nonzero), the do...while loop is terminated.

 

// Program to add numbers until user enters zero
#include <stdio.h>
int main()
{
double number, sum = 0;
//loop body is executed at least once
do
{
printf("Enter a number: ");
scanf("%lf",&number);
sum+= number;
}
while(number!= 0.0);
printf("Sum= %.2lf",sum);
return 0;
}

while	do-while
Condition is checked first then statement(s) is executed.	Statement(s) is executed atleast once, thereafter condition is checked.
It might occur statement(s) is executed zero times, If condition is false.	At least once the statement(s) is executed.
No semicolon at the end of while. while(condition)	Semicolon at the end of while. while(condition);
If there is a single statement, brackets are not required.	Brackets are always required.
Variable in condition is initialized before the execution of loop.	variable may be initialized before or within the loop.
while loop is entry controlled loop.	do-while loop is exit controlled loop.
while(condition) { statement(s); }	do { statement(s); } while(condition);

 

 for loop 

It is an entry controlled looping statement

The syntax of for loop is:
for (initializationStatement; testExpression; updateStatement)
{
// statements
}

The initialization statement is executed only once.

Then, the test expression is evaluated. If the test expression is false (0), for loop is terminated. But if the test expression is true (nonzero), statements inside the body of for loop is executed and the update expression is updated.
This process repeats until the test expression is false.
The for loop is commonly used when the number of iterations is known.

Example: for loop

// Program to calculate the sum of first n natural numbers
// Positive integers 1,2,3...n are known as natural numbers
#include <stdio.h>
int main()
{
int num, count, sum = 0;
printf("Enter a positive integer: ");
scanf("%d",&num);
// for loop terminates when n is less than count
for(count = 1; count <= num; ++count)
{
sum+= count;
}
printf("Sum= %d", sum);
return 0;
}

Summary:

Which loop to Select?

• Selection of a loop is always a tough task for a programmer, to select a loop do the following steps:
• Analyze the problem and check whether it requires a pre-test or a post-test loop.
• If pre-test is required, use a while or for a loop.
• If post-test is required, use a do-while loop.

Summary

• Looping is one of the key concepts on any programming language.
• A block of loop control statements in C are executed for number of times until the condition becomes false.
• Loops are of 2 types: entry-controlled and exit-controlled.
• 'C' programming provides us 1) while 2) do-while and 3) for loop.
• For and while loop C programming are entry-controlled loops.
• Do-while is an exit-controlled loop.

Sample programs ( all university questions)

Print factors of a number
#include <stdio.h>
main()
{
int n,i;
printf("Enter the number...\n");
scanf("%d",&n);
printf("Factors of the number\n");
for(i=1;i<=n;i++)
if(n%i==0)
printf("%d\n",i);
}
Note: an efficient way is to go up to n/2

Check whether the given number is prime or not

#include <stdio.h>
#include <math.h>
main()
{  
  int n,flag=1,i;
  printf("enter a number n >1 for primality test\n");
  scanf("%d",&n);
   for(i=2;i<=sqrt(n);i++)
     if(n%i==0)
      { flag=0;break;}
  if(flag==1)
     printf("%d is prime \n",n);
  else
     printf("%d is not prime \n",n);
}

Print Fibonacci Series up to 100

#include <stdio.h>
main() {
int a=0,b=1,c=1;
  while(c<100)

 {
    printf("%d, ", c);
    c = a+b;
    a= b;
     b=c;
    }

printf("\n");
}

Program to reverse a number
#include <stdio.h>
main()

 {
int n, rev = 0, digit;
printf("Enter an integer: ");
scanf("%d", &n);
while (n != 0) {
digit= n % 10;
rev = rev * 10 + digit;
n /= 10;
}
printf("Reversed number = %d\n", rev);
}

printing the sin series x^1/1!-x^3/3!+x^5/5!.......x^20/20!

#include <stdio.h>
main()
{  
  int i,sign=1;

  for(i=1;i<=20;i+=2)
   {
     printf("x^%d/%d!",i,i);
     sign=-sign;
     if(sign==-1)
        printf("-");
     else
        printf("+");
    }
} 

Check for perfect number ( Eg: 6,28,496 etc..)
#include <stdio.h>
#include <string.h>
int main()
{
int n,i,s=0;
printf("Enter the number\n");
scanf("%d",&n);
for(i=1;i<=n/2;i++)
{
if(n%i==0)
s=s+i;
}
if(s==n)
printf("%d is a perfect number \n",n);
else
printf("%d is a not a perfect number \n",n);
}
Write a C program to find the sum of first and last digit of a number.

 #include <stdio.h>
main()

 {
int n, fdig,ldig;
printf("Enter an integer: ");
scanf("%d", &n);
ldig=n%10;
while (n != 0) {
    fdig=n%10;
n /= 10;
}
printf("Sum of first and last digit = %d\n", fdig+ldig);
}

Write C program to convert the given decimal number into binary number.( university question)

#include <stdio.h>
main()
{
int i,n,bit,bin=0,place=1;
printf("Enter the number:");
scanf("%d",&n);
while(n!=0)
{
bit=n%2;
bin=bin+bit*place;
n=n/2;
place=place*10;
}
printf("Binary=%d\n",bin);
}

Programs to try using Loops

1)Print the even numbers between 0 and 50 (use while)
2)Print the odd numbers between 0 and 50 in the reverse order ( use while)
3)Print the series 5,10,15,….,100 (using for loop)
4)Generate the series 1, 2, 4 ,7, 11 ,16....n ( use while...read n)
5)Generate the Fibonacci series 0 1 1 2 3 5 8…..n ( use do while..read n)
6)Find the factorial of a number ( use for statement do not use built in factorial() function)
7)Print the powers of a number upto 10th power. ( Eg: if n=2 then the program should print 2^0, 2^1, 2^2,2^3,…..,2^10 use for loop)
8)Print the multiplication table of a given number. ( use while)
9)Print the numbers from 1 to 10 and their natural logarithms as a table ( use for)
10)Find the sum of the digits of a number.( use while-university question)
11)Check whether the given 3 digit number is an Armstrong number. (use do while Eg:153,370,371,407) ( uq)
12)Find the factors of a number ( use for)
13)Check whether the given number is prime or not ( use for)
14) Find the GCD or HCF of two numbers ( use Euclid algorithm and while loop)
15) Reverse a number (use while)
16)Find the numbers between 10 and 1000 that are divisible by 13 but not divisible by 3
( use for)
17)Find the sum of series 1-x^2/2+x^4/4-x^6/6........x^n/n ( use while)
18)Check whether the given number is a Krishnamurti number( Krishnamurti Number: It is a number which is equal to the sum of the factorials of all its digits.
For example : 145 = 1! + 4! + 5! = 1 + 24 + 120 = 145)
19)Read N numbers and find the biggest and smallest. ( university question)
20)Read N numbers and find the sum of all odd and even numbers separately.(university question)
21)Count the number of digits in a number.(university question)
22)Write a C program to input a list of n numbers. Calculate and display the sum of cubes of each value in the list.(university question)
23)Write a C program to count the number of zeros and negative terms in a given set of n numbers.
24)Write a C program to input a list of n numbers. calculate and display the average of numbers. Also display the cube of each value in the list.( university question)
25)read mark of n students and print the average mark.Also print the number of students who scored more than 80% mark (mark out of 50)
26)Find the sum s=1+2/3!+3/5!+4/7!... n
27)Write a C program to evaluate the series x-x^3/3!+x^5/5!-x^7/7!....n(uq)

28)Write a program to check whether a number is perfect or not.(sum of factors excluding the number equal to the number :eg 6=1+2+3)( uq)
29)Print the pattern 101010....using for loop(uq) 

30) Print the binary equivalent of a number( eg 12: 1100) ( university question)

31.Write an algorithm to check whether largest of 3 natural numbers is prime or not.(uq)

32.Write a C program to find the sum of first and last digit of a number.( uq)

33.Calculate the sum of the first n odd integers (i.e., 1 + 3 + 5 + - - - + 2n - 1). Test the program by calculating the sum of the first 100 odd integers (note that the last integer will be 199).