Add an if in Mata mean function

[Chen Samulsion]

Dear Stata users,

In the following code, can we add an 'if' expression in mata command? Thank you.
For example, I want to caculate mean of ages only for urban respondents.

mata : all = st_data(., "age1 age2 age3 age4")
mata : mean(vec(all))

[Joseph Coveney]

st_data() has an optional scalar selectvar argument that you can use. If your dataset has an indicator variable in Stata that is 1 for urban respondents and 0 otherwise, then you can reference it in the third argument to st_data()

[Chen Samulsion]

Thank you Joseph Coveney My indicator variable is resident, 1 for city and 2 for urban. Can you make clear how to fix the code in #1? I try all the following codes and failed.

mata : all = st_data(., "y1d1 y1d2 y1d3 y1d4 y1d5 y1d6 y1d7 y1d8 y1d9 y1d10", if resident==1)
mata : all = st_data(., "y1d1 y1d2 y1d3 y1d4 y1d5 y1d6 y1d7 y1d8 y1d9 y1d10", "if resident==1")
mata : all = st_data(., "y1d1 y1d2 y1d3 y1d4 y1d5 y1d6 y1d7 y1d8 y1d9 y1d10", "resident==1")
mata : all = st_data(., "y1d1 y1d2 y1d3 y1d4 y1d5 y1d6 y1d7 y1d8 y1d9 y1d10", resident==1)

[Joseph Coveney]

My indicator variable is resident, 1 for city and 2 for urban. Can you make clear how to fix the code in #1?

Sure:

// One way
mata: stata("generate byte urban = resident == 1")
mata: mean(vec(st_data(., "y1d1 y1d2 y1d3 y1d4 y1d5 y1d6 y1d7 y1d8 y1d9 y1d10", "urban")))

// Another
mata: mean(vec(select(st_data(., "y1d1 y1d2 y1d3 y1d4 y1d5 y1d6 y1d7 y1d8 y1d9 y1d10"), st_data(., "resident") :== 1)))

What's the difference between city and urban?

[Chen Samulsion]

Joseph Coveney Thank you very much. That's great!

What's the difference between city and urban?

Sorry for my typo that has lead to confusion, it should be rural and urban.