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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 19:19:11 +0100 |

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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