Simple Loops

9 min readDec 27, 2025

Sum of Digits

import java.util.Scanner;
public class Main {
 public static void main(String[] args) {
  Scanner scanner = new Scanner(System.in);
  int n = scanner.nextInt(); // 5  // 25
  int sum = 0;
  while (n > 0)
  {
   int lastDigit = n % 10;
   sum = sum + lastDigit;
   n = n / 10;
  }
  System.out.println(sum); // 5  // 7
 }
}

Sum of digits of all numbers from 1 to N.

import java.util.Scanner;
public class Main {
 public static void main(String[] args) {
  Scanner scanner = new Scanner(System.in);
  int n = scanner.nextInt();
  int sum = 0;
  for(i = 1; i <= n; i++)
  {
   sum = sum + i;
  }
  System.out.println(sum); 
 }
}

Sum of Prime Numbers

// Print sum of all prime numbers
// TC - O(n√n)
import java.util.Scanner;
public class Main {
 public static boolean isPrime(int n)
 {
  if(n < 2) 
  {
   return false;
  }
  for(int i = 2; i * i <= n; i++)
  {
   if(n % i == 0)
   {
    return false;
   }
  }
  return true;
 }
 public static void sumPrime(int st, int end)
 {
  long sum = 0;
  for(int i = Math.max(st, 2); i <= end; i++)
  {
   if(isPrime(i))
   {
    sum += i;
   }
  }
  System.out.println(sum);
 } 
 public static void main(String[] args) {
  Scanner in = new Scanner(System.in);
  int st = in.nextInt();
  int end = in.nextInt();
  sumPrime(st,end);
 }
}

// Sieve Summation
// TC - O(n log log n)
import java.util.*;
public static void sumPrime(int st, int end)
{
    if (end < 2 || st > end) {
        System.out.println(0);
        return;
    }

    boolean[] isPrime = new boolean[end + 1];
    Arrays.fill(isPrime, true);

    isPrime[0] = false;
    isPrime[1] = false;

    // even numbers are not prime
    for (int i = 4; i <= end; i+=2) {
        isPrime[i] = false;
    }
    // not all multipliers are prime
    for (int i = 3; i <= end; i+=2) {
        // i = 3, j = 9, 12, ... n
        // i = 5, j = 25, .... n
        // i = 7, j = 49, ... n 
        if (isPrime[i]) {
            for (int j = i * i; j <= end; j += i) {
                isPrime[j] = false;
            }
        }
    }
    long sum = 0;
    for (int i = Math.max(st, 2); i <= end; i++) {
        if (isPrime[i]) {
            sum += i;
        }
    }

    System.out.println(sum);
}

Palindrome or Not

import java.util.Scanner;
public class Main {
  public static void main(String[] args)
 {
  // Reverse the order and see if it is same as original or not 
  // 701 =? 107
  // 101 =? 101
  Scanner scanner = new Scanner(System.in);
  int N = scanner.nextInt();
  int x = N;
  int rev = 0;
  while (x > 0)
  {
   int lastDigit = x % 10;
   rev = rev * 10 + lastDigit;
   x = x / 10; 
  }
  if(N == rev)
  {
   System.out.println("Palindrome");
  }
  else
  {
   System.out.println("Not Palindrome");
  }
 }
}

Armstrong or Not

import java.util.Scanner;
public class Main{
 public static int powerFn(int a, int b)
 {
  int ans = 1;
  for(int i = 1; i <= b; i++)
  {
   ans = ans * a;
  }
  return ans;
 }
 public static void main(String[] args)
 {
  // If sum is same as original number or not
  // Step 1 - Count number of digits present - 704 = 3 -> d
  // Step 2 - (lastDigit)^d = n
  // Step 3 - sum == n ?

  // Example 
  // 7^3 + 0^3 + 4^3 =? 704
  // 1^4 + 6^4 + 3^4 + 4^4 =? 1634
  Scanner in = new Scanner(System.in);
  int n = in.nextInt();
  int copyN = n;
  int numberOfDigits = 0;
  while(n > 0)
  {
   // Calculate how many digits we have
   numberOfDigits ++;
   n = n / 10;
  }
  int sum = 0;
  copyN = n;
  while(n > 0)
  {
   int lastDigit = copyN % 10;
   sum = sum + powerFn(lastDigit, numberOfDigits); // Main logic
   copyN = copyN / 10;
  }
  if(sum == n)
  {
   System.out.println("Armstrong");
  }
  else
  {
   System.out.println("Not Armstrong");
  }
 }
}

Find Armstrong numbers between 1 and N

import java.util.Scanner;
public class Main {
    public static int powerFn(int a , int b)
    {
        int ans = 1;
        for(int i = 1; i <= b; i++)
        {
            ans = ans * a;
        }
        return ans;
    }

    public static boolean Armstrong(int N)
    {
        int x = N;
        int count = 0;
        int sum = 0;

        while(x > 0)
        {
            count++;
            x /= 10;
        }

        x = N;

        while(x > 0)
        {
            int lastDigit = x % 10;
            sum += powerFn(lastDigit, count);
            x /= 10;
        }

        return sum == N;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int st = in.nextInt();
        int end = in.nextInt();

        for(int i = st; i <= end; i++)
        {
            if(Armstrong(i))
            {
                System.out.print(i + " ");
            }
        }
    }
}

Prime Factors

// TC - O(n√n)
import java.util.Scanner;
public class Main {

    public static boolean isPrime(int n) {
        if (n < 2) return false;

        for (int i = 2; i * i <= n; i++) {
            if (n % i == 0) {
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt(); // 24

        System.out.print("Prime factors of " + n + ": "); 

        boolean first = true;
        for (int i = 1; i <= n; i++) {
            if (n % i == 0 && isPrime(i)) {
                if (!first) {
                    System.out.print(", ");
                }
                System.out.print(i);
                first = false;
            }
        }

        System.out.println(); // 2, 3
    }
}

Print all factors of a number.

import java.util.Scanner;
public class Main {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt(); 
        for (int i = 1; i <= n; i++) {
            if (n % i == 0) {
                System.out.print(i + " ");
            }
        }
    }
}

Fibonacci series up to N terms

public class Main {
    public static void main(String[] args) {

        int n = 10;

        if (n <= 0) {
            return;
        }

        int fTerm = 0;
        int sTerm = 1;

        if (n >= 1) System.out.print(fTerm);
        if (n >= 2) System.out.print(" " + sTerm);

        for (int i = 3; i <= n; i++) {
            int tTerm = fTerm + sTerm;
            System.out.print(" " + tTerm);
            fTerm = sTerm;
            sTerm = tTerm;
        }
        // 0 1 1 2 3 5 8 13 21 34
    }
}

Binary to Decimal

public class Main{
 public static void main(String[] args)
 {
  int n = 1010; // Binary
  int power = 1;
  int sum = 0;
  while(n > 0)
  {
   int lastDigit = n % 10;
   sum = sum + lastDigit * power;
   power = power * 2;
   n = n / 10;
  }
  System.out.println(sum); // 10 -> Decimal
 }
}

// Optimised
public class Main{
 public static void main(String[] args)
 {
  String binary = "1010";
  int sum = 0;
  int power = 1;
  
  for(int i = binary.length() - 1; i >= 0; i--) {
      int digit = binary.charAt(i) - '0';
      sum += digit * power;
      power *= 2;
  }
  System.out.println(sum); // 10 -> Decimal
 }
}

Decimal To Binary

public static void decimalToBinary(int n) {
    StringBuilder sb = new StringBuilder();

    while (n > 0) {
        sb.append(n % 2);   // remainder (0 or 1)
        n /= 2;
    }

    System.out.println(sb.reverse().toString());
}

// Another approach
public static void decimalToBinaryBitwise(int n) {
    StringBuilder sb = new StringBuilder();

    while (n > 0) {
        sb.append(n & 1);   // extract last bit
        n = n >> 1;         // shift right
    }

    System.out.println(sb.reverse().toString());
}

Perform input output of all basic data types

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    char ch = in.next().charAt(0);
    int n = in.nextInt();
    float f = in.nextFloat();
    
    System.out.println(ch);
    System.out.println(n);
    System.out.println(f);
  }
}

Perform all arithmetic operations

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n1 = in.nextInt();
    int n2 = in.nextInt();    
    System.out.println(n1 + n2);
    System.out.println(n1 - n2);
    System.out.println(n1 * n2);
    System.out.println(n1 / n2);
    System.out.println(n1 % n2);
  }
}

Find the area and perimeter of the rectangle.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n1 = in.nextInt();
    int n2 = in.nextInt();   
    System.out.println("Area " + (n1 * n2));
    System.out.println("Perimeter " + (2 * (n1 + n2)));
  }
}

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Enter temperature in Fahrenheit and convert to Celsius

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    double F = in.nextDouble();
    double C = ((F - 32) * 5.0) / 9.0;
    System.out.println(C);
  }
}

Calculate total average and percentage of five subjects

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int a = in.nextInt();
    int b = in.nextInt();  
    int c = in.nextInt();
    int d = in.nextInt();
    int e = in.nextInt(); 
    System.out.println("Sum " + (a + b + c + d + e));
    System.out.println("Perimeter " + ((a + b + c + d + e) / 5));
  }
}

Program to check whether a number is negative, positive or zero.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    if(n > 0)
      System.out.println("Number is positive");
    else if(n < 0)
      System.out.println("Number is negative");
    else if(n == 0)
      System.out.println("Number is zero");
    else
      System.out.println("Number is invalid");
  }
}

Program to check whether a number is divisible by 5 and 11 or not.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    if(n % 5 == 0 || n % 11 == 0)
      System.out.println("Number is divisible");
    else
      System.out.println("Number is not divisible");
  }
}

Program to check whether a number is even or odd.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    if(n % 2 == 0)
      System.out.println("Number is even");
    else
      System.out.println("Number is odd");
  }
}

Program to find sum of all even numbers between 1 to n.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    for(int i = 2; i <= n; i+2)
      System.out.println(i + " ");
  }
}

Program to find sum of all odd numbers between 1 to n.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    for(int i = 1; i <= n; i+2)
      System.out.println(i + " ");
  }
}

Program to find HCF (GCD) of two numbers.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    int n1 = in.nextInt();
    int hcf = findHCF(n, n1);
    System.out.println("HCF = " + hcf);
  }
  public static int findHCF(int a, int b)
  {
    if(b == 0)
      return a;
    return (b, a % b);
  }
}

Program to find LCM of two numbers

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    int n1 = in.nextInt();
    int lcm = findLCM(n, n1);
    System.out.println("LCM = " + hcf);
  }
  public static int findLCM(int a, int b)
  {
    if(a == 0 || b == 0)
      return 0;
    return Math.abs(a*b)/ findGCD(a, b);
  }
  public static int findGCD(int a, int b)
  {
    while(b != 0)
    {
      int temp = b;
      b = a;
      a = temp;
    }
    return a;
  }
}

Check whether a year is a leap year or not.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    if((n % 4 == 0 && n % 100 != 0)|| n % 400 == 0)
      System.out.println("Year is a leap year");
    else
      System.out.println("Year is not a leap year");
  }
}

Check whether a character is alphabet or digit.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextChar(0);
    if((ch > = 'a' && ch <= 'z') || (ch > = 'A' && ch <= 'Z'))
      System.out.println("Character is an alphabet");
    if(ch > = '0' && ch <= '9')
      System.out.println("Character is a digit");
    else
      System.out.println("Character is special");
  }
}

Enter week number and print day of the week.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    String dayN = switch(n)
    {
      case 1 -> "Sunday";
      case 1 -> "Monday";
      case 1 -> "Tueday";
      case 1 -> "Wednesday";
      case 1 -> "Thursday";
      case 1 -> "Friday";
      case 1 -> "Saturday";
      default -> "Invalid";
    }
      System.out.println(dayN);
  }
}

Print multiplication table of a given number.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    for(int i = 1; i <= 10; i++)
      System.out.println(n + " * " + i + " = " + (a * i));
  }
}

Print the first and last digit of a number.

import java.util.Scanner;
class Main {
  public static void main(String[] args)
  {
    Scanner in = new Scanner(System.in);
    int n = in.nextInt();
    int last = n % 10;
    int first = n;
    while(first >= 10)
      first = first/10;
    System.out.println(first);
    System.out.println(last);
  }
}

Key takeaways

For Loops: To travel from 1 point to another.
Do While Loops: When talking about digits.
Division: To remove last digit
Modular: Find last digit

Kirti Konka

Simple Loops

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