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 · 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

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

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

User · 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  lt - 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  lt - 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

[r] Pattern matching using a wildcard

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