将列表对象转换为可用的矩阵名称（R）

Question

I want to be able to use a loop to perform the same funtion on a group of data sets without having to recall the name of all of the data sets individually. For example, say I have the following matricies:

a<-matrix(1:5,nrow=5,ncol=2)
b<-matrix(6:10,nrow=5,ncol=2)
c<-matrix(11:15,nrow=5,ncol=2)

I define a vector of set names:

SetNames<- c("a","b","c")

Then I want to sum the second column of all of the matricies without having to call each matrix name. Basically, I would like to be able to call SetNames[1] , have the program return 'a' as USEABLE text which can be used to call apply(a[2],2,sum) .

If apply(SetNames[1][2],2,sum) worked, that would be the basic syntax I was looking for, however I would replace the 1 with a variable I can increase in a loop.

Answer 1

sapply can do that.

sapply(SetNames, function(z) {
  dfz <- get(z)
  sum(dfz[,2])
})
#  a  b  c 
# 15 40 65 

Notice that get() is used here to dynamically access a variable.

a less compact way of writing this would be

sumRowTwo <- function(z) {
  dfz <- get(z)
  sum(dfz[,2])
}
sapply(SetNames, sumRowTwo)

and now you can play around with sumRowTwo and see what eg

sumRowTwo("a")

returns