Numbers In Java : 10 Practice Problems and Solutions
This is the solution for the practice question set on different numbers (like Harshad, Armstrong, Perfect, and so on) and their properties as well as the method for checking given numbers. You can find the corresponding list of questions here. Try solving the questions yourself before you go through the solutions.
Palindrome Number
Problem: Write a program that takes an integer and checks if it’s a palindrome (the number reads the same forwards and backwards).
Example: `121` is a palindrome, but `123` is not.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,copyn,num=0;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//storing a copy of n
copyn=n;
//reversing n
while(n>0)
{
num = num * 10 + (n % 10);
n = n / 10;
}
//comparing original n and reversed n
if(copyn==num)
System.out.println("Palindrome Number!");
else
System.out.println("Not a Palindrome Number!");
}
}
Perfect Number
Problem: A perfect number is one where the sum of its proper divisors (excluding the number itself) equals the number. Write a program to check if a given number is perfect.
Example: `6` (1 + 2 + 3 = 6) is a perfect number, but `10` is not.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,i,sum=0;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//finding sum of the divisors of n
for(i=1;i<n;i++)
{
if(n%i==0)
sum+=i;
}
//comparing n and sum of divisors
if(n==sum)
System.out.println("Perfect Number!");
else
System.out.println("Not a Perfect Number!");
}
}
Armstrong Number
Problem: An Armstrong number (or Narcissistic number) for a 3-digit number is one where the sum of each digit raised to the power of 3 equals the number. Extend this definition to check if any `n`-digit number is an Armstrong number.
Example: `153` is an Armstrong number because 1³ + 5³ + 3³ = 153.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,copyn,sum=0;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//storing a copy of n
copyn=n;
//finding the required sum
while(n>0)
{
sum += (int)Math.pow(n%10,3);
n = n / 10;
}
//comparing original n and sum of each digit raised to the power of 3
if(copyn == sum)
System.out.println("Armstrong Number!");
else
System.out.println("Not an Armstrong Number!");
}
}
Amicable Number
Problem: Two numbers are amicable if the sum of the proper divisors of one equals the other, and vice versa. Write a program to check if two numbers are amicable.
Example: `220` and `284` are amicable because the sum of divisors of `220` is `284`, and vice versa.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n1,n2,i,sum1=0,sum2=0;
//taking input
System.out.print("Enter the first number : ");
n1=sc.nextInt();
System.out.print("Enter the second number : ");
n2=sc.nextInt();
//finding the sum of divisors for n1
for(i=1;i<n1;i++)
{
if(n1%i==0)
sum1+=i;
}
//finding the sum of divisors for n2
for(i=1;i<n2;i++)
{
if(n2%i==0)
sum2+=i;
}
//comparing the sum of divisors with the numbers
if(sum1 == n2 && sum2 == n1)
System.out.println("Amicable Numbers!");
else
System.out.println("Not Amicable Numbers!");
}
}
Automorphic Number
Problem: An automorphic number is one whose square ends in the number itself. Write a program to check if a given number is automorphic.
Example: `5` is automorphic because 5² = 25, and `76` is also automorphic because 76² = 5776.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,copyn,digits=0,sqN,powerof10,lastOfSq;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//storing a copy of n
copyn=n;
//finding the number of digits
while(copyn>0)
{
digits++;
copyn = copyn / 10;
}
//finding last digits of square of n
sqN=(int)Math.pow(n,2);
powerof10=(int)Math.pow(10,digits);
lastOfSq=sqN%powerof10;
//comparing n and last digits of square of n
if(n == lastOfSq)
System.out.println("Automorphic Number!");
else
System.out.println("Not an Automorphic Number!");
}
}
Harshad Number
Problem: A Harshad (or Niven) number is divisible by the sum of its digits. Write a program to check if a given number is a Harshad number.
Example: `18` is a Harshad number because 1 + 8 = 9, and `18` is divisible by `9`.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,sum=0;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//finding the sum of digits
while(n>0)
{
sum += (n % 10);
n = n / 10;
}
//is the sum of digits equal to 9?
if(sum == 9)
System.out.println("Harshad Number!");
else
System.out.println("Not a Harshad Number!");
}
}
Happy Number
Problem: A number is called "happy" if repeatedly summing the squares of its digits eventually leads to `1`. Write a program to check if a given number is a happy number.
Example: `19` is a happy number because 1² + 9² = 82, 8² + 2² = 68, 6² + 8² = 100, and 1² + 0² + 0² = 1.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,sum=0;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//while the sum of the square digits is not a single digit
while(n>9)
{
//resetting sum
sum = 0;
//finding the sum of the square of digits
while(n>0)
{
int num = (int)Math.pow(n % 10, 2);
sum += num;
n = n / 10;
}
//new number is the sum of the square of digits of previous number
n = sum;
}
//is the final sum of digits equal to 1?
if(n == 1)
System.out.println("Happy Number!");
else
System.out.println("Not a Happy Number!");
}
}
Kaprekar Number
Problem: A Kaprekar number is a number whose square can be split into two parts that add up to the original number. Write a program to check if a given number is a Kaprekar number.
Example: `45` is a Kaprekar number because 45² = 2025, and `20 + 25 = 45`.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,copyn,digits=0,sqN,powerof10,sumOfSq;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//storing a copy of n
copyn=n;
//finding the number of digits
while(copyn>0)
{
digits++;
copyn = copyn / 10;
}
//finding sum of parts of square of n
sqN=(int)Math.pow(n,2);
powerof10=(int)Math.pow(10,digits);
sumOfSq=(sqN/powerof10)+(sqN%powerof10);
//comparing n and sum of parts of square of n
if(n == sumOfSq)
System.out.println("Kaprekar Number!");
else
System.out.println("Not a Kaprekar Number!");
}
}
Fascinating Number
Problem: A number `n` is considered "fascinating" if the concatenation of `n`, `2*n`, and `3*n` contains all digits from 1 to 9 exactly once. Write a program to check if a number is fascinating.
Example: `192` is fascinating because 192 x 1 = 192, 192 x 2 = 384, and 192 x 3 = 576, and concatenating `192384576` contains all digits from `1` to `9`.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,copyn,powerof10,rem,newNumber,i,digits=0,count=0;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//storing a copy of n
copyn=n;
//finding number of digits in n
while(copyn>0)
{
digits++;
copyn = copyn / 10;
}
//finding power of 10 for new number
powerof10 = (int)Math.pow(10,digits);
//finding the new number
// eg. 192 -> 192000000 + 384000 + 576
newNumber = (n * powerof10 * powerof10) + ((n*2) * powerof10) + (n*3);
//checking for all digits
while(newNumber>0)
{
//finding the last digit
rem = newNumber % 10;
//comparing to all digits for an occurrence
for(i=1;i<=9;i++)
{
if(rem==i)
{
count++;
}
}
newNumber /= 10;
}
//count should be exactly 9 for one occurrence of each digit
if(count == 9)
System.out.println("Fascinating Number!");
else
System.out.println("Not a Fascinating Number!");
}
}
Pronic Number
Problem: A pronic (or rectangular) number is the product of two consecutive integers. Write a program to check if a given number is pronic.
Example: `12` is a pronic number because it can be expressed as 3 x 4 = 12.
CODE :
import java.io.*;
import java.util.*;
class Main {
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
//declaring all variables
int n,i,count=0;
//taking input
System.out.print("Enter the number : ");
n=sc.nextInt();
//performing calculations
for(i=1;i<n;i++)
{
//checking for divisor
if(n%i==0)
{
//checking product of consecutive numbers
if(i * (i+1) == n)
{
count++;
break;
}
}
}
//at least one pair of consecutive numbers are divisors of n
if(count >= 1)
System.out.println("Pronic Number!");
else
System.out.println("Not a Pronic Number!");
}
}
For While Loops: 10 Practice Problems an...
This is the solution for the practice question set on the concept of iteration statements (for and while loops).
Read More
Switch Case Simplified: 5 Solved Practic...
This is the solution for the practice question set on the concepts of switch case and fall through.
Read More
Mastering If-Else Statements: 8 Solved P...
This is the solution for the practice question set on conditional (if else) statements.
Read More