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?
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.
identity is included. –
Oreilly apply family of functions: ats.ucla.edu/stat/r/library/advanced_function_r.htm#apply . These functions are very powerful for manipulating data sets. –
Oreilly apply, ddply, aggregate, outer and a multitude of others). But I have never had the need for the identify function myself. So my question is still: in functional programming, why would you need an identify function? What is the practical use of it? PS. Your answer doesn't have to be specifically about R, since this seems like a more generic concept. –
Barfuss apply example provided by @gsk3 –
Barfuss 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
identity is used as a no-operation swtich to really make use of the transforming qualities of the function being called. Nice. –
Barfuss One use that appears on a simple code base search is as a convenience for the most basic type of error handling function in tryCatch.
tryCatch(...,error = identity)
which is identical (ha!) to
tryCatch(...,error = function(e) e)
So this handler would catch an error message and then simply return it.
try and tryCatch works. Thanks for the example and reference. –
Barfuss 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().
force –
Overreach 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.)
identity in this context apparently does not actually refer to base::identity. See the code in make.link; it's just matching the name "identity". –
Anthropocentric 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))
}
As this question has already been viewed 8k times it maybe worth updating even 9 years after it has been written.
In a blog post called "Simple tricks for Debugging Pipes (within magrittr, base R or ggplot2)" the author points out how identity() can be very usefull at the end of different kinds of pipes. The blogpost with examples can be found here: https://rstats-tips.net/2021/06/06/simple-tricks-for-debugging-pipes-within-magrittr-base-r-or-ggplot2/
If pipe chains are written in a way, that each "pipe" symbol is at the end of a line, you can exclude any line from execution by commenting it out. Except for the last line. If you add identity() as the last line, there will never be a need to comment that out. So you can temporarily exclude any line that changes the data by commenting it out.
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
I have just used it to split a matrix into the list of its columns. An example:
(m <- matrix(1:9, 3))
# [,1] [,2] [,3]
# [1,] 1 4 7
# [2,] 2 5 8
# [3,] 3 6 9
apply(m, 2, identity, simplify = FALSE)
# [[1]]
# [1] 1 2 3
#
# [[2]]
# [1] 4 5 6
#
# [[3]]
# [1] 7 8 9
Here, the function apply applies the identity function to each column of the matrix m, i.e. it just returns the columns. simplify = FALSE needed to keep it as a list rather than simplify back to an array.
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curve(identity(x))(rather than the slightly (?) more opaquecurve(x*1)orcurve(x+0)... – Exfoliatecurve(x)? – BarfussError in eval(expr, envir, enclos) : could not find function "x") becausecurveuses funny evaluation rules ... – Exfoliate