Using Stata dynamic tags in a text file with the -dyntext- command
==================================================================

Let us consider an example where we study the -mpg- and -weight- variables in
-auto.dta-.  In our examples below, we will first write the commands so that
they will be displayed in our output text file.  Then, we will write the
commands so that Stata will process the Stata dynamic tags, displaying the
results of the Stata commands in the output text file.

We first use the -sysuse- command to load the dataset and then describe
the data using the -describe- command. 

<<dd_do>>
sysuse auto, clear
describe
<</dd_do>>

This produces the following Stata results:

. sysuse auto, clear
(1978 Automobile Data)

. describe

Contains data from /usr/local/stata15/ado/base/a/auto.dta
  obs:            74                          1978 Automobile Data
 vars:            12                          13 Apr 2016 17:45
 size:         3,182                          (_dta has notes)
--------------------------------------------------------------------------------
              storage   display    value
variable name   type    format     label      variable label
--------------------------------------------------------------------------------
make            str18   %-18s                 Make and Model
price           int     %8.0gc                Price
mpg             int     %8.0g                 Mileage (mpg)
rep78           int     %8.0g                 Repair Record 1978
headroom        float   %6.1f                 Headroom (in.)
trunk           int     %8.0g                 Trunk space (cu. ft.)
weight          int     %8.0gc                Weight (lbs.)
length          int     %8.0g                 Length (in.)
turn            int     %8.0g                 Turn Circle (ft.)
displacement    int     %8.0g                 Displacement (cu. in.)
gear_ratio      float   %6.2f                 Gear Ratio
foreign         byte    %8.0g      origin     Car type
--------------------------------------------------------------------------------
Sorted by: foreign


Now, we want to check if -mpg- is always greater than 0 and less than 100.
We use the -assert- command to perform the check.  In this case, we do not
want to include any output in the output text file, so we use the -quietly-
attribute to modify the behavior of the -dd_do- Stata dynamic tag.      

<<dd_do:quietly>>
assert mpg > 0 & mpg < 100
<</dd_do>>


If the data do not satisfy the conditions, -dyntext- will fail with an error
message, which will occur if we run the same -assert- command in a do-file. 

Next, we want to summarize the -weight- variable:

<<dd_do>>
summarize weight
<</dd_do>>

This produces the following in the output text file:

. summarize weight

    Variable |        Obs        Mean    Std. Dev.       Min        Max
-------------+---------------------------------------------------------
      weight |         74    3019.459    777.1936       1760       4840


We want to use the minimum and maximum values of -weight- in a sentence.
Instead of copying and pasting the numbers from the -summarize- output, we can
use the -dd_display- Stata dynamic tag with the -r(min)- and -r(max)-
stored results

The variable weight has minimum value <<dd_display: %4.2f `r(min)'>> and 
has maximum value <<dd_display: %4.2f `r(max)'>>.

which produces the following in the output text file:

> The variable weight has minimum value 1760.00
and has maximum value 4840.00.

The -dd_display- dynamic tag uses Stata's -display- command to evaluate
expressions.  It can be used as a calculator. For example, if we want to
include the range = max - min in a sentence, instead of calculating the
number and then copying and pasting it, we can use 

The variable weight has range <<dd_display: %4.2f `r(max)'-`r(min)'>>.

which produces the following in the output text file:

> The variable weight has range 3080.00.