Executing a repetitive command satisfying conditions

[Fabio Delisio]

Good morning everyone,
I have a dataset that looks like this:

* Example generated by -dataex-. For more info, type help dataex
clear
input str15 new_comm double price
"Apples"                        140
"Apples"                        232
"Apples"                        235
"Apples"                        238
"Apples"                        230
"Apples"                        280
"Apples"                        230
"Apples"                        246
"Apples"                        246
"Apples"                        290
"Apples"                        290
"Apples"                        325
"Apples"                          .
"Apples"                          .
"Apples"                          .
"Apples"                        500
"Apples"                        500
"Apples"                        500
"Apples"                        400
"Bananas"                       180
"Bananas"                       193
"Bananas"                       187
"Bananas"                       174
"Bananas"                       170
"Bananas"                       220
"Bananas"                       200
"Bananas"                       192
"Bananas"                       176
"Bananas"                       200
"Bananas"                       250
"Bananas"                       250
"Bananas"                         .
"Bananas"                         .
"Bananas"                         .
"Bananas"                       240
"Bananas"                       230
"Bananas"                       270
"Bananas"                       240
"Beans"                         260
"Beans"                         204
"Beans"                         187
"Beans"                         190
"Beans"                         160
"Beans"                         180
"Beans"                         180
"Beans"                         160
"Beans"                         160
"Beans"                         150
"Beans"                         190
"Beans"                         160
"Beans"                           .
"Beans"                           .
"Beans"                           .
"Beans"                         170
"Beans"                         205
"Beans"                         190
"Beans"                         180
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                           .
"Bread"                           .
"Bread"                           .
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Bread"                          10
"Oil (vegetable)"               120
"Oil (vegetable)" 115.4000015258789
"Oil (vegetable)" 115.4000015258789
"Oil (vegetable)" 115.4000015258789
"Oil (vegetable)"               116
"Oil (vegetable)"               112
"Oil (vegetable)"               112
"Oil (vegetable)"               114
"Oil (vegetable)"               114
"Oil (vegetable)"               114
"Oil (vegetable)"               115
"Oil (vegetable)"               120
"Oil (vegetable)"                 .
"Oil (vegetable)"                 .
"Oil (vegetable)"                 .
"Oil (vegetable)"               116
"Oil (vegetable)"               112
"Oil (vegetable)"               116
"Oil (vegetable)"               116
"Onions"                         88
"Onions"                        126
"Onions"                        149
"Onions"                         48
"Onions"                         40
end

Now, my aim is to run a command like

mdesc price if new_comm=="Rice"

for all the different categories of commodities. Since I have 20 commodities I thought about using a loop. I tried with

foreach x in new_comm {
          mdesc price
          }

but it doesn't do what I want.

[Nick Cox]

Quite. It's a common error to suppose that a loop in varname is a loop over the distinct values of a variable varname. It isn't.

Your loop is legal but will just execute mdesc once.

mdesc is from SSC, as you are asked to explain. As is prominent in the help it supports by: so

bysort new_comm : mdesc price

should do what you want.

sysuse auto, clear
 bysort foreign : missings report rep78 , percent

is an example of an equivalent command, and missings -- from the Stata Journal -- is more versatile than mdesc.

[Fabio Delisio]

Nick Cox thank you for the clarification!