Search for specific line in R function body - r

Question

I wish to "copy and modify" a function at a specific point in its body. Currently, what I have is
nearest_psd <- function(mat) {
ed <- eigen(mat)
eigvecs <- ed$vectors
eigvals <- ed$values
eigvals[eigvals<0] <- 0
eigvecs %*% diag(eigvals) %*% t(eigvecs)
}
nearest_pd <- nearest_psd
formals(nearest_pd)$pdeps <- 1e-08
body(nearest_pd)[[c(7,3)]] <- quote(pdeps)
, so that nearest_pd is a copy of nearest_psd, except for the line eigvals[eigvals<0] <- pdeps.
However, the line number (7, in this case) is hard-coded, and I would prefer to have a robust way to determine this line number. How can I search for the line that contains the expression eigvals[eigvals<0] <- 0?

Answer 1

You can use identical to compare two expressions; that way, you can identify and replace the expression in question:
to_replace = vapply(body(nearest_pd), function (e) identical(e, quote(eigvals[eigvals < 0] <- 0)), logical(1L))
body(nearest_pd)[to_replace] = list(quote(eigvals[eigvals < pdeps] <- pdeps))
However, this is no more readable, nor more robust, than your code: in both cases you’re forced to hard-code the relevant information; in your code, the indices. In mine, the expression. For that reason I wouldn’t recommend using this.
… of course you could instead use an AST walker to replace all occurrences of 0 in the function’s body with pdeps. But is that better? No, since 0 could be used for other purposes. It currently isn’t, but who knows, once the original function changes. And if the original function can’t be assumed to change, why not hard-code the new function entirely? That is, write this:
nearest_pd <- function (mat, pdeps = 1e-08) {
ed <- eigen(mat)
eigvecs <- ed$vectors
eigvals <- ed$values
eigvals[eigvals < pdeps] <- pdeps
eigvecs %*% diag(eigvals) %*% t(eigvecs)
}
… no need to use metaprogramming just for the sake of it.

Answer 2

The following might do what you want.
nearest_psd <- function(mat) {
ed <- eigen(mat)
eigvecs <- ed$vectors
eigvals <- ed$values
eigvals[eigvals<0] <- 0
eigvecs %*% diag(eigvals) %*% t(eigvecs)
}
nearest_pd <- nearest_psd
formals(nearest_pd)$pdeps <- 1e-08
nearest_psd_body <- body(nearest_psd)
# Find the string we a re looking for and replace it ...
new.code <- gsub("eigvals[eigvals < 0] <- 0",
"MY_NEW_CODE",
nearest_psd_body, fixed = TRUE)
# Buidling the function body as a string.
new.code <- new.code[-1] # delete first { such that ...
new.code <- paste(new.code, collapse = ";") # we can collapse the remaining here ....
new.code <- paste("{", new.code, "}", sep = "", collapse = "") # and then wrap the remaining in { }
# parse returns an expression.
body(nearest_pd) <- parse(text = new.code)
See At a basic level, what does eval-parse do in R? for an explantion of parse. Or In programming, what is an expression? what an expression is.