如何使用apply函数取无名列的意思？

Question

I practiced the ?apply example in R documentation.

x<-cbind(x1=3,x2=c(4:1, 2:5))
dimnames(x)[[1]]<-letters[1:8]
apply(x,2,mean,trim = .2)

It is the outcome.

x1 x2 
 3  3

If i just want to take the x2's mean, like

x2
 3

so i try

apply(x,x[,2],mean)

and it is error:Error in if (d2 == 0L) { : missing value where TRUE/FALSE needed

so how could i do or improvement? thanks.

Answer 1

I'm not sure why you would want to use apply there. You can just do something like:

mean(x[, "x2"], trim = .2)
# [1] 3

If you really wanted to use apply , you can try something like:

apply(x[, "x2", drop = FALSE], 2, mean, trim = .2)
# x2 
#  3 

## apply(x[, 2, drop = FALSE], 2, mean, trim = .2)

In other words, select the columns you're interested in as your dataset, and then apply whatever function you want. Notice the use of drop = FALSE to prevent the single column from becoming a basic vector.