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From |
"Wallace, John" <John_Wallace@affymetrix.com> |

To |
"'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: forvalues and _N |

Date |
Fri, 3 Sep 2004 09:31:17 -0700 |

Thanks for the explanation, Nick. What I was seeking to do isn't all that different from how -encode- works, but I wanted to be in specific control of the association between the numeric variable and the alphanumeric label. I believe -encode- generates its new numeric variable in ascending alphabetic order of the encoded string values. The order the string values had in my original data set was structure I needed to retain, which I did using the following: . insheet using [tab-delimited text file] . keep v1 descriptions . gen pset = _n /*to record the initial order of observations*/ . move pset v1 . rename v1 probesetname /*the string data comes in with the v1 generic variable name*/ . forval e = 1 /`=_N'{ /*thanks to Fred Wolfe*/ .local pval = probesetname[`e'] .label define psetlab `e' "`pval'", add .} .label values pset psetlab /*the desired association*/ -JW -----Original Message----- Nick Cox Why this behaviour? In essence, -forvalues- expects a range, as defined by example in its syntax diagram. What is not quite spelled out in that syntax diagram is that all the elements of the range must be explicit. So forval i = 1/_N { ... } doesn't qualify. _N is not explicit enough; you are expecting Stata to evaluate _N. It is happy enough to do that in other contexts, but not this one. forval i = 1/`=_N' { ... } is a work-around. Stata's parser catches `=_N' and evaluates it and passes the result to -forvalues-, so what -forvalues- actually sees is 1/74, or whatever. This feature was introduced in Stata 7, but not documented until Stata 8. -display- is different, as part of its job is to evaluate before displaying. A quite different comment is that almost all loops over observations can be avoided, but I can't say whether your problem is like that. Nick n.j.cox@durham.ac.uk John Wallace > I have a problem using a forval loop to assign values to a > numeric variable > (for brevity, assume that -encode- doesn't work here). I > want to loop > through the entire dataset, addressing each value in turn, so > > .forval e = 1/_N > > would appear to be the appropriate form. I get an "invalid > syntax" error > when I do so. > > .di _N > > gives me the expected result for this dataset, 2339. > > .forval e = 1/2339 > > works as expected. > How am I screwing this up? Fred Wolfe > This will work: > > forval e = 1/`=_N' Nichols, Austin > That's a very handy tool, indeed. Cf. > > #delimit; > forval e = 1/5 {; di `e'; }; > forval e = 1/2+3 {; di `e'; }; > forval e = 1/`=2+3' {; di `e'; }; > > where the second example bombs for the same (?) reason > as forval e = 1/_N but the third runs fine. * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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