CODEWITHSUNDEEP
java
Daveguy
Daveguy

Reputation: 255

Computing a series in Java?

I am trying to create a recursive method to compute a series in java 1/i.

Here is my code:

public static double computeSeries(int n) {
    if (n == 1)
        return 1;
    else
        return (1/n) + computeSeries(n - 1);

}

where 'n' is passed through the main method. However it doesn't exactly work correctly. FE when I type in 3, it returns 2.0, where I calculated it to be 1.8, and if I use 2, it gives me 1.0

Upvotes: 2

Views: 92

Answers (5)

Calvin P.
Calvin P.

Reputation: 1232

As most others have said, you need to convert the values to double before performing operations. Also, and this may just be personal preference, you may not want to have n calls of computeSeries running before they can all be completed.

Edit: After thinking more, using the for loop as I did below, there is no need for an extra method to calculate each term in the series. You can simply do the following

public static double computeSeries(int n) {
    double sum = 0.0;
    for(int i=1; i<=n; i++){
        sum += (1.0/(double)i);
    }
    return sum;
}

Upvotes: 0

Yassin Hajaj
Yassin Hajaj

Reputation: 21975

While you're going to work with decimals, you might at least want to have a double as input

Solution

public static double computeSeries(double n)

However, if you only want the method to have an int as input, you might want to change 1/n to 1.0/n this will result to an operation of type double instead of int

This is called Promotion

JLS §5.6.2

If either operand is of type double, the other is converted to double.

Upvotes: 4

Marvin
Marvin

Reputation: 14255

Your first division will be 0 in most of the cases, since you're calculating with integers.

Instead use one of

  • return (1.0 / n) + computeSeries(n - 1);
  • return (1 / (double) n) + computeSeries(n - 1);
  • public static double computeSeries(double n) {

Bonus: You should take care of n = 0 to prevent a java.lang.ArithmeticException.

Upvotes: 0

Keammoort
Keammoort

Reputation: 3075

First part of your result (1/n) is truncated to an int:

    return (1/n) + computeSeries(n - 1);

Force calculations to be done in a double type by changing 1 to 1.0 (then it's a double):

    return (1.0/n) + computeSeries(n - 1);

Upvotes: 3

Jim Garrison
Jim Garrison

Reputation: 86774

1/n

You are calculating in integers. This is always ZERO unless n == 1

Upvotes: 0

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