Practice Writing Loops (Repetitive Blocks)

Topics

1. Sum and it’s Variants
2. Product and it’s Variants
3. Max/Min and it’s Variants
4. Counting and it’s Variants
5. Reverse and it’s Variants

Practice Exercises

Exercise 1: Sum of Natural Numbers

Problem: Write a for loop to calculate the sum of the first n natural numbers.

Solution

   int sum = 0;
   for (int i = 1; i <= n; i++) {
      sum += i;
   }
   System.out.println("Sum: " + sum);

Variants

1. Sum of Even Numbers
   Problem: Write a for loop to calculate the sum of the first n even numbers.
2. Sum of Odd Numbers
   Problem: Write a for loop to calculate the sum of the first n odd numbers.
3. Sum of Multiples of 3
   Problem: Write a for loop to calculate the sum of the first n multiples of 3.
4. Sum of Squares of Numbers
   Problem: Write a for loop to calculate the sum of the squares of the first n natural numbers.

Exercise 2: Factorial of a Number

Problem: Write a while loop to calculate the factorial of a number n.

Solution

   int factorial = 1;
   int i = n;
   while (i > 0) {
      factorial *= i;
      i--;
   }
   System.out.println("Factorial: " + factorial);

Variants

1. Product of Even Numbers
   Problem: Write a while loop to calculate the product of the first n even numbers.
2. Product of Odd Numbers
   Problem: Write a while loop to calculate the product of the first n odd numbers.
3. Double Factorial
   Problem: Write a while loop to calculate the double factorial of a number n, which is the product of all integers of the same parity as n.
   If n is odd, calculate n * (n - 2) * (n - 4) * ... * 1.
   If n is even, calculate n * (n - 2) * (n - 4) * ... * 2.

Exercise 3: Find Maximum Element in Array

Problem: Write a for loop to find the maximum value in an array.

Solution

   int[] arr = {10, 50, 30, 70, 20};
   int max = arr[0];
   for (int i = 1; i < arr.length; i++) {
      if (arr[i] > max) {
         max = arr[i];
      }
   }
   System.out.println("Max value: " + max);

Variants

1. Find Minimum Element in Array
   Problem: Write a for loop to find the minimum value in an array.
2. Find Second Maximum Element in Array
   Problem: Write a for loop to find the second maximum value in an array.
3. Find Index of Maximum Element
   Problem: Write a for loop to find the index of the maximum value in an array.

Exercise 4: Count Even Numbers in Array

Problem: Write an enhanced for loop (for-each) to count how many even numbers are in an array.

Solution

   int[] arr = {1, 2, 3, 4, 5, 6};
   int evenCount = 0;
   for (int num : arr) {
      if (num % 2 == 0) {
         evenCount++;
      }
   }
   System.out.println("Even numbers count: " + evenCount);

Variants

1. Count Odd Numbers in Array
   Problem: Write an enhanced for loop to count how many odd numbers are in an array.
2. Count Prime Numbers in Array
   Problem: Write an enhanced for loop to count how many prime numbers are in an array.
3. Count Numbers Greater Than a Threshold
   Problem: Write an enhanced for loop to count how many numbers in an array are greater than a given threshold value k.

Exercise 5: Reverse an Array

Problem: Write a for loop to reverse the elements of an array.

Solution

   int[] arr = {1, 2, 3, 4, 5};
   for (int i = 0; i < arr.length / 2; i++) {
      int temp = arr[i];
      arr[i] = arr[arr.length - i - 1];
      arr[arr.length - i - 1] = temp;
   }
   System.out.println(Arrays.toString(arr));  // Prints reversed array

Variants

1. Reverse a String
   Problem: Write a for loop to reverse a string.
2. Reverse Digits of a Number
   Problem: Write a while loop to reverse the digits of a number.
3. Reverse Every Word in a Sentence
   Problem: Write a for loop to reverse every word in a given sentence.

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Summary

Loops are essential for repeating tasks, traversing data structures, and automating calculations. By understanding the different types of loops (for, while, do-while, enhanced for), their syntax, and common use cases, you can efficiently solve a variety of problems in programming. These practice exercises help reinforce these concepts.

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