Identifying variables based on their position in the dataset

[Aaditya Dar]

Hi,

I have multiple files and each has a different name for the first variable that needs to be renamed. How does one rename a variable based on its position in the dataset? (If I can do this, I can loop over the files.) The variable cannot be uniquely identified by it's type or value or format so I cannot use -ds-. I also tried -findname- but wasn't able to find a fix. Should I be looking at the -char- option more carefully? Or is there another way to solve this problem?

Thank you.

[William Lisowski]

Here is an example that defines the macro var3 as the third variable in the dataset. It is a start toward what you want, I believe.

. sysuse auto, clear
(1978 Automobile Data)

. describe, varlist

Contains data from /Applications/Stata/ado/base/a/auto.dta
  obs:            74                          1978 Automobile Data
 vars:            12                          13 Apr 2013 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

. local var3 : word 3 of `r(varlist)'

. display "`var3'"
mpg

.

[Mike Lacy]

Here's an illustration of one approach:

sysuse auto
local position = 4 // for example
// Get name of variable at this position in the data file
quiet ds
return list // just for pedagogical purposes
local name: word `position' of `r(varlist)'
di "Variable at position `position' is: `name'"

Regards, Mike

[Aaditya Dar]

I wasn't aware of -word-. Thanks once again!

[Nick Cox]

Another way to do it:

 
. sysuse auto
(1978 Automobile Data)

. unab varlist : *

. tokenize "`varlist'"

. di "`4'"
rep78

[daniel klein]

Also

sysuse auto
m : st_varname(1)

Best
Daniel

[Aaditya Dar]

Thanks, Nick and Daniel. A quick follow up question: when using -st_varname- what does 'm:' do? I looked at the help file but didn't understand the use of the prefix.

[Nick Cox]

 
m:

is minimal syntax for invoking single calls to Mata.

[daniel klein]

Thanks for clarifying, Nick. Sorry about the sloppy syntax.

Best
Daniel

[Aaditya Dar]

Got it, thanks!