grep using a character vector with multiple patterns

Question

I am trying to use grep to test whether a vector of strings are present in an another vector or not, and to output the values that are present (the matching pat

Answer 1

Have you tried the match() or charmatch() functions?

Example use:

match(c("A1", "A9", "A6"), myfile$Letter)

Answer 2

In addition to @Marek's comment about not including fixed==TRUE, you also need to not have the spaces in your regular expression. It should be "A1|A9|A6".

You also mention that there are lots of patterns. Assuming that they are in a vector

toMatch <- c("A1", "A9", "A6")

Then you can create your regular expression directly using paste and collapse = "|".

matches <- unique (grep(paste(toMatch,collapse="|"), 
                        myfile$Letter, value=TRUE))

Answer 3

I suggest writing a little script and doing multiple searches with Grep. I've never found a way to search for multiple patterns, and believe me, I've looked!

Like so, your shell file, with an embedded string:

 #!/bin/bash 
 grep *A6* "Alex A1 Alex A6 Alex A7 Bob A1 Chris A9 Chris A6";
 grep *A7* "Alex A1 Alex A6 Alex A7 Bob A1 Chris A9 Chris A6";
 grep *A8* "Alex A1 Alex A6 Alex A7 Bob A1 Chris A9 Chris A6";

Then run by typing myshell.sh.

If you want to be able to pass in the string on the command line, do it like this, with a shell argument--this is bash notation btw:

 #!/bin/bash 
 $stingtomatch = "${1}";
 grep *A6* "${stingtomatch}";
 grep *A7* "${stingtomatch}";
 grep *A8* "${stingtomatch}";

And so forth.

If there are a lot of patterns to match, you can put it in a for loop.

Answer 4

Based on Brian Digg's post, here are two helpful functions for filtering lists:

#Returns all items in a list that are not contained in toMatch
#toMatch can be a single item or a list of items
exclude <- function (theList, toMatch){
  return(setdiff(theList,include(theList,toMatch)))
}

#Returns all items in a list that ARE contained in toMatch
#toMatch can be a single item or a list of items
include <- function (theList, toMatch){
  matches <- unique (grep(paste(toMatch,collapse="|"), 
                          theList, value=TRUE))
  return(matches)
}