Thread: Can't find mistake in my code

Hey guys,

I have a little problem here. The program runs and everything is fine except the actual result. The question is:

Design Java code to calculate and print ∑ (n! + 5) / (n+1)! ; n is an integer from 1 to 10.

I have written the code and I it's here:

/**
*Question #2 
*/
 
public class Sum {
  public static void main (String [] args) {	
    long a = 1;
    long b = 2 * a;
    double sum = (a + 5.0) / b;
 
    long n = 3;	
    while(n<=11) {
      sum = sum + (double)(a + 5.0) / b;
      b = n * b; 
      a = a * (n - 1); //probably mistake here
      n++;
    }//end of a loop
 
    System.out.println("The answer is " + sum);
  }
}

I checked on my calculator and the result is 5.611286476

But my output is 8.520377259700176

Please, guys. I don't know where should I go. Help me find my mistake!

note the difference between (n - 1) and (n + 1). It's a critical distinction.

In pseudo code, I've done something like,

      declare int variable, maxI  and set it to 10. This will be upper limit of for loop.
      declare int variable, n, and set it to 0.
      declare long (or probably could be int) variable, nPerm, that represents n! and set it to 1.
      declare long (or probably could be int) variable, nPlus1Perm, that represents (n + 1)! and set it to 1.
      declare a double sum variable and set to 0.
      for loop with int i going from 0 to less than maxI (it will loop 10 times)
         set n to i + 1.
         set nPerm  to hold the current value of nPlus1Perm;
         advance nPlus1Perm  by multiplying it by (n + 1) and assigning the value back into the nPlus1Perm variable
 
         calculate a double prod variable as per the given equastion.
         add the prod to the sum
 
      end of for loop
      System.out.println("sum: " + sum);

Originally Posted by curmudgeon

note the difference between (n - 1) and (n + 1). It's a critical distinction.

Yes, sir. I've tried it too, but it doesn't give the right answer either. I can't quite understand whether I should add something else to it.
Sorry.

see edit above.

Thanks for very clear algorithm. The answer is correct when I don't put factorial "!" into my calculator(18.07950938), but when I put "!" in the calculator it gives me a different answer with the code you helped me out with.
Does the "!" play role?

Code:

public class AnotherTry
{
public static void main (String [] args)
{
int maxI = 10;
int n = 0;
long nPerm = 1;//n
long nPlus1Perm = 1;//(n+1)
double sum = 0;

for (int i = 0; i < maxI; i++)
{
n = i + 1;
nPerm = nPlus1Perm;
nPlus1Perm = nPlus1Perm *(n+1);
double prod = (n + 5.0) / (n+1.0);
sum += prod;
}
System.out.println("sum: " + sum);
}
}

Thanks a lot for your support!!! I am just a little bit confused here

--- Update ---

I've finally got where I have made a mistake! My code was correct but the instructions in the loop weren't ordered on the right place.
Here is a code:

/**
 *Question #2 
 */
 
public class Sum 
{
	public static void main (String [] args)
	{	
		long a = 1;
		long b = 2 * a;
		double sum = (a + 5.0) / b;
		long n = 3;	
 
		while(n<=11)
		{
			a = a * (n - 1);
			b =  n * b;
			sum = sum + (double)(a + 5.0) / b; 
			n++;
		}//end of a loop
 
		System.out.println("The answer is " + sum);
	}
}

The output is 5.611286475869809 as it should be.

Thanks for the effort anyway!