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From |
Nick Cox <[email protected]> |

To |
[email protected] |

Subject |
Re: st: trying to round values and store in a macro, problem with double/floating precision number storage |

Date |
Thu, 5 May 2011 19:22:04 +0100 |

There is yet a further bug, although it has been removed for other reasons. If the sd is put into a variable and then -summarize-d, it will not be returned in r(sd), but rather in r(mean). Woolton should look out for such bugs elsewhere in his code. Nick On Thu, May 5, 2011 at 7:19 PM, Nick Cox <[email protected]> wrote: > Maarten gave positive advice but some pitfalls here deserve more explanation. > > First, although -round(,)- allows non-integer second arguments, most > are useless or in practice misused. > > In fact I would say that the Stata documentation is very misleading on > this point. Tthe help for -round()- gives as an example (in Stata > 10.1; I am away from my main machine) > > "The rounding definition is generalized for y != 1. With y = .01, for > instance, x is rounded to two decimal places; round(sqrt(2),.01) is > 1.41." > > This just perpetuates a fallacy that is elsewhere painfully explained. > There can be no sense in which sqrt(2) _is_ 1.41 as such a number > cannot be held exactly in Stata or indeed any machine based on binary > arithmetic. > > It is true that e.g. > > . di round(sqrt(2),.01) > 1.41 > > but that is at least in part because the default display format works > as you want here. > > When people want to show a given number of decimal places, there is > only one way to do it directly and reliably: use an appropriate > display format. -round()- works poorly if at all for such purposes. > > Second, putting a numeric constant into a variable and then using > -summarize- to get it out again is unnecessary in two senses. Maarten > gave alternative code, but even setting aside the misuse of -round()- > these seven lines can be reduced to two: > > local mn`v'_snh`i' = round(r(mean),0.01); > local std`v'_snh`i' = round(r(sd),0.01); > > On Thu, May 5, 2011 at 4:44 PM, Woolton Lee <[email protected]> wrote: > > [snip] > gen float mn = round(r(mean),0.01); > gen float sd = round(r(sd),0.01); > sum mn; > local mn`v'_snh`i'= r(mean); > sum sd; > local std`v'_snh`i' = r(sd); > drop mn sd; > > [snip] > >> This loop creates the variables mn and sd and rounds the result of the >> numbers I want to two decimal places then uses summarize to store >> these values into macros. I wonder if there is another more efficient >> way to do this? I originally was doing all of the computations with >> only macros but I found that I could not round to two decimals without >> running into problems where a number like 55.87000000000000000001 >> would show up in my tables. Is there anyway to make this work doing >> the computations with just macros? > * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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