alternating series sum java help

For my assignment I have to create a program that calculates the following:


x - x^3/3! + x^5/5! - x^7/7! + ..... + - x^n/n!

Where the user inputs the value of x they want to calculate and inputs the value of n for the maximum exponent value (the higher the exponent the user inputs the more accurate the calculation will be).

Here's my attempt to the problem:

Code:

---

import java.io.*;
public class s {
public static void main(String args[]) throws IOException
{
BufferedReader keybd = new BufferedReader(new InputStreamReader(System.in));
       
        double x;
        int n;
       
        System.out.print("Input a value for x: ");
        x = Double.parseDouble(keybd.readLine());
       
        System.out.print("Input max exponent: ");
        n = Integer.parseInt(keybd.readLine());
       
        int factorial=1;
        for (int i=2; i<=n; i++)                // this for loop finds factorials (ie 5!=120)
        {
                factorial = factorial * i;
        }
       
        int exp=1;
        double sum = 0;
        for (int i=1; i<=n; i=i+2)
{
sum = sum + (Math.pow(-1,i))*(Math.pow(x,i))/(factorial);
// i don't think I did the factorial part correctly.
// Also I can't get the alternating between positive and negative. it starts with negative because (-1)^1= -1 but the first term of series has to be positive.
}
       
        System.out.println("The result is: " + sum);
}

}

---

Please help! Thanks!

This wont really work easily without a static method to fomput the factorial.

You can more easily break each expression down to

Code:

---

x^((2n + 1)/((2n + 1)!)

---

so you would want to do something like

Code:

---

Math.pow(x, ((2n + 1)/yourFactorialMethod(2n + 1)));

---

I gave you a decent amount here, but I left you with figuring a bit out, create a static factorial class and post the code for it.(start small, work up)

Does your code compile? If not and you can't understand the messages post them.

If it runs but the runtime behaviour is not what you expect (ie it gives a strange answer or throws an exception), say what it does.

And please use code tags: put [code] at the start of your code and [/code] at the end: that way the indentation is preserved.

-----

Quote:

---

// i don't think I did the factorial part correctly.

---

One way to see to to print out the value produced by your factorial loop.

Code:

---

int factorial=1;
for (int i=2; i<=n; i++) // this for loop finds factorials (ie 5!=120)
{
    factorial = factorial * i;
}
[color=blue]System.out.println("n=" + n);
System.out.println("n!=" + factorial);[/color]

---

Quote:

---

Originally Posted by pbrockway2

Does your code compile? If not and you can't understand the messages post them.

---

code compiles, but does not output correct answer

Is it possible to create a for loop within a for loop? Would that work?

Best way to find out is to try it and see for yourself!

db

By the way, @op, were you able to do the last problem?
E(2n + 1)!

If you were this is sort of just a step up from that as I pointed out above.

Quote:

---

Originally Posted by sunde887

I gave you a decent amount here, but I left you with figuring a bit out, create a static factorial class and post the code for it.(start small, work up)

---

Is it possible to do this without creating a new class?

yes. One class can have many methods, I am wanting you to have one of the methods in the class be a factorial method. Below I have created a static method to print hello.

Code:

---

public class StaticHello{
  public static void hello(){
    System.out.println("Hello, World");
  }
  public static void main(String[] args){
    hello();
  }
}

---

Quote:

---

Originally Posted by sunde887

yes. One class can have many methods, I am wanting you to have one of the methods in the class be a factorial method. Below I have created a static method to print hello.

Code:

---

public class StaticHello{
  public static void hello(){
    System.out.println("Hello, World");
  }
  public static void main(String[] args){
    hello();
  }
}

---

---

Ok I think I get it now, but under the factorial method it says "the local variable n may not have initialized". Says something similar for FactorialMethod (check comments within the code):

Code:

---

import java.io.*;
public class s{
public static void main(String args[]) throws IOException
{
BufferedReader keybd = new BufferedReader(new InputStreamReader(System.in));
       
        double x;
        int n;
       
        System.out.print("Input a value for x: ");
        x = Double.parseDouble(keybd.readLine());
       
        System.out.print("Input max exponent: ");
        n = Integer.parseInt(keybd.readLine());
       
        double sum;
        for (int i=0; i<=n; i++)
        {
                sum += (Math.pow(-1,i))*(Math.pow(x,((2*n + 1))/((2*n + 1)FactorialMethod); // "FactorialMethod cannot be resolved to a variable"
        }
        System.out.println("The result is: " + sum);
}

public static int FactorialMethod;{
        int n;
        int factorial=1;
        for (int i=2; i<=n; i++) //"the local variable n may not have initialized"
        {
                factorial *= i;
        }
}

}

---

close, to help you out a bit more I will make a static method to determine if a number is prime.

Code:

---

public class IsPrime{
  public static boolean isPrime(int n){
    boolean flag = true; //assume the number is prime
    for(int i = 2; i < n; i++){
      if(n % i == 0){//if number is evenly divisble by i, switch flag to false, end loop.
        flag = false;
        break;
      }
    }
    return flag;//return the flag(true if nothing divides evenly into n, otherwise returns false)
  }
  public static void main(String[] args){
    System.out.println(isPrime(10));
    System.out.println(isPrime(11));
  }
}

---

Quote:

---

Originally Posted by sunde887

close, to help you out a bit more I will make a static method to determine if a number is prime.

Code:

---

public class IsPrime{
  public static boolean isPrime(int n){
    boolean flag = true; //assume the number is prime
    for(int i = 2; i < n; i++){
      if(n % i == 0){//if number is evenly divisble by i, switch flag to false, end loop.
        flag = false;
        break;
      }
    }
    return flag;//return the flag(true if nothing divides evenly into n, otherwise returns false)
  }
  public static void main(String[] args){
    System.out.println(isPrime(10));
    System.out.println(isPrime(11));
  }
}

---

---

It still says n can't be initialized to a variable:

Code:

---

public static int FactorialMethod;{
        int n;
        boolean flag = true;
        int factorial=1;
        for (int i=2; i<=n; i++) // "n has not been initialized to variable"
        {
                if ((n % i == 0)){
                        flag = false;
                break;
                }
                else
                {
                factorial *= i;
        }
}

}
}

---

How to I make the n equal to the value that the user inputs into the main method (or whatever it's called :D)

Your method is incorrect(you don't need a flag to find the factorial)

The reason I used the flag was to show you a bit more complex example of a static method(one that takes an argument)

Compare my method declaration and yours

Mine:

Code:

---

public static boolean isPrime(int n)

---

yours:

Code:

---

public static int FactorialMethod;

---

Besides the name and return type there is one major difference. Where does my static method to determine if a number is prime declare n?

Quote:

---

Originally Posted by sunde887

Your method is incorrect(you don't need a flag to find the factorial)

The reason I used the flag was to show you a bit more complex example of a static method(one that takes an argument)

Compare my method declaration and yours

Mine:

Code:

---

public static boolean isPrime(int n)

---

yours:

Code:

---

public static int FactorialMethod;

---

Besides the name and return type there is one major difference. Where does my static method to determine if a number is prime declare n?

---

Here it says "Syntax error on 'FactorialMethod', delete this token"

Code:

---

sum += (Math.pow(-1,i))*(Math.pow(x,2*n + 1))/((2*n + 1)FactorialMethod);

---

Also have another error below
Here it says. "This method must return a result of type boolean"

Code:

---

public static boolean FactorialMethod(int n){

---

Ill give you the factorial method since you seem to be genuinely trying but I want you to understand how it works, instead of simply copying and pasting it.

Code:

---

public static int factorial(int n){  //it's public and static, it returns and int, and takes an int as an argument
  for(int i = n; i > 0; i--){
    sum *= i;
  }
}

---

I left two bugs in the code which will make it not compile, you have to find them(this forces you to think about it instead of copy/pasting).

Once you find the bugs you want to work on the actual heart of the problem.

Which really breaks down to
Math.pow(x, ((2n + 1)/(factorial(2n + 1))));

Quote:

---

Originally Posted by sunde887

Ill give you the factorial method since you seem to be genuinely trying but I want you to understand how it works, instead of simply copying and pasting it.

Code:

---

public static int factorial(int n){  //it's public and static, it returns and int, and takes an int as an argument
  for(int i = n; i > 0; i--){
    sum *= i;
  }
}

---

I left two bugs in the code which will make it not compile, you have to find them(this forces you to think about it instead of copy/pasting).

Once you find the bugs you want to work on the actual heart of the problem.

Which really breaks down to
Math.pow(x, ((2n + 1)/(factorial(2n + 1))));

---

It took me several hours (looking through tutorials and figuring it out) to get this done. I'm glad you helped me instead of just giving me the answer because I actually learned something. Thank you :)

Did you get the whole problem solved?

Yes. The program works fine and calculates correct answers

I'm glad you solved it, please mark your your thread solved with the thread tools.