What is the practical use of the identity function

Question

Base R defines an identity function, a trivial identity function returning its argument (quoting from ?identity).

It is defined as :

identity <- function (x){x}

Why would such a trivial function ever be useful? Why would it be included in base R?

Answer 1

I use it from time to time with the apply function of commands.

For instance, you could write t() as:

dat <- data.frame(x=runif(10),y=runif(10))
apply(dat,1,identity)

       [,1]      [,2]      [,3]      [,4]      [,5]      [,6]       [,7]
x 0.1048485 0.7213284 0.9033974 0.4699182 0.4416660 0.1052732 0.06000952
y 0.7225307 0.2683224 0.7292261 0.5131646 0.4514837 0.3788556 0.46668331
       [,8]      [,9]      [,10]
x 0.2457748 0.3833299 0.86113771
y 0.9643703 0.3890342 0.01700427

Answer 2

Here is usage example:

    Map<Integer, Long> m = Stream.of(1, 1, 2, 2, 3, 3)
            .collect(Collectors.groupingBy(Function.identity(),
                    Collectors.counting()));
    System.out.println(m);
    output:
    {1=2, 2=2, 3=2}

here we are grouping ints into a int/count map. Collectors.groupingBy accepts a Function. In our case we need a function which returns the argument. Note that we could use e->e lambda instead

Answer 3

I just used it like this:

fit_model <- function(lots, of, parameters, error_silently = TRUE) {

  purrr::compose(ifelse(test = error_silently, yes = tryNA, no = identity),
                 fit_model_)(lots, of, parameters)
}

tryNA <- function(expr) {
  suppressWarnings(tryCatch(expr = expr,
                            error = function(e) NA,
                            finally = NA))
}

Answer 4

Stepping away from functional programming, identity is also used in another context in R, namely statistics. Here, it is used to refer to the identity link function in generalized linear models. For more details about this, see ?family or ?glm. Here is an example:

> x <- rnorm(100)
> y <- rpois(100, exp(1+x))
> glm(y ~x, family=quasi(link=identity))

Call:  glm(formula = y ~ x, family = quasi(link = identity))

Coefficients:
(Intercept)            x
      4.835        5.842

Degrees of Freedom: 99 Total (i.e. Null);  98 Residual
Null Deviance:      6713
Residual Deviance: 2993         AIC: NA

However, in this case parsing it as a string instead of a function will achieve the same: glm(y ~x, family=quasi(link="identity"))

EDIT: As noted in the comments below, the function base::identity is not what is used by the link constructor, and it is just used for parsing the link name. (Rather than deleting this answer, I'll leave it to help clarify the difference between the two.)

Answer 5

Don't know about R, but in a functional language one often passes functions as arguments to other functions. In such cases, the constant function (which returns the same value for any argument) and the identity function play a similar role as 0 and 1 in multiplication, so to speak.

Answer 6

For whatever it's worth, it is located in funprog.R (the functional programming stuff) in the source of the base package, and it was added as a "convenience function" in 2008: I can imagine (but can't give an immediate example!) that there would be some contexts in the functional programming approach (i.e. using Filter, Reduce, Map etc.) where it would be convenient to have an identity function ...

r45063 | hornik | 2008-04-03 12:40:59 -0400 (Thu, 03 Apr 2008) | 2 lines

Add higher-order functions Find() and Position(), and convenience
function identity().