Explanation of this statement

Question

Ashutosh Shukla 2014-6-30

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/139506-explanation-of-this-statement

评论： dpb 2014-7-1

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.

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

dpb 2014-6-30

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/139506-explanation-of-this-statement#answer_143034

在 MATLAB Online 中打开

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 个评论
显示 4更早的评论隐藏 4更早的评论

dpb 2014-6-30

编辑：dpb 2014-6-30

在 MATLAB Online 中打开

A) Don't believe that one at all...unless you've got infinite storage :)

B) That's possible; but that's easily-enough fixed if both are column vectors. The question is whether there's any need at all for the ' operator -- I'm guessing not but it's possible w could be complex in which case it makes a difference computationally.

I'm guessing this is a remnant left over from development that never got cleaned up and started out intended to be something as

y=w.' * x;

but inadvertently wrote the complex transpose ' instead of the non-conjugate version .' which would be the cheaper computationally again assuming w isn't actually complex, but just a weight function. then for some reason decided need the individual values instead of just the weighted sum. But, of course, in isolation there's no way to really tell what the intent really was/is...

Example--

>> w=[0.1:.1:.3].';
>> x=1:3;x=x.';
>> w.' * x
ans =
  1.4000
>> s=0;for i=1:3;z=w(i)*x(i);s=s+z;disp([z s]),end
  0.1000    0.1000
  0.4000    0.5000
  0.9000    1.4000
>>

Ashutosh Shukla 2014-7-1

no complex numbers in calculation....

simple float(decimal) numbers...

dpb 2014-7-1

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.

请先登录，再进行评论。

Answer 2

Robert Cumming 2014-6-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/139506-explanation-of-this-statement#answer_143033

在 MATLAB Online 中打开

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 个评论
显示 2更早的评论隐藏 2更早的评论

Ashutosh Shukla 2014-7-1

Also what is meant by w(i)'. how is it transposing single element??

dpb 2014-7-1

在 MATLAB Online 中打开

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.

请先登录，再进行评论。

Explanation of this statement

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

回答（2 个）

6 个评论
显示 4更早的评论隐藏 4更早的评论

4 个评论
显示 2更早的评论隐藏 2更早的评论

另请参阅

类别

标签

Community Treasure Hunt

Explanation of this statement

0 个评论 显示 -2更早的评论隐藏 -2更早的评论

回答（2 个）

6 个评论 显示 4更早的评论隐藏 4更早的评论

4 个评论 显示 2更早的评论隐藏 2更早的评论

另请参阅

类别

标签

Community Treasure Hunt

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

6 个评论
显示 4更早的评论隐藏 4更早的评论

4 个评论
显示 2更早的评论隐藏 2更早的评论