Function to subset dataframe using pattern list argument - regex

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.