如何根据R中的两个不同列添加行的值？

Question

How do I make a code in R to add values of one variable for all the same composition of two different variables? For example, I want add all pop of cd: 403 county: 4017 /and all pop of cd :406 and county: 4017 separately.

cd  county  pop
403 4017    1474
403 4017    0
403 4017    869
403 4017    393
403 4017    773
403 4017    1108
403 4017    929
403 4017    730
403 4017    0
406 4017    0
406 4017    2982
406 4017    1254
406 4017    752
406 4017    153
406 4017    0
406 4017    0
406 4017    3775
406 4017    0
406 4017    777
406 4017    5923

If there is already answered question on this topic. What keyword should I use to google it?

Thanks in advance!

Answer 1

require(plyr)
ddply(df,.(cd,county),summarize,total=sum(pop))

   cd county total
1 403   4017  6276
2 406   4017 15616

Answer 2

The answer given by @Troy is probably what most R users would tell you (ie using plyr and ddply() .

However, as my first exposure to data analysis was through database scripting, I remain partial to the sqldf package for these sorts of tasks.

I also find SQL to be more transparent to non-R users (something I frequently encounter in the social science community where I do most of my work).

Here is a solution to your problem producing identical output using sqldf :

#your data assigned to dat
pop <- c(1474,0,869,393,773,1108,929,730,0
        ,0,2982,1254,752,153,0,0,3775,0
        ,777,5923)  
cd <- c(rep(403, 9), rep(406, 11))
county <- rep(4017, 20)

dat <- as.data.frame(cbind(cd, county, pop))

#load sqldf
require(sqldf)

#write a simple SQL aggregate query
#i.e. "select" your fields specifying the aggregate function for the 
#relevant field, "from" a table called dat, and "group by" cd and county
sqldf('select
        cd
        ,county
        ,sum(pop) as total
      from dat
      group by 
        cd
        ,county')

   cd county total
1 403   4017  6276
2 406   4017 15616