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Re: st: Need help understanding stata graphics from http://www.survey-design.com.au/Stata%20Graphs.html.


From   Michael Stewart <michaelstewartresearch@gmail.com>
To   statalist <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Need help understanding stata graphics from http://www.survey-design.com.au/Stata%20Graphs.html.
Date   Mon, 8 Jul 2013 10:15:58 -0400

Dear Nick ,
Thanks a lot for your time and  such detailed answers.They are very educational.
Mike


On Mon, Jul 8, 2013 at 9:39 AM, Nick Cox <njcoxstata@gmail.com> wrote:
> The syntax you ask about is mostly not specific to graphics, so I've
> edited the post down to the core question.
>
> -help macro- indicates that one syntax has the form
>
> `= exp'
>
> where as usual exp is Stata's generic syntax for an expression
> (something that can be evaluated). For example, in
>
> `= 2+2'
>
> the exp is the expression
>
> 2 + 2
>
> The general pattern here is a twofold instruction: first evaluate the
> expression and then use the result of the expression. So, if presented
> with 2 + 2 as an expression within this syntax, Stata would evaluate
> it and place the result 4 at the position indicated.
>
> In terms of your questions 1)...3)
>
> 1) What is the utility of the -foreach- loop?
>
> There is no -foreach- loop. The -forvalues- loop loops over the
> observed values of the variable -x- which are the bar midpoints for
> the histogram.
>
> 2) What does the notation   i=1/`=r(N)' mean? Typically I  see macros
> like `r(N)'
>
> `=r(N)' is another example of the `=exp' syntax. The expression is
> r(N) and the `= '  force evaluation.  `r(N)' would in fact work
> equally well here.
>
> 3) How does local label `label'  `=string(x[`i'],"%8.2f")'  work?
>
> In a loop, take each bar midpoint and format it with two decimal
> places. The syntax uses yet another example of `=exp' and is
> schematically
>
> local label `label' `=exp'
>
> where exp is
>
> string(x[`i'],"%8.2f")
>
> which is an expression yielding a string result. The loop builds up a
> list of labels for the x axis containing the bar midpoints, displayed
> with two decimal places.
>
> Nick
> njcoxstata@gmail.com
>
> Michael Stewart [edited]
>
>> count if !missing(x)
>> forvalues i=1/`=r(N)' {
>>   local label `label'  `=string(x[`i'],"%8.2f")'
>>
>> }
>>
>> histogram mpg , bin(6) xlabel(`label')
>
>> I can't understand Stata commands after "count if !missing(x)"
>>
>> 1) What is the utility of the -foreach- loop?
>
>> 2) What does the notation   i=1/`=r(N)' mean? Typically I  see macros
>> like `r(N)'
>> 3) How does local label `label'  `=string(x[`i'],"%8.2f")'  work?
>>
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-- 
Thank you ,
Yours Sincerely,
Mike.
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