Constructing quines (self-reproducing functions)
Has anyone constructed a quine ("A program that generates a copy of its own source text as its complete output": http://www.nyx.net/~gthompso/quine.htm) in R? (The [quine] tag pulls up lots of examples in Python, Java, ... but apparently none in R.)
f <- function() { body() }
comes close:
> f()
{
body()
}
but lacks the name of the function.
How about the shortest possibility? Most obfuscated?
edit: from the variety of answers below, it seems that there are a variety of ways to define self-referentiality and the environment in which it must occur:
- within the R environment: function
->
function (@bill_080) - within the OS/shell environment: program
->
program [more or less equivalent to program->
text]: (@kohske) - other: function
->
text (@JoshUlrich, @James, problem as defined above)
Notes:
- The thread from R-help pointed out by @Spacedman (which seems to emphasize obfuscation over brevity) suggests that
identical(quine,quine())
is a good test case, although it's tricky because environments get carried along:identical(quine,quine(),ignore.environment=TRUE)
might be easier. - A recent (Oct 2015) blog post provides another answer ...开发者_运维技巧
This is the shortest I can come up with:
> "f" <- function() call("<-", "f", f)
> f()
"f" <- function ()
call("<-", "f", f)
Here is a real Quine, a program (not a function) that generates a copy of its own source text as its complete output.
On console,
# y1.R is a quine program
$ cat y1.R
(function(x){cat(x);cat('(');cat(intToUtf8(0x0022));cat(x);cat(intToUtf8(0x0022));cat(')')})("(function(x){cat(x);cat('(');cat(intToUtf8(0x0022));cat(x);cat(intToUtf8(0x0022));cat(')')})")
# execute y1.R and show output
$ /usr/bin/R --vanilla --slave < y1.R
(function(x){cat(x);cat('(');cat(intToUtf8(0x0022));cat(x);cat(intToUtf8(0x0022));cat(')')})("(function(x){cat(x);cat('(');cat(intToUtf8(0x0022));cat(x);cat(intToUtf8(0x0022));cat(')')})")
# save the output of the execution of y1
$ /usr/bin/R --vanilla --slave < y1.R > y2.R
# compare input and output -- exactly same.
$ diff y1.R y2.R
probably this is not the shortest one.
UPDATED:
and slightly shorter version:
(function(x){cat(x,'(',d<-intToUtf8(0x0022),x,d,')',sep='')})("(function(x){cat(x,'(',d<-intToUtf8(0x0022),x,d,')',sep='')})")
Using what body
does as inspiration, call
can be used to reproduce the calling command:
f <- function ()
{
call("<-", as.name(as.character(sys.calls()[[1]])), sys.function(sys.parent()))
}
Which outputs:
> f()
f <- function ()
{
call("<-", as.name(as.character(sys.calls()[[1]])), sys.function(sys.parent()))
}
If you want a function that returns a function.....maybe this?
junk <- function(...) {
function(...) {
structure(junk(...))
}
}
The output is:
> junk()
function(...) {
structure(junk(...))
}
<environment: 01ef8e50>
> boo <- junk(999)
> boo
function(...) {
structure(junk(...))
}
<environment: 020e1048>
>dput(boo)
function (...)
{
structure(junk(...))
}
While I'm not sure if this "counts" from a quine perspective (I stumbled across this question while trying to verify if it does), the script
function(){}
will output function(){}
. This works on the same principle as Joshua Ulrich's answer, just pared down to the essentials.
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