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Re: Double precision data into caldat [message #91886 is a reply to message #91884] Tue, 08 September 2015 15:22 Go to previous messageGo to previous message
laura.hike is currently offline  laura.hike
Messages: 87
Registered: September 2013
Member
Yes, this answers the question. Thanks!


On Tuesday, September 8, 2015 at 1:58:48 PM UTC-7, Paul van Delst wrote:
> Hello,
>
> On 09/08/15 15:37, Larry H. wrote:
>> Hi,
>>
>> I'm trying to convert some Julian dates back into standard dates
>> using caldat. (In fact, I am testing what I got out of julday in the
>> first place.) There should be hours and minutes in the results. If
>> I take the output of julday as a variable, say TEST, and put it into
>> caldat as
>>
>> caldat, test, m, d, y, h, mm, s
>>
>> I get the right answer. However, if I just use the actual value of
>> test, I get odd results. So, for the Julian day 2456658.56250000, I
>> should get
>>
>> 2014 1 1 1 30 0
>>
>> in year, month, day, hour, min, sec form. If I use
>>
>> caldat, 2456658.56250000D, m, d, y, h, mm, s
>>
>> the results are correct, but if I do the type conversion using
>> double(), it doesn't. So I have
>>
>> caldat, double(2456658.56250000), m, d, y, h, mm, s
>>
>> and
>>
>> p = double(2456658.56250000) caldat, p, m, d, y, h, mm, s
>>
>> both giving the result
>>
>> 2014 1 1 0 0 0
>>
>> Does anyone know why this is? As far as I know, all of those input
>> values are the same.
>
> Nope. They are not.
>
> IDL> p=2456658.56250000
> IDL> help, p
> P FLOAT = 2.45666e+06
> IDL> print, p, format='(f25.10)'
> 2456658.5000000000
>
> Single precision has about 7-8 significant digits.
>
> So for a value you write as "2456658.56250000" -- which is a single
> precision literal constant -- you've only got "2456658.5" of useful digits.
>
> When you use DOUBLE, as in:
>
> IDL> p=DOUBLE(2456658.56250000)
>
> you are taking a SINGLE PRECISION literal constant (2456658.56250000)
> and converting it to double. So you still lose the extra information
> (the "X.X625") stuff, e.g.
>
> IDL> p=DOUBLE(2456658.56250000)
> IDL> help, p
> P DOUBLE = 2456658.5
> IDL> print, p, format='(f25.10)'
> 2456658.5000000000
>
> Now, if you declare a double precision literal constant, like so:
>
> IDL> p=2456658.56250000d0
> IDL> help, p
> P DOUBLE = 2456658.6
>
> you are "declaring" a double precision literal so you have about 15
> digits of precision-y goodness which you see when you print it out:
>
> IDL> print, p, format='(f25.10)'
> 2456658.5625000000
>
>
> Basically, if you ever declare literal constants in code where those
> values can have more than 8 useful digits, always use double precision
> declarations. Converting a single precision 8+ sig fig number to double
> will have no effect.
>
> Hope that helps.
>
> cheers,
>
> paulv

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