在 R 中提取向量中的字符元素

Question

In character vectors like a,b,..., e below, I wonder how to extract the two character elements ie, "bmi" and "ch" ? (ie, desired_output_in_this_case = c("bmi","ch") )

The example below is just a toy example, the character elements can be anything else other than ch and bmi . I'm looking for a general solution.

I have tried the following solution ( unlist(stringr::str_extract_all(a, "bmi|ch")) ). BUT we should manually define "bmi|ch" in it to give the desired output. Thus, it's not a general solution.

a <- "bmi + ch | study"
b <- "bmi * ch | study"
c <- "bmi * ch - 1 | study"
d <- "bmi * ch + 0 | study"
e <- "bmi:ch + 0 | study"

Answer 1

Assume the vector v defined in the Note at the end. Then we can lapply over it using the indicated function. If the number of variables is always the same you could alternately use sapply giving a matrix.

lapply(sub("\\|.*", "", v), function(x) all.vars(parse(text = x)))

giving:

[[1]]
[1] "bmi" "ch" 

[[2]]
[1] "bmi" "ch" 

[[3]]
[1] "bmi" "ch" 

[[4]]
[1] "bmi" "ch" 

[[5]]
[1] "bmi" "ch" 

Note

a <- "bmi + ch | study"
b <- "bmi * ch | study"
c <- "bmi * ch - 1 | study"
d <- "bmi * ch + 0 | study"
e <- "bmi:ch + 0 | study"
v <- c(a, b, c, d, e)

Answer 2

This is a bit more complicated. I will just leave it here in case someone may find it interesting.

vecs<-list(a,b, c,d,e)
split_me<-Map(function(x) gsub("([a-z].*[a-z])(\\W.*)","\\1",x, 

perl=TRUE), vecs)
 lapply(split_me, function(x) 
  unlist(strsplit(gsub("\\s", "",x), "[+*:]")))

Result

[[1]]
[1] "bmi" "ch" 

[[2]]
[1] "bmi" "ch" 

[[3]]
[1] "bmi" "ch" 

[[4]]
[1] "bmi" "ch" 

[[5]]
[1] "bmi" "ch" 

Data

a <- "bmi + ch | study"
b <- "bmi * ch | study"
c <- "bmi * ch - 1 | study"
d <- "bmi * ch + 0 | study"
e <- "bmi:ch + 0 | study"
vecs<-list(a,b, c,d,e)