/* This program would serve to demostrate while and d-while loops. */

#include<iostream>
using namespace std;

int main()
{
	/* 1. Loop to calculate the sum of the first 10 natural numbers. 
	 *  We need to add 1 + 2 + 3 + 4 + ... + 10
	 *  We accumulate the sum in a variable. That is we sum stuff one addition at a time.
	 *  Calculate 0 + 1 first. Then add that sum to 2, then add that sum to 3 and so on.
	 *  At each step, we check if the value we are adding has reached 10. If it has passed 10, we stop.
	 */
	int i = 1, sum = 0; // i is conventionally used for counting variables.
	
	while (i <= 10)
	{
		sum = sum + i;
		i++;	// This moves to the next i at each step
	}

	cout<< "The sum of the first 10 natural numbers is "<< sum << endl;


	/* 2. Loop to print numbers from 20 through 1 in descending order.
	 *  We need to print 20 19 18 ... 2 1
 	 *  We only need a counting variable here.
	 *  This is to demonstrate that loops can count backwards.
	 */

	i = 20; // reusing the old counting variable.
	// using a do - while loop here
	do
	{
		cout<< i << " ";
		i--;
	}while (i>=1);

	cout<<endl<<endl;

	/* 3. Printing the first 100 multiples of 5.
	 * We can use 2 variables here. One is the counting variable. 
	 * The other is the printing variable. 
	 * We need to print 5 10 15 20 25 ... 495 500
	 * This is to demonstrate loops can contain more than just the major operation and an increment/decrement statement
	 */
	
	i = 1; // resuing counting variable.
	int print = 5;
	
	while ( i <= 100)
	{
		cout<< print <<" ";
		print = print + 5; // next number to print
		i = i + 1; // we have printed one more number
	}

	cout<<endl<<endl;
	/* 4. We don't always need to use counter variables. 
	 * We can adjust the test expression to use the print variable, and eliminate the counting variable.
	 * The following loop is equivalent to the above loop.
	 */
	print = 5; // print has been declared before. Reusing.
	
	while ( print <= 5 * 100)
	{
		cout<< print <<" ";
		print = print + 5;
	}
	
	/* 5. The test expression need not be a counting variable. It can depend on user input.
	 * Over here, we're going to ask the user a question and decide if we're going to stay in the loop
	 * or exit depending on the answer to that question.
	 */

	cout<<endl<<endl;

	char ans;
	do
	{
		cout<<"Is your question \"Is this the Krusty Krab?\" (Y or N): ";
		cin>>ans;
		
		// This is to avoid printng "No, this is Patrick!! when the user says no
		if(ans == 'Y')
		{
			cout<<"No, this is Patrick!!"<<endl;
		}
	}while( ans == 'Y');

	/* 6. The above loop had an internal check to print the reply only when the user says yes.
	 * Without the check, it would have printed the reply in the one case when the user says no as well.
	 * We can also use a couple of redundant print and input statements.
	 * The code below produces equivalent output to the code above.
	 */
	
	cout<<endl<<endl;
	cout<<"Is your question \"Is this the Krusty Krab?\" (Y or N): ";
	cin>>ans;
	while( ans == 'Y')
	{
		cout<<"No, this is Patrick!!"<<endl;
		cout<<"Is your question \"Is this the Krusty Krab?\" (Y or N): ";
		cin>>ans;
	}

	cout<<endl<<endl;

	/* 7. Printing the factorial of a number. 
	 * The factorial of a number is the product of all numbers from 1 through the numbers.
	 * For example, 5 factorial is 1 * 2 * 3 * 4 * 5 = 120
	 * We accumulate the factorial in a product variable.
	 */
	long fact = 1; // Using 0 as the starting variable would 0 out the answer.
		// We use long because factorials can ger very big very fast.
	i =1;

	int num;
	cout<<"Enter a positive integer: ";
	cin>> num;

	while (i<=num)
	{
		fact = fact * i;
		i++;
	}
	cout <<"The factorial is "<< fact<<endl;
	 
	return 0;
}