What do the %op% operators in mean? For example "%in%"?

I didn't think GSee's or Sathish's answers went far enough because "%" does have meaning all by itself and not just in the context of the %in% operator. It is the mechanism for defining new infix operators by users. It is a much more general issue than the virtues of the %in% infix operator or its more general prefix ancestor match. It could be as simple as making a pairwise "s"(um) operator:

 `%s%` <- function(x,y) x + y

Or it could be more interesting, say making a second derivative operator:

 `%DD%` <- function(expr, nam="x") { D(D( bquote(.(expr)), nam), nam) }
 expression(x^4) %DD% "x"
 # 4 * (3 * x^2)

The %-character also has importance in the parsing of Date, date-time, and C-type format functions like strptime, formatC and sprintf.

Since that was originally written we have seen the emergence of the magrittr package with the dplyr elaboration that demonstrates yet another use for %-flanked operators.

So the most general answer is that % symbols are handled specially by the R parser. Since the parser is used to process plotmath expressions, you will also see extensive options for graphics annotations at the ?plotmath help page.


%op% denotes an infix binary operator. There are several built-in operators using %, and you can also create your own.

(A single % sign isn't a keyword in R. You can see a list of keywords on the ?Reserved help page.)

How do I get help on binary operators?

As with anything that isn't a standard variable name, you have to to enclose the term in quotes or backquotes.

?"%in%"
?`%in%`

Credit: GSee's answer.

What does %in% do?

As described on the ?`%in%` help page (which is actually the ?match help page since %in% is really only an infix version of match.),

[%in%] returns a logical vector indicating if there is a match or not for its left operand

It is most commonly used with categorical variables, though it can be used with numbers as well.

c("a", "A") %in% letters
## [1]  TRUE FALSE

1:4 %in% c(2, 3, 5, 7, 11)
## [1] FALSE  TRUE  TRUE FALSE

Credit: GSee's answer, Ari's answer, Sathish's answer.

How do I create my own infix binary operators?

These are functions, and can be defined in the same way as any other function, with a couple of restrictions.

  1. It's a binary opertor, so the function must take exactly two arguments.
  2. Since the name is non-standard, it must be written with quotes or backquotes.

For example, this defines a matrix power operator.

`%^%` <- function(x, y) matrixcalc::matrix.power(x, y)

matrix(1:4, 2) %^% 3

Credit: BondedDust's answer, Ari's answer.

What other % operators are there?

In base R:

%/% and %% perform integer division and modular division respectively, and are described on the ?Arithmetic help page.

%o% gives the outer product of arrays.

%*% performs matrix multiplication.

%x% performs the Kronecker product of arrays.

In ggplot2:

%+% replaces the data frame in a ggplot.

%+replace% modifies theme elements in a ggplot.

%inside% (internal) checks for values in a range.

%||% (internal) provides a default value in case of NULL values. This function also appears internally in devtools, reshape2, roxygen2 and knitr. (In knitr it is called %n%.)

In magrittr:

%>% pipes the left-hand side into an expression on the right-hand side.

%<>% pipes the left-hand side into an expression on the right-hand side, and then assigns the result back into the left-hand side object.

%T>% pipes the left-hand side into an expression on the right-hand side, which it uses only for its side effects, returning the left-hand side.

%,% builds a functional sequence.

%$% exposes columns of a data.frame or members of a list.

In data.table:

%between% checks for values in a range.

%chin% is like %in%, optimised for character vectors.

%like% checks for regular expression matches.

In Hmisc:

%nin% returns the opposite of %in%.

In devtools:

%:::% (internal) gets a variable from a namespace passed as a string.

In sp:

%over% performs a spatial join (e.g., which polygon corresponds to some points?)

In rebus:

%R% concatenates elements of a regex object.

More generally, you can find all the operators in all the packages installed on your machine using:

library(magrittr)

ip <- installed.packages() %>% rownames
(ops <- setNames(ip, ip) %>% 
  lapply(
    function(pkg)
    {
      rdx_file <- system.file("R", paste0(pkg, ".rdx"), package = pkg)
      if(file.exists(rdx_file))
      {
        rdx <- readRDS(rdx_file)
        fn_names <- names(rdx$variables)
        fn_names[grepl("^%", fn_names)]
      }
    }
  ) %>% 
  unlist
)

Put quotes around it to find the help page. Either of these work

> help("%in%")
> ?"%in%"

Once you get to the help page, you'll see that

‘%in%’ is currently defined as

‘"%in%" <- function(x, table) match(x, table, nomatch = 0) > 0’

Since time is a generic, I don't know what time(X2) returns without knowing what X2 is. But, %in% tells you which items from the left hand side are also in the right hand side.

> c(1:5) %in% c(3:8)
[1] FALSE FALSE  TRUE  TRUE  TRUE

See also, intersect

> intersect(c(1:5), c(3:8))
[1] 3 4 5

More generally, %foo% is the syntax for a binary operator. Binary operators in R are really just functions in disguise, and take two arguments (the one before and the one after the operator become the first two arguments of the function).

For example:

> `%in%`(1:5,4:6)
[1] FALSE FALSE FALSE  TRUE  TRUE

While %in% is defined in base R, you can also define your own binary function:

`%hi%` <- function(x,y) cat(x,y,"\n")
> "oh" %hi% "my"
oh my

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