For loops: initialize the sum variable

For this code if put the initial value of sum=1 then I get different answer.But I thought the inital values of sum doesn't matter? Can anyone explain?

m = input('Please enter the number of terms in the sum: '); 
n = 0; 

SUM = 0;
disp(' ') 
for n = 1:m 
an = (-1/3)^n/(2*n + 1); 
SUM = SUM + an = (-1/3)^n/(2*n + 1); 
Val_Exp = sqrt(12)*SUM; 
Prec_pi = abs(pi-Val_Exp); 
fprintf('For n = %2i, Value = %.15f to a precision of %.10e.\n', n, Val_Exp, Prec_pi) 
end disp(' ')

Answer 1

[Walter Roberson]

SUM = SUM + an = (-1/3)^n/(2*n + 1); 

That is invalid syntax. You cannot have two = in the same statement.

end disp(' ')

That is invalid syntax. You need a statement separator between end and what follows.

Why whould you think that the initial values did not matter?

I would suggest to you that what you missed is that your summation is really over 0 to m instead of 1 to m, and that there are two ways you can compute that: you can either initialize the sum to 0 and loop from 0, or you can initialize the sum to 1 and loop from 1. Either way works because the term for n = 0 works out to 1.