Tuesday, January 29, 2013

Stata Syntax Command's Syntax

do file

* When programming Stata "programs" (loosely known as functions in R) it is often times very useful to use the "syntax" command to parse arguments.

* This command takes the arguments input into the command and uses them to control the function of the command.

* For example:

cap program drop example1
program define example1, rclass

  syntax [anything]
  * The [] tell stata that this argument is optional
  * The anything tells Stata that this argument can be of any form excluding a comma (,)

  di "`anything'"

  * The local function 1 returns the first argument in the command
  di "`1'"

  * While 2 returns the second
  di "`2'"

end

example1 hello world
example1 *
example1 2123 123213 hello world

* We can see the display changes as a result of how the arguments are structured

* Programmers can force the user to input only inputs of a particular structure.

* For instance:


cap program drop example2
program define example2, rclass

  syntax [varlist]
  * The [] tell stata that this argument is optional
  * The anything tells Stata that this argument can be of any form excluding a comma (,)

  di "`varlist'"

  * The local function 1 returns the first argument in the command
  di "`1'"

  * While 2 returns the second
  di "`2'"

  * Variables targetted in this manner can be modified:
  foreach v of varlist `varlist' {
    rename `v' r_`v'_rename
  }

end

clear

gen hello = "content of variable hello"
gen world = "content of variable world"

example2 hello world
* Which works because we created the variables hello and world

example2 hello world
* This does not work because the above command already renamed the variables hello and world

* There is a lot of trickiness involved in effectively programming the syntax command and unfortunately it can be very frustrating because the only feedback stata gives is "invalid syntax" when something goes wrong.

* Also, unfortunately stata does not have many actual coding examples in which syntax is displayed.

* However, I think the following example will go a long way to answering many questions.

* This is the start of a command that will simulate data and test different estimators on that data.

* It will also set default parameters if the user does not input parameters into the simulation directly.

* Before executing the command, the command will display to the user the parameter set to be estimated

cap program drop example3
program define example3, rclass

*
  syntax [anything] [, NGrp(numlist >0 integer) NInd(numlist >0 integer) ///
                    Rgrp(numlist integer) Udist(string) NEXogenous NENdogenous]
  * The first few letters being capital allows the user to abreviate the option to just those two or three letters.

  * Set defaults if the user has not defined any.
  if "`ngrp'"==""   local ngrp=50
  if "`nind'"==""   local nind=20
  if "`rgrp'"==""   local rgrp=0
  if "`udist'"==""  local udist="rnormal()*5"

  * This sets the local parameter exogenous to be equal to 1 by default unless the user specifies ontherwise
  local exogenous=1
  if "`nexogenous'"!="" local exogenous=0

  local endogenous=1
  if "`nendogenous'"!="" local endogenous=0

  di as text _newline "Simulation Paramaters"
  di "Number of groups: `ngrp'"
  di "Average number of individuals in each group: `nind'"
  di "Group size random (0 no, 1 yes): `rgrp'"
  di "Error distribution: `udist'"

  di _newline _c "Models estimated: "
  if `exogenous' == 1 di _c "EXOGENOUS"
  if `endogenous' == 1 di " ENDOGENOUS"
  if  `exogenous' == 0 & `endogenous' == 0 di "No model estimated!"

  ********************

  * The actual simulation

  ********************

  * End of the command
end

  example3 , ngrp(454)
  example3 , ng(454)
  * See how the abreviation is possible

  example3 , nex  udist(runifrom()*3)
  example3 , nex  udist(runifrom()*3) r(1)

  example3 , nex nen

* It is very useful to check out the "help syntax" file.

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