| Re: Old Question [message #18311 is a reply to message #18178] |
Wed, 15 December 1999 00:00   |
thompson
Messages: 584
Registered: August 1991
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Senior Member |
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Ben Tupper <pemaquidriver@tidewater.net> writes:
> Jacques Basson wrote:
>> Hi all
>>
>> Sorry, this has got to be an old question, but I can't seem to locate
>> the answer. What is the way around the following problem?
>>
>> IDL> a = -1
>> IDL> print, -1^(1./3)
>> -1.00000
>> IDL> print, a^(1./3)
>> NaN
>> % Program caused arithmetic error: Floating illegal operand
>>
>> Thanks
>> Jacques
Um, I don't know if you realize this, but the only reason that the first
example DOESN'T fail is because
IDL> print, -1^(1./3)
is equivalent to
IDL> print, -( 1^(1./3))
In other words, the minus sign doesn't come into play until after the
exponentiation is applied. If you typed in
IDL> print, (-1)^(1./3)
you'd get the NaN result, with the error message, as above. Note that you can
test this by typing in
IDL> print, -1^2, -(1^2), (-1)^2
-1 -1 1
Of course, as is implied by the answer below, the proper way to evaluate your
example would be
IDL> a = -1
IDL> print, a^complex(1./3, 0)
( 0.500000, 0.866025)
William Thompson
> Hello,
> I now know why it happens. In the documentation I see...
> Exponentiation
> The caret (^) is the exponentiation operator. A^B is equal to A raised to
> the B power.
> * If A is a real number and B is of integer type, repeated multiplication
> is applied.
> * If A is real and B is real (non-integer), the formula A^B = e^(B ln A)
> is evaluated.
> * If A is complex and B is real, the formula A^B = (re^(iq))^B = r^B *
> (cosBq + isinBq) (where r is the real part of A and iq is the imaginary
> part) is evaluated.
> * B is complex, the formula A^B = e^(B ln A) is evaluated. If A is
> also complex, the natural logarithm is computed to be ln(A) = ln(re^(iq))
> = ln(r) + iq (where r is the real part of A and iq is the imaginary
> part).
> * A^0 is defined as 1.
> Your example falls into the second type of operation. I don't know how
> to get around that but would like to know also.
> Ben
> --
> Ben Tupper
> Pemaquid River Company
> 248 Lower Round Pond Road
> POB 106
> Bristol, ME 04539
> Tel: (207) 563-1048
> Email: PemaquidRiver@tidewater.net
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