knitr::opts_chunk$set(collapse = TRUE)

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:

- validating whether an expression, such as
`m/s`

is a valid physical unit - verifying whether two units such as
`m/s`

and`km/h`

are convertible - converting values between two convertible units
- providing names and symbols for specific units
- handle different character encodings (utf8, ascii, iso-8859-1 and latin1)

The `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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