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Square Root Complex Numbers

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Josh
Josh on 9 Mar 2013
Commented: Yogan Sganzerla on 6 Aug 2019
Hi
I'm new to Matlab. I have a matrix of data for example 1.04,-5.84,-5.84,8.54 etc. As you can see its a mixture of positive and negative values. I have to square root all the numbers and then add the square root up.
I get an output like this: 0 + 0.2572i 1.4758 0 + 1.0258i 0 + 1.0297i .... etc
How can I add these up because when I do the sum total is displayed like so: 24.1589 +25.8258i I was expecting a real number such as 25.0414 why is it formatted like this. I am new to matlab so not sure what I need to do, works fine if my data has no negative numbers so I assume it's to do with that.
Thanks
  3 Comments
Josh
Josh on 9 Mar 2013
I'd expect 1.41421356i which I would get but say I have -2 and 4 (1.41421356i and 2) now if I add them up I get -2 + 1.41421356 I thought I'd get 0.58578644 though. If you can see were i'm coming from
Youssef  Khmou
Youssef Khmou on 9 Mar 2013
real parts are summed separately from imaginary parts , if you want add both Real and Imag sumsthen try :
r=rand(10);
p=sqrt(r);
S=sum(real(p(:)))+sum(imag(p(:)));

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Answers (2)

Youssef  Khmou
Youssef Khmou on 9 Mar 2013
hi Josh,
the square root of <0 number is complex number , if you only want the real part use "real" function, but if you want imaginary part, use "imag" function, and for the modulus you can use "abs" function , example :
r=randn(10);
p=sqrt(r);
Real_part=sum(real(p(:)));
Imag_part=sum(imag(p(:)));
Modul_part=sum(abs(p(:)));

Walter Roberson
Walter Roberson on 10 Mar 2013
Edited: Walter Roberson on 10 Mar 2013
To get the results you indicate you want (which are not the results one would normally want when taking the square roots of negative numbers),
sum(sqrt(r(r>0))) - sum(sqrt(-r(r<0)))
Remember, sqrt(-A) is not -sqrt(A) but is instead 1i * sqrt(A) where 1i indicates sqrt(-1)
  5 Comments
Torsten
Torsten on 6 Aug 2019
A = sqrtm(M)
Yogan Sganzerla
Yogan Sganzerla on 6 Aug 2019
Perfect! Thank you very much.

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