Pattern matching using a wildcard

Question

How do I identify a string using a wildcard   I ve found glob2rx  but I don t quite understand how to use it  I tried using the following code to pick the rows of the data frame that begin with the word blue     make data frame a  lt - data frame  x    c  red   blue1   blue2    red2       1 result  lt - subset a  x    glob2rx  blue         2 test   ls pattern   glob2rx  blue     result2  lt - subset a  x    test      3 result3  lt - subset a  x    pattern  blue        However  neither of these worked  I m not sure if I should be using a different function to try and do this

User · Accepted Answer

If you want to examine elements inside a dataframe you should not be using ls() which only looks at the names of objects in the current workspace (or if used inside a function in the current environment). Rownames or elements inside such objects are not visible to ls() (unless of course you add an environment argument to the ls(.)-call). Try using grep() which is the workhorse function for pattern matching of character vectors:

result <- a[ grep("blue", a$x) , ]  # Note need to use `a$` to get at the `x`

If you want to use subset then consider the closely related function grepl() which returns a vector of logicals can be used in the subset argument:

subset(a, grepl("blue", a$x))
      x
2 blue1
3 blue2

Edit: Adding one "proper" use of glob2rx within subset():

result <- subset(a,  grepl(glob2rx("blue*") , x) )
result
      x
2 blue1
3 blue2

I don't think I actually understood glob2rx until I came back to this question. (I did understand the scoping issues that were ar the root of the questioner's difficulties. Anybody reading this should now scroll down to Gavin's answer and upvote it.)

User · Answer

You re on the right track - the keyword you should be googling is Regular Expressions  R does support them in a more direct way than this using grep   and a few other alternatives    Here s a detailed discussion  http   www regular-expressions info rlanguage html

User · Answer

If you really do want to use wildcards to identify specific variables  then you can use a combination of ls   and grep   as follows      l   ls     vars with result  lt - l grep  result   l

User · Answer

glob2rx   converts a pattern including a wildcard into the equivalent regular expression  You then need to pass this regular expression onto one of R s pattern matching tools   If you want to match  blue   where   has the usual wildcard  not regular expression  meaning we use glob2rx   to convert the wildcard pattern into a useful regular expression    gt  glob2rx  blue     1    blue    The returned object is a regular expression   Given your data   x  lt - c  red   blue1   blue2    red2     we can pattern match using grep   or similar tools    gt  grx  lt - glob2rx  blue     gt  grep grx  x   1  2 3  gt  grep grx  x  value   TRUE   1   blue1   blue2   gt  grepl grx  x   1  FALSE  TRUE  TRUE FALSE   As for the selecting rows problem you posted   gt  a  lt - data frame x    c  red   blue1   blue2    red2     gt  with a  a grepl grx  x       1  blue1 blue2 Levels  blue1 blue2 red red2  gt  with a  a grep grx  x       1  blue1 blue2 Levels  blue1 blue2 red red2   or via subset      gt  with a  subset a  subset   grepl grx  x          x 2 blue1 3 blue2   Hope that explains what grob2rx   does and how to use it

User · Answer

You can also use package data table and it s Like function  details given below How to select R data table rows based on substring match  a la SQL like

[r] Pattern matching using a wildcard

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