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Re: st: The accuracy of the float data type
From
R Zhang <[email protected]>
To
[email protected]
Subject
Re: st: The accuracy of the float data type
Date
Fri, 24 Jan 2014 11:55:08 -0500
Thanks to you both, Sergiy and Nick .
Nick,
1.are you saying that I should follow Sergiy's advice to change
format? If so, given the large number of observations I have , how do
I automate the process?
2. if I do not change the format, I listed some observations below to
show you that sales and maxsale look the same, however, when I use" l
if sales == maxsale" it does not list all of the observations that
appear equal.
*****************
+--------------------+
| sales maxsale1 |
|--------------------|
1. | 25.395 25.395 |
2. | 32.007 32.007 |
3. | 53.798 53.798 |
4. | 12.748 12.748 |
5. | 13.793 13.793 |
..... omitted to save space
31. | 166.181 166.181 |
32. | 21.927 166.181 |
33. | 26.328 189.897 |
34. | 31.787 189.897 |
35. | 189.897 189.897 |
|--------------------|
36. | 264.582 264.582 |
37. | 33.61 264.582 |
38. | 312.227 312.227 |
39. | 35.413 312.227 |
40. | 406.36 406.36 |
|--------------------|
41. | 444.875 444.875 |
egen maxsale=max(sales), by (gvkey year)
l if sales == maxsale,
the first observation that is listed is 444.875 444.875 ,
why is that?
thanks!
On Fri, Jan 24, 2014 at 11:34 AM, Nick Cox <[email protected]> wrote:
> This is very good advice in general, but in this case the maxima are
> selected from the original values, so that equality is to be expected
> for some observations.
> Nick
> [email protected]
>
>
> On 24 January 2014 16:31, Sergiy Radyakin <[email protected]> wrote:
>> Zhang, avoid comparing floating point numbers for equality. Instead
>> there is a system variable c(epsfloat) , which you can refer to when
>> you need to deal with precision:
>>
>> clear
>> input float sales
>> 25.395
>> 32.007
>> end
>>
>> list
>>
>> display c(epsfloat)
>>
>> list if sales==25.395
>> list if abs(sales-25.395)<=10*c(epsfloat)
>>
>> list if sales==32.007
>> list if abs(sales-32.007)<=10*c(epsfloat)
>>
>>
>> Best, Sergiy Radyakin
>>
>> On Fri, Jan 24, 2014 at 11:23 AM, Maarten Buis <[email protected]> wrote:
>>> I would do this differently:
>>>
>>> *------------------ begin example ------------------
>>> // get some example data
>>> sysuse auto
>>>
>>> // create a variable denoting missing values
>>> gen byte miss = missing(rep78, price)
>>>
>>> // create our indicator variable
>>> bys rep78 miss (price) : gen max = _n == _N if !miss
>>>
>>> // admire the result
>>> list rep78 miss price max in 1/12, sepby(rep78)
>>> *------------------- end example -------------------
>>> * (For more on examples I sent to the Statalist see:
>>> * http://www.maartenbuis.nl/example_faq )
>>>
>>> Hope this helps,
>>> Maarten
>>>
>>>
>>> On Fri, Jan 24, 2014 at 4:53 PM, R Zhang <[email protected]> wrote:
>>>> Dear Statalist,
>>>>
>>>> my data structure is as follows
>>>>
>>>> firmID segmentID sales year
>>>> 1001 1 25.395 1990
>>>> 1001 1 32.007 1991
>>>>
>>>> ............
>>>>
>>>> a firm can operate in multiple segments as identified by segmentID .
>>>> I wanted to identify the largest segment by sales,so I used
>>>>
>>>> bysort firmID year : egen maxsale=max(sales)
>>>>
>>>> then I did
>>>> gen PriSIC=0
>>>> replace PriSIC=1 if sales=maxsale
>>>>
>>>> I got
>>>> firmID segmentID sales year maxsale prisic
>>>> 1001 1 25.395 1990 25.395 0
>>>> 1001 1 32.007 1991 32.007 0
>>>>
>>>> I could not figure out why prisic is 0, so I compute the diffderence
>>>> (sales-maxsale), it shows a very small negative number , and the data
>>>> dictionary shows sales format float %12.0g, and maxsale format float
>>>> %9.0g
>>>>
>>>> what should I do to correct this?
>>>>
>>>> thanks!!!
>>>>
>>>> Rochelle
>>>> *
>>>> * For searches and help try:
>>>> * http://www.stata.com/help.cgi?search
>>>> * http://www.stata.com/support/faqs/resources/statalist-faq/
>>>> * http://www.ats.ucla.edu/stat/stata/
>>>
>>>
>>>
>>> --
>>> ---------------------------------
>>> Maarten L. Buis
>>> WZB
>>> Reichpietschufer 50
>>> 10785 Berlin
>>> Germany
>>>
>>> http://www.maartenbuis.nl
>>> ---------------------------------
>>> *
>>> * For searches and help try:
>>> * http://www.stata.com/help.cgi?search
>>> * http://www.stata.com/support/faqs/resources/statalist-faq/
>>> * http://www.ats.ucla.edu/stat/stata/
>> *
>> * For searches and help try:
>> * http://www.stata.com/help.cgi?search
>> * http://www.stata.com/support/faqs/resources/statalist-faq/
>> * http://www.ats.ucla.edu/stat/stata/
> *
> * For searches and help try:
> * http://www.stata.com/help.cgi?search
> * http://www.stata.com/support/faqs/resources/statalist-faq/
> * http://www.ats.ucla.edu/stat/stata/
*
* For searches and help try:
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* http://www.stata.com/support/faqs/resources/statalist-faq/
* http://www.ats.ucla.edu/stat/stata/