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Re: st: Re-assigning local macro values within loops after each iteration

From   Stephen Cranney <>
Subject   Re: st: Re-assigning local macro values within loops after each iteration
Date   Tue, 21 Aug 2012 11:21:33 -0600

All right, I think I've figured it out. Thanks everyone!

On Mon, Aug 20, 2012 at 2:42 PM, Nick Cox <> wrote:
> Sorry, but this sounds like the same question, and so my answer is
> essentially the same. A -forvalues- loop cannot be extended once it is
> set going; it can only be exited.
> A flavour of the code fragments here is that you are setting up a new
> variable for each individual, which seems weird to me. I get the
> impression that you are more used to programming in some other
> language and are only part-way translating to Stataish thinking. In
> Stata, a variable is also a vector, but Stata has many variable-scale
> operations and functions. Also, in practice, it is quite difficult to
> program moderate-sized problems in Stata unless you have successfully
> programmed several smaller-sized problems; there are too many things
> to learn at once. (I don't think, by the way, that Stata is a
> difficult language; I would say the same of any other language.)
> Yet another thing that may be causing difficulty is some confusion
> over when to use -in-, the -if- command and the -if- qualifier.
> A more positive piece of advice is from what I know of demographic
> theory it is eminently suitable to matrix formulations, which suggests
> that Mata not Stata would be a good vehicle for this.
> Nick
> On Mon, Aug 20, 2012 at 8:15 PM, Stephen Cranney
> <> wrote:
>> Is there any way that I can retain the forvalues i = 1/`N' {
>> syntax while using the while loop to make it continue generating after
>> first generation?  The `i' reference here is how I connect the child
>> to not only the parent, so I would like to keep it if I could, but the
>> parent's date-of-giving-birth to calculate the child's date-of-birth,
>> etc. I have a hard time seeing how I can make these connections while
>> relying on the while loop. Here's an example of the syntax I mean:
>>         replace generationnumber=generationnumber[`i']+1 if newvar`i'==1
>>         replace parentid= id[`i'] if newvar`i'==1
>>         replace birthdate= date[`i'] if newvar`i'==1
>>         replace birthmonth= month[`i'] if newvar`i'==1
>>         replace birthyear= year[`i'] if newvar`i'==1
>> In response to the second concern, since each data point is a unique
>> month/year/id combination, doubling it basically replicates the
>> individual parent's characteristics (which I then change with
>> reference to the `i' value that I want to retain). The Xnewvar
>> variable I create deals with the problem of replicating the
>> replications, so in the end I have, for example, 1000 observations for
>> person 1, and 1000 observations for each of her children.
>> Thanks again,
>> Stephen
>> On Mon, Aug 20, 2012 at 12:36 PM, Nick Cox <> wrote:
>>> The Catch-22 here is that telling us part of your problem and showing
>>> us part of your code is not enough for anyone to get a complete
>>> understanding of your problem, while telling us all of your problem
>>> and showing us all of your code would not really help, as you would be
>>> unlikely to get anyone to invest time in understanding it all.
>>> That said, there is an important misunderstanding here. The effect of
>>> local N = _N
>>> forvalues i = 1/`N' {
>>> is to set up a loop with a particular upper limit that is fixed when
>>> the -forvalues- loop starts. Changing the number of observations after
>>> the loop has begun makes no difference to that, unless you change the
>>> number in such a way that the loop terminates for other reasons. In
>>> effect, you are "compiling" the number of observations into your code
>>> when the loop starts, so that's fixed code.
>>> My suggestion is that you may find it easier to set this up as a -while- loop.
>>> local notdone = 1
>>> local N = _N
>>> while `notdone' {
>>> }
>>> with code within the loop to change
>>> local notdone = 0
>>> as appropriate. That certainly permits the number of observations to change.
>>> That said, I am curious about the line
>>> expand 2, gen(newvar`i')
>>> which doubles the number of observations, just because of one
>>> childbirth. Sounds like something out of the Alien* films to me, but
>>> nothing like human demography? Shouldn't that -expand- be limited to
>>> expansion of certain observations?
>>> Nick
>>> On Mon, Aug 20, 2012 at 7:02 PM, Stephen Cranney
>>> <> wrote:
>>>> I'm running a loop that loops over observations and generates new
>>>> observations with expand when one of the observations has a certain
>>>> value for one of the variables. (It's a population simulation that is
>>>> simulating a new person when each childbirth=1, each observation is a
>>>> month/year combination for a specific id). I initially set the macro
>>>> at N= _N, but I want to re-set the macro every time it loops, this way
>>>> the simulation can run past the second generation. As it is, it saves
>>>> the initial _N value and terminates when that value is reached, but I
>>>> would like to re-assign the macro so that it equals the value of _N
>>>> after each expansion, allowing it to run into multiple generations
>>>> until it terminates because of other parameters I've imposed.
>>>> I have tried to place the macro within the loop to see if it will
>>>> re-do it, but it does not seem to be working. My syntax so far:
>>>> local N = _N
>>>> forvalues i = 1/`N' {
>>>>         local N = _N
>>>>         if childbirth[`i'] == 1 {
>>>>         expand 2, gen(newvar`i')
>>>>         egen Xnewvar= rowtotal (newvar*)
>>>>         drop if Xnewvar>1
>>>>         drop if Xnewvar== 0 & generationnum!= 1
>>>>         drop Xnewvar
>>>>         replace generationnum=generationnum[`i']+1 if newvar`i'==1
>>>> ** A lot of code that differentiates the new child's demographic
>>>> characteristics from the parent's
>>>>     }
>>>> }
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