Question

I have a pattern list

patternlist <- list('one' = paste(c('a','b','c'),collapse="|"), 'two' = paste(1:5,collapse="|"), 'three' = paste(c('k','l','m'),collapse="|"))

that I want to select from to extract rows from a data frame

dataframez <- data.frame('letters' = c('a','b','c'), 'numbers' = 1:3, 'otherletters' = c('k','l','m'))

with this function

pattern.record <- function(x, column="letters", value="one")
{
  if (column %in% names(x))
  {
   result <- x[grep(patternlist$value, x$column, ignore.case=T),]
  }
  else
  {
    result <- NA
  }
  return(result)
}

oddly enough, I get an error when I run it:

> pattern.record(dataframez)
 Error in grep(patternlist$value, x$column, ignore.case = T) : 
  invalid 'pattern' argument

Answer 1

The problem is your use of the `$` operator.

In your function, it is looking a column \ named element called column

It is far simpler here to use `[[`

Then x[[column]] uses what column is defined as, not column as a name.

The relevant lines in ?`$` are

Both [[ and $ select a single element of the list. The main difference is that $ does not allow computed indices, whereas [[ does. x$name is equivalent to x[[“name”, exact = FALSE]]. Also, the partial matching behavior of [[ can be controlled using the exact argument.

You are trying to use value and column as computed indices (i.e. computing what value and column are defined as), thus you need `[[`.

The function becomes

pattern.record <- function(x, column="letters", value="one", pattern_list)
{
  if (column %in% names(x))
  {
    result <- x[grep(pattern_list[[value]], x[[column]], ignore.case=T),]
  }
  else
  {
    result <- NA
  }
  return(result)
}

pattern.record(dataframez, patternlist = pattern_list)

##   letters numbers otherletters
## 1       a       1            k
## 2       b       2            l
## 3       c       3            m

note that I’ve also added an argumentpattern_list so it does not depend on an object named patternlist existing somewhere in the parent environments (in your case the global environment.