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

[Nick Cox <njcoxstata@gmail.com>]

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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