I said > you can attach an attribribute called ".Environment" to your object > that environment(object) will retrieve
You can also use environment(object) <- envir instead of object <- structure(object, .Environment = envir) to set the ".Environment" attribute to envir. This makes the code more symmetric and you don't have remember the magic attribute name. Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On > Behalf > Of William Dunlap > Sent: Monday, January 28, 2013 8:14 AM > To: Johannes Graumann; r-h...@stat.math.ethz.ch > Subject: Re: [R] parse/eval and character encoded expressions: How to deal > with non- > encoding strings? > > Instead of saving a string that can be parsed into a language object (and > later evaluated), I would just save something that that could be evaluated > directly. Note that a literal like "MyFile" or 3.14 evaluates to itself so > you > can save a literal string or a language object and use eval() on either. > Unless > you do this there is no good way to know if "TestDir/TestFile*.txt" means to > do some multiplication and division or to use a glob pattern to find a group > of files. > E.g., > > makeFoo <- function(file, ...) { > structure(c(...), fileExpr=substitute(file), class="Foo") > } > getFileFoo <- function(FooObject) { > eval(attr(FooObject, "fileExpr")) > } > > used as > > > z <- makeFoo(system.file(package="fpp", "DESCRIPTION"), 1, 2, 3) > > z > [1] 1 2 3 > attr(,"fileExpr") > system.file(package = "fpp", "DESCRIPTION") > attr(,"class") > [1] "Foo" > > getFileFoo(z) > [1] "C:/Program Files/R/R-2.15.2/library/fpp/DESCRIPTION" > or > > z <- makeFoo("C:/Temp/File.txt", 1, 2, 3) > > z > [1] 1 2 3 > attr(,"fileExpr") > [1] "C:/Temp/File.txt" > attr(,"class") > [1] "Foo" > > getFileFoo(z) > [1] "C:/Temp/File.txt" > > One thing you might worry about is in which environment the language object is > evaluated (the envir= argument to eval()). If you embed your object in a > formula > then environment(formula) tells you where to evaluate it. If the formula > syntax > is distracting then you can attach an attribribute called ".Environment" to > your object > that environment(object) will retrieve. E.g., > > makeFooEnv <- function(file, ..., envir = parent.frame()) { > fileExpr <- structure(substitute(file), .Environment = envir) > structure(c(...), fileExpr = fileExpr, class="Foo") > } > getFileFoo <- function(FooObject) { > fileExpr <- attr(FooObject, "fileExpr") > eval(fileExpr, envir=environment(fileExpr)) > } > > used as > > fooObjs <- lapply(1:3, function(i)makeFooEnv(paste0("File",i,".txt"), > i^2)) > > fooObjs[[2]] > [1] 4 > attr(,"fileExpr") > paste0("File", i, ".txt") > attr(,"fileExpr")attr(,".Environment") > <environment: 0x000000000b7f8998> > attr(,"class") > [1] "Foo" > > > > i <- 17 > > getFileFoo(fooObjs[[2]]) # Note that eval gets value of i as 2, not 17 > [1] "File2.txt" > > Bill Dunlap > Spotfire, TIBCO Software > wdunlap tibco.com > > > -----Original Message----- > > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On > > Behalf > > Of Johannes Graumann > > Sent: Sunday, January 27, 2013 11:29 PM > > To: r-h...@stat.math.ethz.ch > > Subject: [R] parse/eval and character encoded expressions: How to deal with > > non- > > encoding strings? > > > > Hi, > > > > I am intending to save a path-describing character object in a slot of a > > class I'm working on. In order to have the option to use "system.file" etc > > in such string-saved path definitions, I wrote this > > > > ExpressionEvaluator <- function(x){ > > x <- tryCatch( > > expr=base::parse(text=x), > > error = function(e){return(as.expression(x))}, > > finally=TRUE) > > return(x) > > } > > > > This produces > > > > > ExpressionEvaluator("system.file(\"INDEX\")") > > expression(system.file("INDEX")) > > > eval(ExpressionEvaluator("system.file(\"INDEX\")")) > > [1] "/usr/lib/R/library/base/INDEX" > > > > Which is what I want. However, > > > > > eval(ExpressionEvaluator("Test")) > > Error in eval(expr, envir, enclos) : object 'Test' not found > > > > prevents me from general usage (also in cases where "x" does NOT encode an > > expression). > > > > I don't understand why it is that > > > base::parse(text="Test") > > will return > > [1] expression(Test) > > while > > > as.expression("Test") > > produces > > [1] expression("Test") > > which would work with the eval call. > > > > Can anyone point out to me how to solve this generally? How can I feed the > > function a character object and get back an eval-able expression independent > > of whether there was an expression "encoded" in the input or not. > > > > Thank you for any hints. > > > > Sincerely, Joh > > > > ______________________________________________ > > 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. > > ______________________________________________ > 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. ______________________________________________ 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.
