This document intends to be a guide on how to work with quantities
data (magnitudes with units and/or uncertainty) in two distinct
workflows: R *base* and the so-called *tidyverse*. Units and
errors (and, by extension, quantities) objects essentially are numeric
vectors, arrays and matrices with associated metadata. This metadata is
not always compatible with some functions, and thus we here explore the
most common operations in data wrangling (subsetting, ordering,
transformations, aggregations…) to identify potential issues and propose
possible workarounds.

Let us consider the traditional `iris`

data set for this
exercise. According to its documentation,

`iris`

is a data frame with 150 cases (rows) and 5 variables (columns) named`Sepal.Length`

,`Sepal.Width`

,`Petal.Length`

,`Petal.Width`

, and`Species`

.

And values are provided in centimeters. If we consider, for instance, a 2% of uncertainty, the first step is to define proper quantities. Then we will work on the resulting data frame for the rest of this article.

```
library(quantities)
#> Loading required package: units
#> udunits database from /usr/share/udunits/udunits2.xml
#> Loading required package: errors
iris.q <- iris
for (i in 1:4)
quantities(iris.q[,i]) <- list("cm", iris.q[,i] * 0.02)
head(iris.q)
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> 1 5.1(1) [cm] 3.50(7) [cm] 1.40(3) [cm] 0.200(4) [cm] setosa
#> 2 4.9(1) [cm] 3.00(6) [cm] 1.40(3) [cm] 0.200(4) [cm] setosa
#> 3 4.70(9) [cm] 3.20(6) [cm] 1.30(3) [cm] 0.200(4) [cm] setosa
#> 4 4.60(9) [cm] 3.10(6) [cm] 1.50(3) [cm] 0.200(4) [cm] setosa
#> 5 5.0(1) [cm] 3.60(7) [cm] 1.40(3) [cm] 0.200(4) [cm] setosa
#> 6 5.4(1) [cm] 3.90(8) [cm] 1.70(3) [cm] 0.400(8) [cm] setosa
```

Note that, throughout this document, and unless otherwise stated, we
will talk about *quantities objects* as a shortcut for
*quantities, units and errors objects*.

In this section, we consider all the methods and functions included in the default packages, i.e., those that are automatically installed along with any R distribution:

```
#> [1] "base" "compiler" "datasets" "graphics" "grDevices" "grid"
#> [7] "methods" "parallel" "splines" "stats" "stats4" "tcltk"
#> [13] "tools" "utils"
```

Quantities objects have all the subsetting methods defined
(`[`

, `[[`

, `[<-`

,
`[[<-`

). Therefore they can be used in the same way as
with plain numeric vectors, and in conjunction with `which`

and other functions to perform subsetting. The `subset`

function is very handy too and achieves the same result:

```
iris.q[which(iris.q$Sepal.Length > set_quantities(75, mm)), ]
#> Warning: In '>' : boolean operators not defined for 'errors' objects,
#> uncertainty dropped
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> 106 7.6(2) [cm] 3.00(6) [cm] 6.6(1) [cm] 2.10(4) [cm] virginica
#> 118 7.7(2) [cm] 3.80(8) [cm] 6.7(1) [cm] 2.20(4) [cm] virginica
#> 119 7.7(2) [cm] 2.60(5) [cm] 6.9(1) [cm] 2.30(5) [cm] virginica
#> 123 7.7(2) [cm] 2.80(6) [cm] 6.7(1) [cm] 2.00(4) [cm] virginica
#> 132 7.9(2) [cm] 3.80(8) [cm] 6.4(1) [cm] 2.00(4) [cm] virginica
#> 136 7.7(2) [cm] 3.00(6) [cm] 6.1(1) [cm] 2.30(5) [cm] virginica
subset(iris.q, Sepal.Length > set_quantities(75, mm))
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> 106 7.6(2) [cm] 3.00(6) [cm] 6.6(1) [cm] 2.10(4) [cm] virginica
#> 118 7.7(2) [cm] 3.80(8) [cm] 6.7(1) [cm] 2.20(4) [cm] virginica
#> 119 7.7(2) [cm] 2.60(5) [cm] 6.9(1) [cm] 2.30(5) [cm] virginica
#> 123 7.7(2) [cm] 2.80(6) [cm] 6.7(1) [cm] 2.00(4) [cm] virginica
#> 132 7.9(2) [cm] 3.80(8) [cm] 6.4(1) [cm] 2.00(4) [cm] virginica
#> 136 7.7(2) [cm] 3.00(6) [cm] 6.1(1) [cm] 2.30(5) [cm] virginica
```

Note that another quantities object is defined for the comparison.
This is needed because different units are incomparable. Also note that
the first line throws a warning telling us that the uncertainty was
dropped for this operation. This kind of warning is thrown once, and
this is why `subset`

succeeds silently.

The `sort`

function, as its name suggests, *sorts*
vectors, and it is compatible with quantities:

```
iris.q$Sepal.Length[1:5]
#> Units: [cm]
#> Errors: 0.102 0.098 0.094 0.092 0.100
#> [1] 5.1 4.9 4.7 4.6 5.0
sort(iris.q$Sepal.Length[1:5])
#> Units: [cm]
#> Errors: 0.092 0.094 0.098 0.100 0.102
#> [1] 4.6 4.7 4.9 5.0 5.1
```

More generally, the `order`

function can be used for data
frame ordering:

```
head(iris.q[order(iris.q$Sepal.Length), ])
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> 14 4.30(9) [cm] 3.00(6) [cm] 1.10(2) [cm] 0.100(2) [cm] setosa
#> 9 4.40(9) [cm] 2.90(6) [cm] 1.40(3) [cm] 0.200(4) [cm] setosa
#> 39 4.40(9) [cm] 3.00(6) [cm] 1.30(3) [cm] 0.200(4) [cm] setosa
#> 43 4.40(9) [cm] 3.20(6) [cm] 1.30(3) [cm] 0.200(4) [cm] setosa
#> 42 4.50(9) [cm] 2.30(5) [cm] 1.30(3) [cm] 0.300(6) [cm] setosa
#> 4 4.60(9) [cm] 3.10(6) [cm] 1.50(3) [cm] 0.200(4) [cm] setosa
```

The `transform`

function is able to modify variables in a
data frame or to create new ones. The `within`

function
provides a similar but more flexible approach though. Both are fully
compatible with quantities:

```
head(within(iris.q, {
Sepal.Area <- Sepal.Length * Sepal.Width
Petal.Area <- Petal.Length * Petal.Width
rm(Sepal.Length, Sepal.Width, Petal.Length, Petal.Width)
}))
#> Species Petal.Area Sepal.Area
#> 1 setosa 0.280(8) [cm^2] 17.8(5) [cm^2]
#> 2 setosa 0.280(8) [cm^2] 14.7(4) [cm^2]
#> 3 setosa 0.260(7) [cm^2] 15.0(4) [cm^2]
#> 4 setosa 0.300(8) [cm^2] 14.3(4) [cm^2]
#> 5 setosa 0.280(8) [cm^2] 18.0(5) [cm^2]
#> 6 setosa 0.68(2) [cm^2] 21.1(6) [cm^2]
```

Row aggregation is the process of summarising data based on some grouping variable(s). There are several ways of working with data split by factors in R base, and, although they tend to preserve classes, they are generally not very kind to other metadata (i.e., attributes) by default.

In the following example, the average `Sepal.Length`

is
computed per `Species`

, but the metadata gets dropped:

```
tapply(iris.q$Sepal.Length, iris.q$Species, mean)
#> setosa versicolor virginica
#> 5.006 5.936 6.588
```

Many of these functions include a `simplify`

parameter
which, if set to `FALSE`

, preserves quantities metadata:

```
(sepal.length.agg <-
tapply(iris.q$Sepal.Length, iris.q$Species, mean, simplify=FALSE))
#> $setosa
#> 5.0(1) [cm]
#>
#> $versicolor
#> 5.9(1) [cm]
#>
#> $virginica
#> 6.6(1) [cm]
```

The only drawback is that the result is a list, and such a list must be unlisted with care, otherwise, metadata gets dropped again:

```
# drops quantities
unlist(sepal.length.agg)
#> setosa versicolor virginica
#> 5.006 5.936 6.588
# preserves quantities
do.call(c, sepal.length.agg)
#> Units: [cm]
#> Errors: 0.10012 0.11872 0.13176
#> setosa versicolor virginica
#> 5.006 5.936 6.588
```

The `by`

function is an object-oriented wrapper for
`tapply`

applied to data frames which also provides a
`simplify`

parameter. A more convenient way of working with
summary statistics is the `aggregate`

generic, from the
`stats`

namespace. Although there is a
`aggregate.data.frame`

method, there is a more intuitive
interface to it through the `aggregate.formula`

method.
Again, it is necessary to set `simplify=FALSE`

to keep
quantities:

```
(iris.q.agg <- aggregate(. ~ Species, data = iris.q, mean, simplify=FALSE))
#> Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> 1 setosa 5.006 3.428 1.462 0.246
#> 2 versicolor 5.936 2.77 4.26 1.326
#> 3 virginica 6.588 2.974 5.552 2.026
```

Apparently, the output has no metadata associated, but what really happens is that the resulting columns are lists:

Therefore, as in the `tapply`

/`by`

case, they
must be unlisted with care to still preserve the metadata:

```
unlist_quantities <- function(x) {
stopifnot(is.list(x) || is.data.frame(x))
unlist <- function(x) {
if (any(class(x[[1]]) %in% c("quantities", "units", "errors")))
do.call(c, x)
else x
}
if (is.data.frame(x))
as.data.frame(lapply(x, unlist), col.names=colnames(x))
else unlist(x)
}
unlist_quantities(iris.q.agg)
#> Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> 1 setosa 5.0(1) [cm] 3.43(7) [cm] 1.46(3) [cm] 0.25(1) [cm]
#> 2 versicolor 5.9(1) [cm] 2.77(6) [cm] 4.26(9) [cm] 1.33(3) [cm]
#> 3 virginica 6.6(1) [cm] 2.97(6) [cm] 5.6(1) [cm] 2.03(4) [cm]
```

And this method works for the `tapply`

/`by`

case too:

Joining data frames by common columns can done with the
`merge`

generic. Such operations are based on appending
columns, which may be subset or replicated to fit the length of the
merged observations. Therefore, quantities should be preserved in all
cases. In the following example, we generate a data frame with the
height per species and then merge it with the main data set:

```
height <- data.frame(
Height = set_quantities(c(55, 60, 45), cm, c(45, 30, 35)),
Species = c("setosa", "virginica", "versicolor")
)
head(merge(iris.q, height))
#> Species Sepal.Length Sepal.Width Petal.Length Petal.Width Height
#> 1 setosa 5.1(1) [cm] 3.50(7) [cm] 1.40(3) [cm] 0.200(4) [cm] 60(40) [cm]
#> 2 setosa 4.9(1) [cm] 3.00(6) [cm] 1.40(3) [cm] 0.200(4) [cm] 60(40) [cm]
#> 3 setosa 4.70(9) [cm] 3.20(6) [cm] 1.30(3) [cm] 0.200(4) [cm] 60(40) [cm]
#> 4 setosa 4.60(9) [cm] 3.10(6) [cm] 1.50(3) [cm] 0.200(4) [cm] 60(40) [cm]
#> 5 setosa 5.0(1) [cm] 3.60(7) [cm] 1.40(3) [cm] 0.200(4) [cm] 60(40) [cm]
#> 6 setosa 5.4(1) [cm] 3.90(8) [cm] 1.70(3) [cm] 0.400(8) [cm] 60(40) [cm]
```

The `reshape`

function, from the `stats`

namespace, provides an interface for both pivoting and unpivoting (i.e.,
*tidyfying* data). In the case of the `iris`

data set,
we would say that it is in the *wide format*, because each row
has more than one observation.

This function has a quite peculiar nomenclature. First of all, the
unpivoting operation is accessed by providing the argument
`direction="long"`

. We need to define the
`varying`

columns (columns to unpivot), as character or
indices, and they are unpivoted based on their names. By default, the
separator `sep="."`

is used, which means that
`Sepal.Width`

will be broken down into `Sepal`

and
`Width`

, and the former will be unpivoted with the latter as
grouping variable. We can specify the name of the grouping variable with
the `timevar`

argument.

Putting everything together, this is how to unpivot the data set by
the dimension (which we will call it `dim`

) of the
petal/sepal:

```
long.1 <- reshape(iris.q, varying=1:4, timevar="dim", idvar="dim.id", direction="long")
head(long.1)
#> Species dim Sepal Petal dim.id
#> 1.Length setosa Length 5.1(1) [cm] 1.40(3) [cm] 1
#> 2.Length setosa Length 4.9(1) [cm] 1.40(3) [cm] 2
#> 3.Length setosa Length 4.70(9) [cm] 1.30(3) [cm] 3
#> 4.Length setosa Length 4.60(9) [cm] 1.50(3) [cm] 4
#> 5.Length setosa Length 5.0(1) [cm] 1.40(3) [cm] 5
#> 6.Length setosa Length 5.4(1) [cm] 1.70(3) [cm] 6
```

It can be noted that the unpivoting also generates an index to
indentify multiple records from the same group. We have changed the name
of that identifier to `dim.id`

(just `id`

by
default).

We can further unpivot sepal and petal as the `part`

of
the flower. First, we need to prepend a common identifier to columns 3
and 4, which are to be unpivoted:

```
names(long.1)[3:4] <- paste0("value.", names(long.1)[3:4])
long.2 <- reshape(long.1, varying=3:4, timevar="part", idvar="part.id", direction="long")
head(long.2)
#> Species dim dim.id part value part.id
#> 1.Sepal setosa Length 1 Sepal 5.1(1) [cm] 1
#> 2.Sepal setosa Length 2 Sepal 4.9(1) [cm] 2
#> 3.Sepal setosa Length 3 Sepal 4.70(9) [cm] 3
#> 4.Sepal setosa Length 4 Sepal 4.60(9) [cm] 4
#> 5.Sepal setosa Length 5 Sepal 5.0(1) [cm] 5
#> 6.Sepal setosa Length 6 Sepal 5.4(1) [cm] 6
```

And the final result has one tidy observation per row.

The pivoting operation can be accessed by providing the argument
`direction="wide"`

. The process is almost symmetrical, but we
need to specify `v.names`

, as character, instead of
`varying`

columns. First, we can pivot by flower part:

```
wide.1 <- reshape(long.2, v.names="value", timevar="part", idvar="part.id", direction="wide")
head(wide.1)
#> Species dim dim.id part.id value.Sepal value.Petal
#> 1.Sepal setosa Length 1 1 5.1(1) [cm] 1.40(3) [cm]
#> 2.Sepal setosa Length 2 2 4.9(1) [cm] 1.40(3) [cm]
#> 3.Sepal setosa Length 3 3 4.70(9) [cm] 1.30(3) [cm]
#> 4.Sepal setosa Length 4 4 4.60(9) [cm] 1.50(3) [cm]
#> 5.Sepal setosa Length 5 5 5.0(1) [cm] 1.40(3) [cm]
#> 6.Sepal setosa Length 6 6 5.4(1) [cm] 1.70(3) [cm]
```

Then, we remove `"value."`

from the column names and pivot
by dimension (note that indices are removed to match the initial data
frame):

```
names(wide.1)[5:6] <- sub("value\\.", "", names(wide.1)[5:6])
wide.2 <- reshape(wide.1, v.names=c("Sepal", "Petal"), timevar="dim", idvar="dim.id", direction="wide")
#> Warning in reshapeWide(data, idvar = idvar, timevar = timevar, varying =
#> varying, : some constant variables (part.id) are really varying
wide.2$dim.id <- NULL
wide.2$part.id <- NULL
head(wide.2)
#> Species Sepal.Length Petal.Length Sepal.Width Petal.Width
#> 1.Sepal setosa 5.1(1) [cm] 1.40(3) [cm] 3.50(7) [cm] 0.200(4) [cm]
#> 2.Sepal setosa 4.9(1) [cm] 1.40(3) [cm] 3.00(6) [cm] 0.200(4) [cm]
#> 3.Sepal setosa 4.70(9) [cm] 1.30(3) [cm] 3.20(6) [cm] 0.200(4) [cm]
#> 4.Sepal setosa 4.60(9) [cm] 1.50(3) [cm] 3.10(6) [cm] 0.200(4) [cm]
#> 5.Sepal setosa 5.0(1) [cm] 1.40(3) [cm] 3.60(7) [cm] 0.200(4) [cm]
#> 6.Sepal setosa 5.4(1) [cm] 1.70(3) [cm] 3.90(8) [cm] 0.400(8) [cm]
```

We have seen that quantities have been correctly preserved through the whole process. Finally, we can check whether both data frames are identical. Given that the order of columns have changed, we can simply check this column name by column name and then put everything together:

Quantities support R base scatterplots out of the box: errors are displayed as segments around each point and units are automatically added to the corresponding axis label.

Here is an example of a simple plot of a single quantity, where each value is automatically indexed in the x-axis:

X-Y scatterplots support units and errors in both axes:

```
# x-y plots
with(iris.q, plot(Sepal.Length, Sepal.Width, col=Species))
# dataframe plots
plot(iris.q[, c("Sepal.Length", "Sepal.Width")], col=iris.q$Species)
```

which are equivalent, and produce the same result, as the formula method:

There is a fundamental limitation in R base for mixed quantities and non-quantities data due to S3 dispatch. It is possible, for instance, to plot quantities in the x-axis and numeric data in the y-axis:

However, when the x-axis has numeric data, quantities methods will not be dispatched for the y-axis:

One way to overcome this limitation is to set unitless and errorless quantities in the x-axis:

The *core
tidyverse* includes the following packages:
`ggplot2`

, `dplyr`

, `tidyr`

,
`readr`

, `purrr`

, `tibble`

,
`stringr`

and `forcats`

. This section covers use
cases for `dplyr`

(everything except for pivoting and unpivoting), `tidyr`

(for pivoting
and unpivoting) and `ggplot2`

(for plotting).

```
library(dplyr); packageVersion("dplyr")
#> [1] '1.1.4'
library(tidyr); packageVersion("tidyr")
#> [1] '1.3.1'
```

Although not strictly necessary, we will convert the data frame to
`tibble`

format to enjoy the formatting provided by
`pillar`

.

```
iris.q <- as_tibble(iris.q)
head(iris.q)
#> # A tibble: 6 × 5
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> (err) [cm] (err) [cm] (err) [cm] (err) [cm] <fct>
#> 1 5.1(1) 3.50(7) 1.40(3) 0.200(4) setosa
#> 2 4.9(1) 3.00(6) 1.40(3) 0.200(4) setosa
#> 3 4.70(9) 3.20(6) 1.30(3) 0.200(4) setosa
#> 4 4.60(9) 3.10(6) 1.50(3) 0.200(4) setosa
#> 5 5.0(1) 3.60(7) 1.40(3) 0.200(4) setosa
#> 6 5.4(1) 3.90(8) 1.70(3) 0.400(8) setosa
```

Since `dplyr`

1.0.0, as we will see, there is enhanced
support for custom S3 classes thanks to the new implementation based on
`vctrs`

>= 0.3.0. Packages `units`

>= 0.6-7,
`errors`

>= 0.3.4 and `quantities`

>= 0.1.5
add support for this approach.

The `filter`

generic finds observations where conditions
hold. The main difference with base subsetting is that, if a condition
evaluates to `NA`

for a certain row, it is dropped. As in the
base case, another quantities object must be defined for the
comparison:

```
iris.q %>%
filter(Sepal.Length > set_quantities(75, mm)) %>%
head()
#> # A tibble: 6 × 5
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> (err) [cm] (err) [cm] (err) [cm] (err) [cm] <fct>
#> 1 7.6(2) 3.00(6) 6.6(1) 2.10(4) virginica
#> 2 7.7(2) 3.80(8) 6.7(1) 2.20(4) virginica
#> 3 7.7(2) 2.60(5) 6.9(1) 2.30(5) virginica
#> 4 7.7(2) 2.80(6) 6.7(1) 2.00(4) virginica
#> 5 7.9(2) 3.80(8) 6.4(1) 2.00(4) virginica
#> 6 7.7(2) 3.00(6) 6.1(1) 2.30(5) virginica
```

There are also three scoped variants available
(`filter_all`

, `filter_if`

,
`filter_at`

) and a subsetting function by row number called
`slice`

. All of them preserve quantities.

The `arrange`

generic sorts variables in a straightforward
way, and it is compatible with quantities:

```
iris.q %>%
arrange(Sepal.Length) %>%
head()
#> # A tibble: 6 × 5
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> (err) [cm] (err) [cm] (err) [cm] (err) [cm] <fct>
#> 1 4.30(9) 3.00(6) 1.10(2) 0.100(2) setosa
#> 2 4.40(9) 2.90(6) 1.40(3) 0.200(4) setosa
#> 3 4.40(9) 3.00(6) 1.30(3) 0.200(4) setosa
#> 4 4.40(9) 3.20(6) 1.30(3) 0.200(4) setosa
#> 5 4.50(9) 2.30(5) 1.30(3) 0.300(6) setosa
#> 6 4.60(9) 3.10(6) 1.50(3) 0.200(4) setosa
```

The `desc`

function can be applied to individual variables
to arrange in descending order.

There are two generics for column transformations:
`mutate`

modifies or adds new variables preserving the
existing ones, while `transmute`

drops the existing
variables. The syntax is very similar to base functions
`transform`

and `within`

, and equally compatible
with quantities:

```
iris.q %>%
transmute(
Species = Species,
Petal.Area = Petal.Length * Petal.Width,
Sepal.Area = Sepal.Length * Sepal.Width
) %>%
head()
#> # A tibble: 6 × 3
#> Species Petal.Area Sepal.Area
#> <fct> (err) [cm^2] (err) [cm^2]
#> 1 setosa 0.280(8) 17.8(5)
#> 2 setosa 0.280(8) 14.7(4)
#> 3 setosa 0.260(7) 15.0(4)
#> 4 setosa 0.300(8) 14.3(4)
#> 5 setosa 0.280(8) 18.0(5)
#> 6 setosa 0.68(2) 21.1(6)
```

`dplyr`

breaks down aggregation operations in two distinct
parts: grouping (with `group_by`

) and summarising (using
`summarise`

and others). Since `dplyr`

>=
1.0.0, operations on aggregated data is now fully compatible with
quantities and,compared to base methods, no fancy unlisting is
required:

```
iris.q %>%
group_by(Species) %>%
summarise_all(mean)
#> # A tibble: 3 × 5
#> Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> <fct> (err) [cm] (err) [cm] (err) [cm] (err) [cm]
#> 1 setosa 5.0(1) 3.43(7) 1.46(3) 0.25(1)
#> 2 versicolor 5.9(1) 2.77(6) 4.26(9) 1.33(3)
#> 3 virginica 6.6(1) 2.97(6) 5.6(1) 2.03(4)
```

Several verbs are provided for different types of joins, such as
`inner_join`

, `left_join`

, `right_join`

or `full_join`

. Internally, they use the same grouping
mechanism than summaries. Therefore, since `dplyr`

>=
1.0.0, these are fully compatible with quantities too:

```
iris.q %>%
left_join(data.frame(
Height = set_quantities(c(55, 60, 45), cm, c(45, 30, 35)),
Species = c("setosa", "virginica", "versicolor")
)) %>%
head()
#> Joining with `by = join_by(Species)`
#> # A tibble: 6 × 6
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species Height
#> (err) [cm] (err) [cm] (err) [cm] (err) [cm] <chr> (err) [cm]
#> 1 5.1(1) 3.50(7) 1.40(3) 0.200(4) setosa 60(40)
#> 2 4.9(1) 3.00(6) 1.40(3) 0.200(4) setosa 60(40)
#> 3 4.70(9) 3.20(6) 1.30(3) 0.200(4) setosa 60(40)
#> 4 4.60(9) 3.10(6) 1.50(3) 0.200(4) setosa 60(40)
#> 5 5.0(1) 3.60(7) 1.40(3) 0.200(4) setosa 60(40)
#> 6 5.4(1) 3.90(8) 1.70(3) 0.400(8) setosa 60(40)
```

The only difference with base `merge`

here is that
`dplyr`

does not reorder columns with respect to the
left-hand side.

Finally, pivoting and unpivoting is handled by a separate package,
`tidyr`

. Historically, this was managed using the verbs
`spread`

(pivot) and `gather`

(unpivot). These
verbs, which are not compatible with quantities, are deprecated and no
longer maintained.

Instead, there are new and more straightforward verbs for
(un)pivoting data frames called `pivot_wider`

(equivalent to
`spread`

) and `pivot_longer`

(equivalent to
`gather`

). These verbs do make use of the new approach
brought by `vctrs`

and therefore are fully compatible with
quantities.

Compared to base R, the unpivoting operation is substantially more
straightforward. In the next example, we directly merge the four columns
of interest into the `value`

column, and the correspoding
column names are gathered into the `name`

column. Such a
column is then separated into flower `part`

(sepal, petal)
and `dim`

(length, height):

```
iris.q %>%
pivot_longer(1:4) %>%
separate(name, c("part", "dim")) %>%
head()
#> # A tibble: 6 × 4
#> Species part dim value
#> <fct> <chr> <chr> (err) [cm]
#> 1 setosa Sepal Length 5.1(1)
#> 2 setosa Sepal Width 3.50(7)
#> 3 setosa Petal Length 1.40(3)
#> 4 setosa Petal Width 0.200(4)
#> 5 setosa Sepal Length 4.9(1)
#> 6 setosa Sepal Width 3.00(6)
```

In the following example, we first unpivot the original data set,
then we assign quantities and try to pivot it to obtain
`iris.q`

back, and it just works:

```
iris %>%
# first gather, with row numbers as row_id
mutate(row_id = 1:n()) %>%
pivot_longer(1:4) %>%
# assign quantities
mutate(value = set_quantities(value, cm, value * 0.05)) %>%
# now spread and remove the row_id
pivot_wider() %>%
select(-row_id) %>%
head()
#> # A tibble: 6 × 5
#> Species Sepal.Length Sepal.Width Petal.Length Petal.Width
#> <fct> (err) [cm] (err) [cm] (err) [cm] (err) [cm]
#> 1 setosa 5.1(3) 3.5(2) 1.40(7) 0.20(1)
#> 2 setosa 4.9(2) 3.0(1) 1.40(7) 0.20(1)
#> 3 setosa 4.7(2) 3.2(2) 1.30(6) 0.20(1)
#> 4 setosa 4.6(2) 3.1(2) 1.50(7) 0.20(1)
#> 5 setosa 5.0(2) 3.6(2) 1.40(7) 0.20(1)
#> 6 setosa 5.4(3) 3.9(2) 1.70(8) 0.40(2)
```

Quantities packages provide `ggplot2`

elements to make
scatterplots straightforward:

`units`

provides automatic detection of units scale type, with optional conversion and customization via`scale_x_units()`

and`scale_y_units()`

.`errors`

provides automatic placement of errorbars via`geom_errors()`

.`quantities`

provides a compatibility layer between them, so that conversions from`scale_[x|y]_units`

affect errorbars too.

By default, units are automatically placed in the axes:

```
p0 <- ggplot(iris.q) + aes(Sepal.Length, Sepal.Width, color=Species) +
geom_point()
p0
#> Sepal.Length * ~group("[", cm, "]")
#> Sepal.Width * ~group("[", cm, "]")
```

And errobars can be requested via `geom_errors()`

:

Errors may be dropped from any axis:

```
p0 + geom_errors(aes(x=drop_errors(Sepal.Length)))
#> Sepal.Length * ~group("[", cm, "]")
#> Sepal.Width * ~group("[", cm, "]")
```

And units can be converted for display:

R base works smoothly with quantities in most cases. The only
shortcoming is that some care must be applied to aggregations. In
particular, simplification must be explicitly disabled
(`simplify=FALSE`

), and such a simplification (i.e.,
converting lists to vectors of quantities) must be applied manually
while avoiding `unlist`

.

Since `dplyr`

1.0.0 and `tidyr`

1.1.0 (for
`units`

>= 0.6-7, `errors`

>= 0.3.4 and
`quantities`

>= 0.1.5), the new `vctrs`

-based
approach brings us full compatibility with quantities for all the
operations considered in this document, including grouped operations,
joining and pivoting, which did not work for previous versions.

Both R base and `ggplot2`

plots work out of the box,
although the latter provides much more flexibility and can be used
independently of the tidyverse.

`data.table`

The `data.table`

package is another popular data tools, which provides *a
high-performance version of base R’s data.frame with syntax
and feature enhancements for ease of use, convenience and programming
speed*.

Long story short, we have not included a section on
`data.table`

because *currently* (v1.11.4) it does not
work well with vectorised attributes. The underlying problem is similar
to `dplyr`

’s issue, but unfortunately it affects more
operations, including row subsetting and ordering. Only column
transformation seems to work, and other operations generate corrupted
objects.

We have found that defining quantities columns as lists (where each element consists of a single value, with unit and uncertainty) may be a workaround, but this probably would be a serious performance penalty for a package that is typically chosen for speed reasons.