Split and re-concatenate a string

Question

I am trying to get the host of an IP address from a list of strings.

ips <- c('140.112.204.42', '132.212.14.139', '31.2.47.93', '7.112.221.238')

I want to get the first 2 digits from the ips. output:

ips <- c('140.112', '132.212', '31.2', '7.112')

This is the code that I wrote to convert them:

cat(unlist(strsplit(ips, "\\.", fixed = FALSE))[1:2], sep = ".")

When I check the type of individual ips in the end I get something like this:

140.112 NULL

Not sure what I am doing wrong. If you have some other ideas completely different from this that is completely fine too.

Answer 1

With sub:

ips <- c('140.112.204.42', '132.212.14.139', '31.2.47.93', '7.112.221.238')

sub('\\.\\d+\\.\\d+$', '', ips)
# [1] "140.112" "132.212" "31.2"    "7.112"

With str_extract from stringr:

library(stringr)
str_extract(ips, '^\\d+\\.\\d+')
# [1] "140.112" "132.212" "31.2"    "7.112"

With strsplit + sapply:

sapply(strsplit(ips, '\\.'), function(x) paste(x[1:2], collapse = '.'))
# [1] "140.112" "132.212" "31.2"    "7.112"

With read.table + apply:

apply(read.table(textConnection(ips), sep='.')[1:2], 1, paste, collapse = '.')
#[1] "140.112" "132.212" "31.2"    "7.112"

Notes:

1. sub('\\.\\d+\\.\\d+$', '', ips):

   i. \\.\\d+\\.\\d+$ matches a literal dot, a digit one or more times, a literal dot again, and a digit one or more times at the end of the string

   ii. sub removes the above match from the string

2. str_extract(ips, '^\\d+\\.\\d+'):

   i. ^\\d+\\.\\d+ matches a digit one or more times, a literal dot and a digit one or more times in the beginning of the string

   ii. str_extract extracts the above match from the string

3. sapply(strsplit(ips, '\\.'), function(x) paste(x[1:2], collapse = '.')):

   i. strsplit(ips, '\\.') splits each ip using a literal dot as the delimiter. This returns a list of vectors after the split

   ii. With sapply, paste(x[1:2], collapse = '.') is applied to every element of the list, thus taking only the first two numbers from each vector, and collapsing them with a dot as the separator. sapply then coerces the list to a vector, thus returning a vector of the desired ips.

4. apply(read.table(textConnection(ips), sep='.')[1:2], 1, paste, collapse = '.'):

   i. read.table(textConnection(ips), sep='.')[1:2] treats ips as text input and reads it in with dot as a delimiter. Only taking the first two columns.

   ii. apply enables paste to be operated on each row, and collapses with a dot.