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I have been working with R for about 2 months and have had a little bit of trouble getting a hold of how the $ and %% terms.

I understand I can use the $ term to pull a certain value from a function (e.g. t.test(x)$p.value), but I'm not sure if this is a universal definition. I also know it is possible to use this to specify to pull certain data.

I'm also curious about the use of the %% term, in particular, if I am placing a value in between it (e.g. %x%) I am aware of using it as a modulator or remainder e.g. 7 %% 5 returns 2. Perhaps I am being ignorant and this is not real?

Any help or links to literature would be greatly appreciated.

Note: I have been searching for this for a couple hours so excuse me if I couldn't find it!

user227710
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Sean
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  • I am not entirely educated on all uses of '%%' but I know %in% is used for element inclusion and %*% is matrix multiplication. So maybe it is used for vector and matrix operations? – Lauren Goodwin Jul 09 '15 at 20:14
  • http://www.statmethods.net/management/operators.html - wrt 7 %% 5 – user2864740 Jul 09 '15 at 20:14
  • also %>% + variants are used as `pipes` (a coding method influenced by FSharp) in the `magrittr` package. Searching for that should get you started. – Chris Jul 09 '15 at 20:20
  • `'%in%` is defined on the `?match` page. Function with flanking %-signs are called "specials". The authors of magrittr and other packages that define "+" methods for graphical objects have defined additional functions which are being dispatched using the R class system which was originally being used for math or logical operations. This is usually called "overloading". The operator precedence is detailed in `?Syntax` help page. And the `?Ops` page might be interesting reading as well. – IRTFM Jul 09 '15 at 20:32
  • The `$` doesn't really fit, but the `%%` and `%*%` elements of this question are a dupe of [this r-faq](http://stackoverflow.com/q/12730629/903061). – Gregor Thomas Jul 09 '15 at 20:44

3 Answers3

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You are not really pulling a value from a function but rather from the list object that the function returns. $ is actually an infix that takes two arguments, the values preceding and following it. It is a convenience function designed that uses non-standard evaluation of its second argument. It's called non-standard because the unquoted characters following $ are first quoted before being used to extract a named element from the first argument.

 t.test  # is the function
 t.test(x) # is a named list with one of the names being "p.value"

The value can be pulled in one of three ways:

 t.test(x)$p.value
 t.test(x)[['p.value']]  # numeric vector
 t.test(x)['p.value']  # a list with one item

 my.name.for.p.val <- 'p.value'
 t.test(x)[[ my.name.for.p.val ]]

When you surround a set of characters with flanking "%"-signs you can create your own vectorized infix function. If you wanted a pmax for which the defautl was na.rm=TRUE do this:

 '%mypmax%' <- function(x,y) pmax(x,y, na.rm=TRUE)

And then use it without quotes:

> c(1:10, NA) %mypmax% c(NA,10:1)
 [1]  1 10  9  8  7  6  7  8  9 10  1
IRTFM
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First, the $ operator is for selecting an element of a list. See help('$').

The %% operator is the modulo operator. See help('%%').

Chris Watson
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The '$' operator is used to select particular element from a list or any other data component which contains sub data components. For example: data is a list which contains a matrix named MATRIX and other things too. But to get the matrix we write, Print(data$MATRIX)

The %% operator is a modulus operator ; which provides the remainder. For example: print(7%%3) Will print 1 as an output

Sunit Kumar
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