Element-wise power resulting in imaginary values and NAN

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MathN00b 2020-1-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/501726-element-wise-power-resulting-in-imaginary-values-and-nan

评论： Walter Roberson 2020-1-25

I am using element-wise power .^,which for A.^B should yield a matrix with elements A(i,j) raised to the power B(i,j). However, for the simple example:

A = [-1, 0; 0, -1];
B = [1, 0; 0, 1];
A.^B

I'm getting

ans =
  -1.0000 + 0.0000i      NaN + 0.0000i
      NaN + 0.0000i  -1.0000 + 0.0000i

If I use a loop, there is no error.

for i=1:2
    for j =1:2
        AB(i,j) = A(i,j)^B(i,j);
    end
end
AB =
    -1     1
     1    -1

I can also select a single power coefficient and get the correct values, such as

A.^B(1,2)
ans =
     1     1
     1     1

What's wrong with the matrix element-wise power? Why is it giving out imaginary numbers and NaN for essentially 0^0, which should equal 1. Thanks.

EDIT: More Information on Replicating the Error

I tried to figure out why others can't replicate the errors I'm getting. After much frustration, I restarted the computer (Windows 10, R2018a) and tried it from a fresh instance, just copying and pasting the code into the Command Window, I get no error. However, if I run it from a script (e.g., script.m with the exact same code), then I get the error, and once it's errored, it can't be undone. Did some system settings change once I run it from an M file? I hope this helps to pinpoint the problem.

>> A = [-1, 0; 0, -1];
B = [1, 0; 0, 1];
A.^B
ans =
    -1     1
     1    -1
>> script
ans =
  -1.0000 + 0.0000i      NaN + 0.0000i
      NaN + 0.0000i  -1.0000 + 0.0000i
>> A = [-1, 0; 0, -1];
B = [1, 0; 0, 1];
A.^B
ans =
  -1.0000 + 0.0000i      NaN + 0.0000i
      NaN + 0.0000i  -1.0000 + 0.0000i

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Walter Roberson 2020-1-24

Could you attach the script that is triggering the problem?

MathN00b 2020-1-25

Thank you for your help, Walter! The script contains the exact same code

 A = [-1, 0; 0, -1];
B = [1, 0; 0, 1];
A.^B

I figured out the problem, thanks to James. One of the libraries contained a directory named @double with an alternative power.m file, which overloaded the default power function. I had no idea this is called when I used .^. It defined

o=power(s,t)

o=exp(log(s)*t);

o=exp(log(s).*t);

depending on the arguments. Thanks again for your patience and help, I really appreciate it.

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采纳的回答

James Tursa 2020-1-24

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/501726-element-wise-power-resulting-in-imaginary-values-and-nan#answer_411883

Looks like the problem calculation is being done in the background as exp(B.*log(A)), but I don't know why it does this sometimes and not other times.

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MathN00b 2020-1-25

Thanks, James! One of the libraries contained a directory named @double with an alternative power.m file, which overloaded the default power function. I had no idea this is called when I used .^. It defined

o=power(s,t)

o=exp(log(s)*t);

o=exp(log(s).*t);

depending on the arguments. Much thanks!