Using symsum within a loop

hello, i am trying to implement a moving average for a recording 'rec' by taking 3 consecutive data points at each iteration and averaging them. i tried to use symsum in my code below but i keep getting an error:

error using sym/subsindex (line 737)

Invalid indexing or function definition. When defining a function, ensure that the arguments are symbolic variables and the body of the function is a SYM expression. When indexing, the input must be numeric, logical, or ':'.

here is my code:

%%%%%
for i=2:length(rec) -1
          syms k
          filtered_rec(i) = symsum(rec(i+k),k,(m-1)/2,(m+1)/2);
 end
%%%%%

Any ideas,thanks ?

Answer 1

[Walter Roberson]

symbolic variables can never be used as indices, including not in symsum.

You should just construct the terms and sum() them

sum( rec(i+(m-1)/2:i+(m+1)/2) )

Or you could skip all of the looping and symbolic variables and just use

filtered_rec = rec(2:end-1) + rec(3:end);

This makes assumptions about the value of m. However, if those assumptions are not justified, then rec(i+(m-1)/2) would be out of range considering that you are going to length(rec)-1 on the for loop.

[Fatima Ramadan]

thanks,that was way simpler! i'll just make sure the summation terms are not out of range.