Explanation of this statement

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Ashutosh Shukla
Ashutosh Shukla on 30 Jun 2014
Commented: dpb on 1 Jul 2014
for i=1:N
yd(i) = sum(w(i)' .* x(i));
end
Can anyone explain the above statement to me?? I can't understand the use of sum as acc. to me w(i) and x(i) are just numbers and not whole vector. So what's the sum for. Don't doubt the code. It is correct. Just need an explanation.

Answers (2)

dpb
dpb on 30 Jun 2014
Did you just look and see??? But, yes, you're evaluation is correct of the code as written--now whether that's what was intended is another question.
The above loop could be rewritten as
yd=w' .* x;
Try
all(yd)==all(w' .* x)
to prove it.
  6 Comments
Ashutosh Shukla
Ashutosh Shukla on 1 Jul 2014
no complex numbers in calculation....
simple float(decimal) numbers...
dpb
dpb on 1 Jul 2014
no complex numbers in calculation....
Which I presumed was likely the case but just pointing out there is a difference if w were to be complex.

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Robert Cumming
Robert Cumming on 30 Jun 2014
Looking at the code, unless sum is an overloaded function that does something other than the standard inbuild sum - it looks redundant.
To check what sum is - type:
which sum
To check its not doing anything - remove it and run the code again!
  4 Comments
Ashutosh Shukla
Ashutosh Shukla on 1 Jul 2014
Also what is meant by w(i)'. how is it transposing single element??
dpb
dpb on 1 Jul 2014
That's why I brought up the complex number issue; the ' is the complex transpose which has no net effect if the values are real but does if they're not. Newbies often mistakenly use it when the non-complex transpose operator .' is intended instead, which is the case I presumed happened here.
See
doc punct
for details.
As noted, if it's written as a loop on an individual element for a real value it has no effect, just as the sum of a single value is simply the value.

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