How to avoid a loop in R: selecting items from a list

And one more approach:

t <- c("bob_smith","mary_jane","jose_chung","michael_marx","charlie_ivan")
pieces <- strsplit(t,"_")
sapply(pieces, "[", 1)

In words, the last line extracts the first element of each component of the list and then simplifies it into a vector.

How does this work? Well, you need to realise an alternative way of writing x[1] is "["(x, 1), i.e. there is a function called [ that does subsetting. The sapply call applies calls this function once for each element of the original list, passing in two arguments, the list element and 1.

The advantage of this approach over the others is that you can extract multiple elements from the list without having to recompute the splits. For example, the last name would be sapply(pieces, "[", 2). Once you get used to this idiom, it's pretty easy to read.

You can use apply (or sapply)

t <- c("bob_smith","mary_jane","jose_chung","michael_marx","charlie_ivan")
f <- function(s) strsplit(s, "_")[[1]][1]
sapply(t, f)

bob_smith    mary_jane   jose_chung michael_marx charlie_ivan 

       "bob"       "mary"       "jose"    "michael"    "charlie" 

See: A brief introduction to “apply” in R

How about:

tlist <- c("bob_smith","mary_jane","jose_chung","michael_marx","charlie_ivan")
fnames <- gsub("(_.*)$", "", tlist)
# _.* matches the underscore followed by a string of characters
# the $ anchors the search at the end of the input string
# so, underscore followed by a string of characters followed by the end of the input string

for the RegEx approach?