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

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

Subject |
RE: st: RE: if and if |

Date |
Fri, 14 Nov 2008 17:19:28 -0000 |

Further thoughts: Suppose _hypothetically_ that blocks if exp { } were to be interpreted observation by observation when <exp> in principle takes on a different value for each observation, as for example if varname == 2 { } Now 1. What happens if the data change during the { }, say some or all of the values of -varname- are changed, or -varname- is dropped? 2. What happens if you nest two or more of these say if varname == 2 { ... if anothervarname > 3 { ... } ... } Which observations are to be used in the inner loop? 3. What happens if we have two types of -if-s say if varname == 2 { ... ... if varname == 3 ... } Which observations are to be used in the inner statement? In essence, the key point is that no language feature is totally independent of any others. It's not that it would be impossible to work out rules for these circumstances; it's just that I suspect that on balance they would bite users -- often with highly mysterious bugs -- more than they helped them. Also, the more complicated code you wrote, the more likely you would be to get lost on what you are doing -- not that it's impossible now. Ashim in effect wants Stata to act as smartly as he can think when he is on form -- and so do we all. The trouble is that software has to be written that does not make too easy for us to do very weird stupid things by accident. Nick n.j.cox@durham.ac.uk Nick Cox Dear Ashim: You make the same point and you should not be surprised that my reply is the same. Sure, you find this "natural" but you did not design Stata. The ambiguity of if exp ... meaning one thing when the result of exp is a single true or false value and another thing when the result is a variable's worth is not in my view a desirable feature. What's much more important is that Bill Gould disagreed with you in 1985. Those of us who grew up with Fortran, C or similar languages I suggest find it natural that the expression in if exp { is single-valued. So, whose taste prevails? It may not help much now, but my prediction is that once you have been using Stata for 17 years, you will have come to agree with Bill Gould's decision. (It should take much less.) Nick n.j.cox@durham.ac.uk Ashim Kapoor Dear Nick, I think it is lacking because : it seemed to me that it is natural to think that if ( var1==2) { replace var2==3 replace var3==4 } in this case the program should be SMART enough to do the replacements in corresponding observations and not only work in the 1st observations,etc . I mean if someone DOES NOT know Stata (but he does know that each var in stata is a column vector ) and the 1st observation rule, what would he think on the FIRST look at this code. Surely it would seem that the changes would be made in the corresponding observations. Seems intuitive to me. And it has everyday use in my programming. Maybe for other reasons as you point out the people who programmed it did not let it work that way and they may be right. On Thu, Nov 13, 2008 at 8:01 PM, Nick Cox <n.j.cox@durham.ac.uk> wrote: > This business of the two -if-s has confused many people. > > (Incidentally, I think -cond()- is a red herring as far as this question > is concerned. It is without doubt very useful, but it has no bearing on > the matter.) > > But what Ashim wants is, I think, not "in between" at all. He wants what > he calls Type 2 -- the -if- command -- to do the same as what he calls > Type 1 -- the -if- qualifier -- when both are based on tests on > variables. > > However, it doesn't, and no amount of hoping, wishing, pushing or > shoving will make it so -- apart from one borderline exception. > > His query raises the question of why Stata has the two constructs if > they are really identical. But they aren't. > > It often happens that people used to some construct in other languages > would like to see Stata behave that way, and vice versa, but that's the > way it is. If you go to users' meetings (e.g. today and tomorrow in San > Francisco), you can confront the developers and say "Why is Stata > designed like that?". Sometimes there is a compelling argument against > one syntax and for another; sometimes the answer is "We had to make a > choice, and we just liked that syntax". But no syntax can be based on > users knowing that they mean one thing in one context and one thing in > another. All syntaxes have to be based on the language implementation > having only one interpretation of what is meant. > > I have got to say, however, that the Stata documentation is not 100% > innocent here. If you look at -help ifcmd- it gives among other details > > ================= > Typical use: Example 3 > > program ... > ... > if x==1 local word "one" > else if x==2 local word "two" > else if x==3 local word "three" > else if x==4 local word "four" > else local word "big" > ... > end > ================== > > But this example is _only_ good programming style if -x- is a scalar. > (Arguably not even then, as a temporary name would be preferable.) And > that is not explained, or (to me) self-evident. > > Also, the fact that (again, for a variable x) > > if x == 1 ... > > will be taken as > > if x[1] == 1 ... > > is not explained in the help, although it is in [P] -if- and the FAQ > cited earlier in this thread. I think several people would find out > about this matter earlier and easier if the point were made in the help. > > > The borderline exception is thus that for a variable x > > if x == 1 ... > > is exactly equivalent to > > ... if x == 1 > > if and only if you have a single-observation dataset. > > There are languages that support something like > > if x == 1 ... > > as equivalent to > > ... if x == 1 > > -- doesn't SAS support something similar -- but that is in a context > that implies a loop over observations. Stata's way of supporting loops > over observations (think of the ways that -generate- and -replace- work) > is more usually implicit. > > As Ashim pointed out, Stata offers a concise alternative to > > replace j=2 if k==2 > replace m=2 if k==2 > replace n=2 if k==2 > > It is > > foreach v of var j m n { > replace `v' = 2 if k == 2 > } > > (Ashim's own code here is illegal, by virtue of his omitting "var".) > > Alternatively, you can go > > foreach v in j m n { > > Short of Stata re-inventing itself as an ultra-terse language like APL > or J either seems to me about as concise as anyone might want. I really > don't understand why Ashim wants a "better way". What could be better, > or how is this lacking? > > Nick > n.j.cox@durham.ac.uk > > Ashim Kapoor > > I realize that there are 2 kinds if's in Stata. > > Type 1 : would be something like replace j = 2 if k==2 > here the replace in j would happen ONLY in the corresponding > observation of k. THIS IS WHAT I WANT. > > Type 2 : the programming if something like > > local j > > if `j'==2 { > > do something. > > } > > ************************************************************** > > I guess I want to do something which is in between the above 2. > > I want to say the following : -- > > replace j=2 if k==2 > replace m=2 if k==2 > replace n=2 if k==2 > > in ONE shot. > > so I try : - > ************************************** Block A > if k==2 { > replace j=2 > replace m=2 > replace n=2 > } > ********************************************* > This does not work. Because k==2 would mean k==2 in ALL observations. > While I mean to say make j / m / n = 2 in those observations where k > is 2. > > How do I do this in a quick manner in Block A ? > > I understand that I can always do :- > > foreach var of j m n { > replace `var'=2 if k==2 > } > > But is there a better way ? > > The reason I want this is the following : - > > I want > > if k==2 { > replace j=1 > replace m=5 > replace n=89 > } > > so the above method fails as I do not have the SAME value 2 to be put > in each of j / m /n. > > Any slick way of doing this ? * * 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/

**Follow-Ups**:**Re: st: RE: if and if***From:*"Ashim Kapoor" <ashimkapoor@gmail.com>

**Re: st: RE: if and if***From:*"Ashim Kapoor" <ashimkapoor@gmail.com>

**References**:**st: if and if***From:*"Ashim Kapoor" <ashimkapoor@gmail.com>

**st: RE: if and if***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**Re: st: RE: if and if***From:*"Ashim Kapoor" <ashimkapoor@gmail.com>

**RE: st: RE: if and if***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

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