How to sum only the positive elements in a series?

I was asked to write a program to calculate only the positive elements of a series until the sum has reached the required value.

A picture for the series:

I have managed to write a code but the code repeats the loop infinite times until I force it to stop.

sum = 0.0;
n = 0;
disp('      n         Sum');

while sum < 4
    n = n + 1;
    sum = sum + f(n);

      if sum > 0;
          sum = sum + f(n);
          disp([n sum]);
      else
          disp('skip');
      end
  end

f(n) in the code represent this:

function y = f(x)
y = ((-1)^(x+1))*(1/x);

Im not confident about the 'else' part of the loop.

Any help is appreciated.

Answer 1

[Roger Stafford]

Sultan, I think you are interpreting your instructions incorrectly. It says, "calculate only the positive elements of a series until the sum has reached the required value" which I think means to sum only the positive elements. You are summing every number. The test you make is useless because that sum, even if it includes negative values, can never itself be negative. Just keep adding the numbers which are positive and skip over the negative ones until the sum of the positives reaches or exceeds the desired value.

Answer 2

[Walter Roberson]

what is f(n) exactly? If f(1) = (-1)^(n+1)*1/n then you are going to get negative values and you are adding those negative values into "sum" before you are testing to find out if the term was negative.

Note: do not use "sum" as the name of a variable or you will run into problems.

Answer 3

[Jos (10584)]

This piece of pseudocode might get you started:

while CurrentSum < maxixumSum
   n = n + 1 
   CurrentValue = (-1^n)/n 
   if CurrentValue > 0
    % … etcetera

Answer 4

[Sultan]

Thanks guys, Ive managed to solve it, thanks to your hints.

[Sagar Damle]

Can you put your correct code here to help others?