R：根据向量中具有相同名称的值从列表中选择元素

Question

[Probably this question already has an answer here, but I didn't manage to find one, also because I have some difficulty in formulating it concisely. Suggestions for reformulating the title of the question are appreciated.]

I have

• a list of matrices with different numbers of rows,
• a vector of integer values with the same names as the list's,
• a list of names that appear in the list and vector above,
• an integer variable telling which column to choose from those matrices.

Let's construct, as a working example:

mynames <- c('a', 'c')

mylist <- list(a=matrix(1:4,2,2), b=matrix(1:6,3,2), c=matrix(1:8,4,2))

myvec <- 2:4
names(myvec) <- names(mylist)

chooseCol <- 2

I'd like to construct a vector having as elements the rows taken from myvec and column chooseCol , for the names appearing in mynames . My attempt is

sapply(mynames, function(elem){mylist[[elem]][myvec[elem], chooseCol]})

which correctly yields

a c 
4 8 

but I was wondering if there's a faster, base (non- tidyverse ) method of doing this.

Also important or relevant: the order of the names in mylist and myvec can be different, so I can't rely on position indices.

Answer 1

I would use mapply -

mapply(function(x, y) x[y, chooseCol], mylist[mynames], myvec[mynames])

#a c 
#4 8