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Re: st: loop with egen


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: loop with egen
Date   Fri, 8 Feb 2013 14:00:47 +0000

If that's true, then

foreach v of var <varlist> {
       qui su `v'
       gen mean_`v' = r(mean)
       gen sd_`v' = r(sd)
}

is a more efficient way of doing it. -egen- has lots of stuff that is
irrelevant for your purpose.

I've found that for occasional graphics tricks it is helpful to put
variables into constants. Otherwise I think my "usually" is still
true.

On Fri, Feb 8, 2013 at 1:51 PM, Nahla Betelmal <nahlaib@gmail.com> wrote:
> Dear all
>
> many thanks Rich, and yes I had the variables in my command already.
> this was not the problem. The issue was with each variable new name.
>
> Thanks Nick, the loop works perfectly. I need to generate the
> constants as variables to use in further equations and calculations
> instead of writing them manually.
>
> Many thanks again
>
> Nahla
>
>
> On 8 February 2013 13:38, Nick Cox <njcoxstata@gmail.com> wrote:
>> I don't believe that was your code.
>>
>> I suspect that you wrote something of the form
>>
>> foreach v of var <varlist> {
>>       egen mean = mean(`v')
>>       egen sd = sd(`v')
>> }
>>
>> where instead of <varlist> you had a list of variable names.
>>
>> Second time around the loop, the problem is that -mean- already
>> exists, and so the loop fails.
>>
>> You could make this loop legal by ensuring that each new variable has
>> a different name
>>
>> foreach v of var <varlist> {
>>       egen mean_`v' = mean(`v')
>>       egen sd_`v'  = sd(`v')
>> }
>>
>> but that would still be a bad idea. There is usually little point in
>> generating lots of variables that just contain constants.
>>
>> su <varlist> will give you a list of means and standard deviations.
>>
>> For more precise control, consider
>>
>> . sysuse auto
>> (1978 Automobile Data)
>>
>> . tabstat mpg weight, s(mean sd) c(s) format(%2.1f)
>>
>>     variable |      mean        sd
>> -------------+--------------------
>>          mpg |      21.3       5.8
>>       weight |    3019.5     777.2
>> ----------------------------------
>>
>> Nick
>>
>> On Fri, Feb 8, 2013 at 1:29 PM, Nahla Betelmal <nahlaib@gmail.com> wrote:
>>
>>> I am trying to generate the mean and standard deviation for more than
>>> 30 variables. I tried to do that using the following loop
>>>
>>> foreach v of var {
>>> egen mean = mean(`v')
>>> egen sd = sd(`v')
>>> }
>>>
>>> however, it only generate the mean and standard deviation for the
>>> first variable only.
>>>
>>> I got this message :mean already defined
>>> r(110);
>>>
>>> what should I add to the loop , so it can generate the mean and
>>> standard deviation for each variable in separate column?
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