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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

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

Subject |
st: RE: RE: RE: create many variables at once |

Date |
Tue, 23 Jul 2002 16:46:01 +0100 |

Joly.Patrick@ic.gc.ca commented on various solutions with -for*-: > The above solutions reminded me that -for-'s functionality > hasn't quite been > subsumed by the introduction of -foreach- and -forvalues- > as I have been > prone to believe. In version 6, I was quite content using > the set of > looping commands (tokenize, while, etc.) and hadn't got to > learning -for- > just yet. Thus, when -foreach- and -forvalues- came about > with Stata 7, I > saw no reason to bother with it. But the solutions are excellent > illustrations that -for-'s sometimes results in more > succinct and, arguably, > elegant solutions as it avoids unnecessary looping. > > To right the wrong, I looked up for.hlp once more and noted small > inconsistency the examples provided. The last example reads: > > (begin excerpt) > . for any male female: save c:\data\X if sex=="X" > > will not work. for will think there are three commands: > > save c: > data > X if sex=="X" > > Instead, you must type > > . for any male female: save "c:\data\X" if sex=="X" > (end of excerpt) > > The last line won't work either since the syntax of save > doesn't allow the > [if exp] condition -- a non-feature that I, among others, > wouldn't mind > seeing implemented in -save- BTW. ;) (NB: I know it's been > discussed on the > list before) > I agree that the -for- solution is neater here for the problem posed. However, much of the engineering work behind -for- was done precisely to achieve this result. That is, -for- is optimised for what I think of as "parallel" problems: do something with the ith member of this list [ and the ith member of that list [ and the ith member of THAT list ... ] ] for i = 1, ..., I so that there are I results. In contrast, -foreach- and -forvalues- grow very well indeed with "nested" problems: do something with the ith member of this list [ and the jth member of that list [ and the kth member of that list ... ] ] for i = 1, ..., I j = 1, ..., J k = 1, ..., K ... so that there are I * J * K * ... results. On the whole, I think nested problems are more common in (my) Stata practice. On the face of it, it is difficult to devise a simple syntax to cope with both kinds of problems. Some would argue that it is difficult to devise a simple syntax to cope with parallel problems. Having said that, note that Kit Baum provided an approach using -foreach- to the original problem. Nick n.j.cox@durham.ac.uk * * 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/

**Follow-Ups**:**st: -for- and its fortes***From:*Ronan Conroy <rconroy@rcsi.ie>

**References**:**st: RE: RE: create many variables at once***From:*Joly.Patrick@ic.gc.ca

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