knitr::opts_chunk$set(collapse = TRUE, fig.asp = 0.7, fig.width = 7)
units:::units_options(negative_power = FALSE)
R has little support for physical measurement units. The exception
is formed by time differences: time differences objects of class
difftime
have a units
attribute that can be modified:
t1 = Sys.time() t2 = t1 + 3600 d = t2 - t1 class(d) units(d) d units(d) = "secs" d
We see here that the units
method is used to retrieve and modify the
unit of time differences.
The units
package generalizes this idea to other physical units, building upon the
udunits2 C library.
The udunits2
library provides the following operations:
m/s
is a valid physical unitm/s
and km/h
are convertibleThe units
R package uses the
udunits2 C library to extend
R with functionality for manipulating numeric vectors that have
physical measurement units associated with them, in a similar way as
difftime
objects behave.
We can set units to numerical values by set_units
:
library(units) (a <- set_units(runif(10), m/s))
the result, e.g.
set_units(10, m/s)
literally means "10 times 1 m divided by 1 s". In writing, the "1" values are omitted, and the multiplication is implicit.
When conversion is meaningful, such as hours to seconds or meters to kilometers, conversion can be done explicitly by setting the units of a vector
b = a units(b) <- make_units(km/h) b
Arithmetic operations verify units, and create new ones
a + a a * a a ^ 2 a ** -2
and convert to the units of the first argument if necessary:
a + b # m/s + km/h -> m/s
Currently, powers are only supported for integer powers, so using a ** 2.5
would result in an error.
There are some basic simplification of units:
t <- make_units(s) a * t
which also work when units need to be converted before they can be simplified:
t <- make_units(min) a * t
Simplification to unit-less values gives the "1" as unit:
m <- make_units(m) a * t / m
Allowed operations that require convertible units are +
, -
, ==
,
!=
, <
, >
, <=
, >=
. Operations that lead to new units are
*
, /
, and the power operations **
and ^
.
Mathematical operations allowed are: abs
, sign
, floor
,
ceiling
, trunc
, round
, signif
, log
, cumsum
, cummax
, cummin
.
signif(a ** 2 / 3, 3) cumsum(a) log(a) # base defaults to exp(1) log(a, base = 10) log(a, base = 2)
Summary functions sum
, min
, max
, and range
are allowed:
sum(a) min(a) max(a) range(a) make_units(min(m/s, km/h)) # converts to first unit:
Following difftime
, printing behaves differently for length-one vectors:
a a[1]
The usual subsetting rules work:
a[2:5] a[-(1:9)]
c(a,a)
concatenation converts to the units of the first argument, if necessary:
c(a,b) # m/s, km/h -> m/s c(b,a) # km/h, m/s -> km/h
difftime
From difftime
to units
:
t1 = Sys.time() t2 = t1 + 3600 d = t2 - t1 (du = as_units(d))
vice versa:
(dt = as_difftime(du)) class(dt)
matrix
objectsset_units(matrix(1:4,2,2), m/s) set_units(matrix(1:4,2,2), m/s * m/s)
but
set_units(matrix(1:4,2,2), m/s) %*% set_units(4:3, m/s)
strips units.
data.frame
sunits in data.frame
objects are printed, but do not appear in summary
:.
set.seed(131) d <- data.frame(x = runif(4), y = set_units(runif(4), s), z = set_units(1:4, m/s)) d summary(d) d$yz = with(d, y * z) d d[1, "yz"]
Units are often written in the form m2 s-1
, for square meter per second. This
can be defined as unit, and also parsed by as_units
:
(x = 1:10 * as_units("m2 s-1"))
udunits understands such string, and can convert them
y = 1:10 * make_units(m^2/s) x + y
Printing units in this form is done by
deparse_unit(x)
Base scatter plots and histograms support automatic unit placement
in axis labels. In the following example we first convert to
SI units. (Unit in
needs a bit special treatment, because in
is a
reserved word in R.)
mar = par("mar") + c(0, .3, 0, 0) displacement = mtcars$disp * as_units("in")^3 units(displacement) = make_units(cm^3) weight = mtcars$wt * 1000 * make_units(lb) units(weight) = make_units(kg) par(mar = mar) plot(weight, displacement)
We can change grouping symbols from [ ]
into ( )
:
units_options(group = c("(", ")") ) # parenthesis instead of square brackets par(mar = mar) plot(weight, displacement)
We can also remove grouping symbols, increase space between variable name and unit by:
units_options(sep = c("~~~", "~"), group = c("", "")) # no brackets; extra space par(mar = mar) plot(weight, displacement)
More complex units can be plotted either with negative powers, or as divisions,
by modifying one of units
's global options using units_options
:
gallon = as_units("gallon") consumption = mtcars$mpg * make_units(mi/gallon) units(consumption) = make_units(km/l) par(mar = mar) plot(displacement, consumption) # division in consumption units_options(negative_power = TRUE) # division becomes ^-1 plot(displacement, consumption) # division in consumption
As usual, units modify automatically in expressions:
units_options(negative_power = TRUE) # division becomes ^-1 par(mar = mar) plot(displacement, consumption) plot(1/displacement, 1/consumption)
units_options(negative_power = FALSE) # division becomes /
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