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

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

Subject |
st: -foreach- and -for-, nested and parallel |

Date |
Wed, 16 Oct 2002 19:15:38 +0100 |

Buzz Burhans started a thread on problems with code including nested -foreach- structures, resolved to his satisfaction by Scott Merryman's suggestion. This is a postscript to that thread making some general and some specific points. Nested and parallel structures ============================== 1. If you nest two or more -foreach- loops, the total number of steps is the _product_ of the number of steps in the loop, so foreach i of num 1/2 { foreach j of num 1/3 { di "i is `i' and j is `j'" } } entails 2 * 3 steps in total: i is 1 and j is 1 i is 1 and j is 2 i is 1 and j is 3 i is 2 and j is 1 i is 2 and j is 2 i is 2 and j is 3 2. You can try doing this with nested -for-, but success is not guaranteed, for the kind of reasons discussed in another recent thread. It works in this case: for X in num 1/2 , noheader : for Y in num 1/3, noheader: di "i is X and j is Y" i is 1 and j is 1 i is 1 and j is 2 i is 1 and j is 3 i is 2 and j is 1 i is 2 and j is 2 i is 2 and j is 3 3. -for- by contrast is optimised for stepping through parallel structures. The total number of steps is just the number in any of the lists, as all must have the same length: for X in any i j k \ Y in num 1/3, noheader: di "X is Y" i is 1 j is 2 k is 3 4. How do you this with -foreach-? Sometimes one list is just a transformation of the other, so you can do the mapping on the fly. Suppose variables v3-v14 are to be renumbered v1-v12: foreach n of num 3/14 { local new = `v' - 2 rename v`n' v`new' } (Note here that -v3- has already been -rename-d -v1- when we try to -rename- -v5- as -v3-.) Perhaps more commonly, some such trick cannot be used, or cannot be identified. In essence then, -foreach- will step through one list and you have to ensure that it steps through any other list(s) at the same time. Buzz's example was a nice little illustration of a nasty little problem. To recap, he wanted to -rename- v3-v14 as preday18 preday15 preday12 preday9 preday7 preday6 preday5 preday4 preday3 preday2 preday1 preday0 for good reasons. One fairly transparent way to do that is local i = 0 local suffixes "18 15 12 9 7 6 5 4 3 2 1 0" foreach v of var v3-v14 { local i = `i' + 1 local new : word `i' of `suffixes' rename `v' preday`new' } But the same idea can be expressed more briefly, at the cost of introducing more programming stuff: tokenize "18 15 12 9 7 6 5 4 3 2 1 0" foreach v of var v3-v14 { rename `v' preday`1' mac shift } To understand the underlying machinery, you need to know or learn about local macros, -macro shift- and -tokenize-. The main idea is that -tokenize- puts the elements of 18 ... 0 in macros `1', `2', ... We use the first value and then shift them all up one, and loop around repeating that. 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**:**Re: st: -foreach- and -for-, nested and parallel***From:*Buzz Burhans <wsb2@cornell.edu>

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