christophe dutang wrote:
Dear UseRs,
I'm trying to find variable names (string after the "mydata$") in a
expression. For example,
myexpr <- expression( ( mydata$variable1 / mydata$variable2 ) ^ 2 - 1 + 3 *
4 )
I would like to get "variable1" and "variable2". The following few lines
split the original character string into pieces.
mystring <- as.character(myexpr)
mydatapositions <- gregexpr("mydata", as.character(myexpr))[[1]]
mydataend <- mydatapositions + attr(mydatapositions, "match.length") +1
mydatabegin <- c(mydatapositions, nchar(mystring))
In this loop, I try to remove operator signs, spaces and brackets. But I
could not match the hat ^ in the string.
for(i in 1:length(mydatapositions ))
{
nomydata <- substr(mystring, mydataend[i], mydatabegin[i+1]-1)
nomydata <- gsub("\ ","",nomydata)
print(nomydata)
cat("_____\n")
res <- gregexpr("[+]",nomydata)
print(c(gsub("[+]","",nomydata), unlist(res)))
res <- gregexpr("[-]",nomydata)
print(c(gsub("[-]","",nomydata), unlist(res)))
res <- gregexpr("[/]",nomydata)
print(c(gsub("[/]","",nomydata), unlist(res)))
res <- gregexpr("[/]",nomydata)
print(c(gsub("[*]","",nomydata), unlist(res)))
res <- gregexpr(")",nomydata)
print(c(gsub(")","",nomydata), unlist(res)))
res <- gregexpr("\^",nomydata)
print(c(gsub("\^","",nomydata), unlist(res)))
print(gsub("[0-9]","",nomydata))
cat("-------------\n")
}
I get the following warnings telling me the character is not recognized but
I don't know how to solve the problem...
Warning messages:
1: '\^' is an unrecognized escape in a character string
2: unrecognized escape removed from "\^"
3: '\^' is an unrecognized escape in a character string
4: unrecognized escape removed from "\^"
Any help is welcome.
You need to put a backslash into the pattern. To do that, you double it
when you enter the string, e.g.
res <- gregexpr("\\^",nomydata)
For your original problem, you might find it more reliable to look
through the original expression code. mydata$variable1 parses to a list
with three elements: $, mydata, variable1, so a recursive search
through the expression might be more successful against strangely
formatted code or extremely long
expressions which may not deparse completely.
Duncan Murdoch
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
