Better way of extracting string from expression

[hadley]

Here's a sketch from me and @wch

x <- quote({
  # This is a comment
  a +      b
})

src <- attr(x, "srcref")
src_1 <- src[[1]]
src_n <- src[[length(src)]]

lines <-  attr(src_1, "srcfile")$lines
lines <- lines[seq(src_1[1], src_n[3])]
lines[1] <- substring(lines[1], src_1[5] + 1, nchar(lines[1]))

if (lines[1] == "" || grepl("^\\s+$", lines[1])) {
  lines <- lines[-1]
}

[wch]

Note that attr(src_1, "srcfile")$lines gives a string with \n characters in it instead of a char vector where each element represents one line. Running the code in the above example has someNAs:

lines
# [1] "\n  # This is a comment\n  a +      b\n})\n"
# [2] NA                                           
# [3] NA  

So you need to add a strsplit():

x <- quote({
  # This is a comment
  a +      b
})

src <- attr(x, "srcref")
src_1 <- src[[1]]
src_n <- src[[length(src)]]

lines <-  attr(src_1, "srcfile")$lines
lines <-  strsplit(lines, "\n")[[1]]     # <-- Here
lines <- lines[seq(src_1[1], src_n[3])]
lines[1] <- substring(lines[1], src_1[5] + 1, nchar(lines[1]))

if (lines[1] == "" || grepl("^\\s+$", lines[1])) {
  lines <- lines[-1]
}
lines
# [1] "  # This is a comment" "  a +      b"         

Oddly, if you access the source ref before the lines, it causes the string to be split up into a char vector, as a side effect -- just like if you called strsplit() on it.

x <- quote({
  # This is a comment
  a +      b
})

src <- attr(x, "srcref")
src_1 <- src[[1]]
src_n <- src[[length(src)]]

srcref(attr(src_1, "srcfile"), c(1,1,1,1,1,1,1,1))  # <--- Here
lines <-  attr(src_1, "srcfile")$lines
lines <- lines[seq(src_1[1], src_n[3])]
lines[1] <- substring(lines[1], src_1[5] + 1, nchar(lines[1]))

if (lines[1] == "" || grepl("^\\s+$", lines[1])) {
  lines <- lines[-1]
}
lines
# [1] "  # This is a comment" "  a +      b"         

[jennybc]

It appears that attr(src[[1]], "srcfile")$fixedNewlines can be consulted to determine the split status.