问题
What's the most straight forward way of overloading '+' for characters?
I have defined '%+%' <- function(...) paste(...,sep=""):
str <- "aa"%+%"bb"%+%"cc" #str="aabbcc"
But I don't like the syntax. I think str <- "aa"+"bb"+"cc" would be nicer.
(I am building long SQL queries to use with RODBC, the usual paste is not very handy in such situations. Any suggestions?)
回答1:
I think that using two arguments is better than the dots:
'%+%' <- function(x,y) paste(x,y,sep="")
"a"%+%"b"%+%"C"
[1] "abC"
If you really really want to you can overwrite +, but be veeeeery careful when doing this as you will break one of the most important functions in R. I can't think of any reason why you would want to do that over %+%:
# '+' <- function(x,y) paste(x,y,sep="")
# "a"+"b"+"C"
# [1] "abC"
rm('+')
commented it out to be sure I don't accidently break someones R:)
回答2:
You may try something like that :
R> oldplus <- `+`
R> `+` <- function(e1, e2) {
R> if (is.character(e1) && is.character(e2)) {
R> paste(e1,e2,sep="")
R> }
R> else {
R> oldplus(e1,e2)
R> }
R> }
Which gives :
R> 2+3
[1] 5
R> "aa"+"bb"
[1] "aabb"
But as Sacha pointed out, overloading such a basic function is very dangerous, and I can't assure you it will not break your R session and make your computer explode :-)
回答3:
Why is the usual 'paste' not very handy? It's what it's meant for. Suggestions:
Write yourself an unusual paste function that does what you want. Maybe you just don't like typing 'sep=""' all the time. So write a function that calls paste with sep="". Or whatever.
Building long SQL queries with string concatenation is potential fail anyway. See http://xkcd.com/327/ for the canonical example.
Another possibility is some kind of templating solution. I've used the brew package in the past and it's great for that.
回答4:
You can find this operator in stringi package.
http://docs.rexamine.com/R-man/stringi/oper_plus.html
来源：https://stackoverflow.com/questions/5322546/operation-overloading-in-r