/* This program serves to demonstrate while and do-while loops.
 * Loops are repetitive structures that are a cornerstone of programming
 */
import java.util.Scanner;
class LoopExercise
{
	public static void main(String [] args)
	{
		Scanner input = new Scanner(System.in);
		int num,times,limit;
		//Here, num holds a number, times serves as a counter for the number of times something has been done.
		//Limit is the upper limit for the counter.

		//This part prints the multiplication tables for a number entered by the user,
		// for 'limit' number of lines.

		System.out.print("Enter the number for the tables: ");
		num = input.nextInt();

		System.out.print("Enter the upper limit: ");
		limit = input.nextInt();

		times = 1;
		while(times<=limit)
		{
			System.out.println(num +" * "+times+" = "+num*times);
			times++; // This line is called the increment line. This is very important. If this is missing, we get an infinte loop.
		}
		/* The above structure is a while loop. The program checks if the conditon is true, the statements inside the loop,
		 * called the "body of the loop", are executed. Then the program checks the condition again. This process is repeated
  		 * until the condition becomes false. The program then moves on to the first line after the loop.
		 */

		//We can use loops to add numbers from 1 till the limit.
		//We use the same limit that we used for the previous loop.
		
		int count = 1; //This us another loop counter. We can also reuse 'times' and 'num'

		int sum = 0; //This is the accumulator variable. This keeps a running total of the numbers we've encountered so far.
		while(count<=limit)
		{
			sum = sum + count; //Add the current number to count
			count++;
		}
		System.out.println("The sum of all natural numbers up to "+limit+" is "+sum);

		//We can use if statements inside loops.
		// In this part we calculate the sum of all the odd natural numbers up to 'limit'
		// Here, we start of at 1, and increment the counter by 2 at each iteration of the loop.
		
		count =1;
		sum =0; //Reset the values for count and sum

		while(count<=limit)
		{
			sum = sum+count;
			count = count + 2; //Since we start off with an odd number and keep adding 2, count will always be odd.
		}
		System.out.println("The sum of all odd natural numbers up to "+limit+" is "+sum);

		/* Loops can be used in Stings as well. Here, we read in one letter at a time and concatenate the letter we just
		 * read in to the string we have so far to make one long string. We use strings to read in the letters because
		 * using chars would require some conversions.
		 */
		String text =""; //We declare an empty string
		String letter="";
		count =0;
		// We are building a string of 'limit' number of letters
		while(count < limit)
		{
			System.out.print("Enter a letter: ");
			letter = input.next();
			text=text+letter;
			count++;
		}
		System.out.println("The string we just constructed is "+text);
		
		/* do-while loops are very similar to while loops, except where while loops run a minimum of 0 times, do-while loops run
		 * at least once.
		 */

		// Here, we calulate an alternating sum.
		// 1 - 2 + 3 - 4 + 5 - 6 ....
		//So, if the current number is odd, we add that number to the sum. Otherwise (it's even), subtract.
		count =1;
		sum=0;
		do
		{
			if(count%2==1) // number is odd if, after division by 2, we have a remainder of 1
				sum = sum + count;
			else 	sum = sum - count;
			count++;
		}while(count<=limit);
		System.out.println("The alternating sum is: "+ sum);
	}
}