[R] Comparing complex numbers

[David Stoffer]

Thanks- I was hoping I wouldn't have to loop, but using abs() is better than 
comparing Re()s and Im()s, which was my original thought.  I have to compare
all the roots, so I used something like this:

> for (i in 1:length(z1))  if(any(abs(z1[i]-z2[1:length(z2)])<1e-15))
> print("ouch")

... thanks again for your help.



Duncan Murdoch-2 wrote:
> 
> On 7/11/2008 11:51 AM, David Stoffer wrote:
>> Is there an easy way to compare complex numbers?
>> 
>> Here is a small example: 
>> 
>>>   (z1=polyroot(c(1,-.4,-.45)))
>> [1]  1.111111-0i -2.000000+0i
>>>   (z2=polyroot(c(1,1,.25)))
>> [1] -2+0i -2+0i
>>>   x=0
>>>   if(any(identical(z1,z2))) x=99
>>>    x
>> [1] 0
>> 
>> #  real and imaginary parts:
>> 
>>>    Re(z1); Im(z1)
>> [1]  1.111111     -2.000000
>> [1] -8.4968e-21   8.4968e-21
>> 
>>>    Re(z2); Im(z2)
>> [1] -2 -2
>> [1]  0  0
>> 
>> Both z1 and z2 have a root of -2, but I guess Im(z1)
>> isn't close enough to zero for identical().
> 
> == is the test you had in mind, I think, but like identical it requires 
> exact equality.  identical() wants the whole vector to be identical. 
> all.equal() checks for element by element approximate equality.
> 
> So you can get element by element approximate equality by something like
> 
>  > for (i in 1:length(z1)) print(all.equal(z1[i], z2[i]))
> [1] "Mean relative Mod difference: 2.8"
> [1] TRUE
> 
> The trouble with this approach is that it will use a different scale for 
> each comparison.  I think you really need to fashion your own test, e.g.
> 
>  > abs(z1-z2) < 1e-15
> [1] FALSE  TRUE
> 
> Duncan Murdoch
> 
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> 
> 


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