How to get line number of a function call in R? - r

Question

For debug purposes, I want to print a line number (and function name) of the place the current function was called from. How do I get this in R?
I've seen a solution of getting the source file name
But how to get the line number and function name?]
EDIT: I found how to get this data from traceback() in some form, traceback is able to print it out, but I am not sure how to decode the information out of it:
f <- function () {
traceback(x = 3, max.lines = 1)
}
g <- function()
{
f()
}
x <- g()
source("file.R") # file with this code
# 5: g() at file.R#20
# 4: eval(ei, envir)
# 3: eval(ei, envir)
# 2: withVisible(eval(ei, envir))
# 1: source("file.R")
str(x[[1]])
# chr "g()"
# - attr(*, "srcref")= 'srcref' int [1:8] 20 1 20 8 1 8 20 20
# ..- attr(*, "srcfile")=Classes 'srcfilecopy', 'srcfile' <environment: 0x0000000013a31700>

Answer 1

Found a solution! Got it from the code of traceback():
f <- function ()
{
x <- .traceback(x = 1)
srcloc <- if (!is.null(srcref <- attr(x[[1]], "srcref"))) {
srcfile <- attr(srcref, "srcfile")
paste0("Called from ", x[[2]], ", at ", basename(srcfile$filename), "#", srcref[1])
}
cat(srcloc, "\n")
}
g <- function()
{
f()
}
g()
# Called from g(), at file.R#15
Wrote a nice wrapper function for it:
# returns a list, unless fmtstring is specified
# level: 1 - caller of the caller of this function; 2 - its parent, 3 - its grand-parent etc.
# fmtstring: return format string: %f (function), %s (source file), %l (line)
#
# example: str <- caller_info("Called from %f at %s#%l\n")
# !!! it won't work with e.g. cat(caller_info("Called from %f at %s#%l\n"))
# or cat(paste0(caller_info("Called from %f at %s#%l\n"))) !!!
caller_info <- function (fmtstring = NULL, level = 1) # https://stackoverflow.com/q/59537482/684229
{
x <- .traceback(x = level + 1)
i <- 1
repeat { # loop for subexpressions case; find the first one with source reference
srcref <- getSrcref(x[[i]])
if (is.null(srcref)) {
if (i < length(x)) {
i <- i + 1
next;
} else {
warning("caller_info(): not found\n")
return (NULL)
}
}
srcloc <- list(fun = getSrcref(x[[i+1]]), file = getSrcFilename(x[[i]]), line = getSrcLocation(x[[i]]))
break;
}
if (is.null(fmtstring))
return (srcloc)
fmtstring <- sub("%f", paste0(srcloc$fun, collapse = ""), fmtstring)
fmtstring <- sub("%s", srcloc$file, fmtstring)
fmtstring <- sub("%l", srcloc$line, fmtstring)
fmtstring
}
This is how it's used:
f <- function ()
{
str <- caller_info("Called from %f at %s#%l\n")
cat(str)
}
The only (minor) limitation is that when called in subexpressions like cat(caller_info("Called from %f at %s#%l\n")) or cat(paste0(caller_info("Called from %f at %s#%l\n"))), R confusingly counts these subexpression things as stack levels, which messes it up. So better avoid the use of this wrapper in expressions.

Answer 2

There aren't easy functions to give you what you're asking for, but for debugging purposes you can call browser() in a function, then run the where command to see the current call stack. For example, you might see something like this:
where 1: calls()
where 2 at ~/temp/test.R#6: print(calls())
where 3 at ~/temp/test.R#9: f()
where 4: eval(ei, envir)
where 5: eval(ei, envir)
where 6: withVisible(eval(ei, envir))
where 7: source("~/temp/test.R", echo = TRUE)
This gives locations for a couple of the calls, but not all of them.
If you really want something that prints as you go (like the __LINE__ and __FILE__ macros in C/C++), it's a little harder. This prints the current location:
cat("This is line ", getSrcLocation(function() {}, "line"),
" of ", getSrcFilename(function() {}))
Not all functions have names, and R functions don't know what name you called them under, but you can see the current call using sys.call(). So this prints everything:
cat("This is line ", getSrcLocation(function() {}, "line"),
" of ", getSrcFilename(function() {}),
" called as", deparse(sys.call()),
"\n")
which might print
This is line 3 of test.R called as f()
sys.call has an argument to move up the stack, but I don't know of a way to get the line number information.
You can get the location of the start of the function that made the current call using
cat("Called from ", getSrcFilename(sys.function(-1)), " line ", getSrcLocation(sys.function(-1), "line"),
" as ", deparse(sys.call()), "\n")
which will show you the code that made the call, but the line number is only for the function it came from. It's a good argument for keeping your functions short!