MATLAB: Partial Sums Using For Loop

for loopsummation

Hey all,
I'm trying to write a general form to compute the partial sum of e^x. So far I've gotten a function that spits out the value of the nth term, but not the sum of that term and all the terms before it. (Ex, if M is 2, the computer gives me 0.5 as the output, but it should be 1+1+0.5 = 2.5 for terms n=0, n=1, and n=2.).
The problem definitely has to do with the summation line– how do I fix that? Thanks!
    function y = Pset0_P5_fxn(x,M)
x = 1; M = 2;
   for n = 0:M
       a = ((x.^n)./(factorial(n)))
       y = sum(a)
   end
end
  end
EDIT: The function seems to work fine, but won't plot– for all x values, the y value is the same (whatever the output is on the last iteration). How can I store the output at each iteration in a vector?

Best Answer

You replace the "a" variable with each iteration, so only the last iteration is present when you do "sum(a)". So either make "a" a vector (if you want to save the intermediate terms), or just sum as you go. E.g.,
Make "a" a vector:
function y = Pset0_P5_fxn(x,M)
   x = 1; M = 2; % test values

   for n = 0:M
       a(n+1) = ((x.^n)./(factorial(n))); % Note the added (n+1) subscript
   end
   y = sum(a);
end
Sum as you go:
function y = Pset0_P5_fxn(x,M)
   x = 1; M = 2; % test values
   y = 0; % Initialize y
   for n = 0:M
       a = ((x.^n)./(factorial(n)));
       y = y + a;
   end
end
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