Partial Sums Using For Loop

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?

Answer 1

[James Tursa]

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

[CLow]

Ah-ha, thank you! I figured there was something up since I wasn't getting a vector out in the workspace.