---
title: "A quick summary of selection syntax in `{datawizard}`"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{A quick summary of selection syntax in `{datawizard}`}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r setup, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

pkgs <- c(
  "datawizard",
  "dplyr",
  "htmltools"
)

if (!all(vapply(pkgs, requireNamespace, quietly = TRUE, FUN.VALUE = logical(1L)))) {
  knitr::opts_chunk$set(eval = FALSE)
}
```

```{r load, echo=FALSE, message=FALSE}
library(datawizard)
library(dplyr)
library(htmltools)

set.seed(123)
iris <- iris[sample(nrow(iris), 10), ]
row.names(iris) <- NULL

row <- function(...) {
  div(
    class = "custom_note",
    ...
  )
}
```

```{css, echo=FALSE}
.custom_note {
  border-left: solid 5px hsl(220, 100%, 30%);
  background-color: hsl(220, 100%, 95%);
  padding: 5px;
  margin-bottom: 10px
}
```

This vignette can be referred to by citing the following:

Patil et al., (2022). datawizard: An R Package for Easy Data Preparation and Statistical Transformations. *Journal of Open Source Software*, *7*(78), 4684, https://doi.org/10.21105/joss.04684

# Selecting variables

## Quoted names

This is the most simple way to select one or several variables. Just use a character
vector containing variables names, like in base R.

```{r}
data_select(iris, c("Sepal.Length", "Petal.Width"))
```

## Unquoted names

It is also possible to use unquoted names. This is useful if we use the `tidyverse`
and want to be consistent about the way variable names are passed.

```{r}
iris %>%
  group_by(Species) %>%
  standardise(Petal.Length) %>%
  ungroup()
```


## Positions

In addition to variable names, `select` can also take indices for the variables 
to select in the dataframe.

```{r}
data_select(iris, c(1, 2, 5))
```


## Functions

We can also pass a function to the `select` argument. This function will be applied
to all columns and should return `TRUE` or `FALSE`. For example, if we want to 
keep only numeric columns, we can use `is.numeric`.

```{r}
data_select(iris, is.numeric)
```

Note that we can provide any custom function to `select`, *provided it returns `TRUE` or `FALSE`* when applied to a column.

```{r}
my_function <- function(i) {
  is.numeric(i) && mean(i, na.rm = TRUE) > 3.5
}

data_select(iris, my_function)
```


## Patterns

With larger datasets, it would be tedious to write the names of variables to select,
and it would be fragile to rely on variable positions as they may change later. 
To this end, we can use four select helpers: `starts_with()`, `ends_with()`,
`contains()`, and `regex()`. The first three can take several patterns, while
`regex()` takes a single regular expression.
 
```{r}
data_select(iris, starts_with("Sep", "Peta"))

data_select(iris, ends_with("dth", "ies"))

data_select(iris, contains("pal", "ec"))

data_select(iris, regex("^Sep|ies"))
```

```{r echo=FALSE}
row("Note: these functions are not exported by `datawizard` but are detected and
applied internally. This means that they won't be detected by autocompletion
when we write them.")
```

```{r echo=FALSE}
row("Note #2: because these functions are not exported, they will not create
conflicts with the ones that come from the `tidyverse` and that have the same name.
So we can still use `dplyr` and its friends, it won't change anything for selection
in `datawizard` functions!")
```


# Excluding variables

What if we want to keep all variables except for a few ones? There are two ways
we can invert our selection.

The first way is to put a minus sign `"-"` in front of the `select` argument.

```{r}
data_select(iris, -c("Sepal.Length", "Petal.Width"))

data_select(iris, -starts_with("Sep", "Peta"))

data_select(iris, -is.numeric)
```

Note that if we use numeric indices, we can't mix negative and positive values. 
This means that we have to use `select = -(1:2)` if we want to exclude the first
two columns; `select = -1:2` will *not* work:

```{r}
data_select(iris, -(1:2))
```

Same thing for variable names:

```{r}
data_select(iris, -(Petal.Length:Species))
```

The second way is to use the argument `exclude`. This argument has the same 
possibilities as `select`. Although this may not be required in most contexts, 
if we wanted to, we could use both `select` and `exclude` arguments at the same 
time.

```{r}
data_select(iris, exclude = c("Sepal.Length", "Petal.Width"))

data_select(iris, exclude = starts_with("Sep", "Peta"))
```

# Programming with selections

Since `datawizard` 0.6.0, it is possible to pass function arguments and loop indices
in `select` and `exclude` arguments. This makes it easier to program with
`datawizard`.

For example, if we want to let the user decide the selection they want to use:

```{r}
my_function <- function(data, selection) {
  extract_column_names(data, select = selection)
}
my_function(iris, "Sepal.Length")
my_function(iris, starts_with("Sep"))

my_function_2 <- function(data, pattern) {
  extract_column_names(data, select = starts_with(pattern))
}
my_function_2(iris, "Sep")
```

It is also possible to pass these values in loops, for example if we have a list 
of patterns and we want to relocate columns based on these patterns, one by one:

```{r}
new_iris <- iris
for (i in c("Sep", "Pet")) {
  new_iris <- new_iris %>%
    data_relocate(select = starts_with(i), after = -1)
}
new_iris
```

In the loop above, all columns starting with `"Sep"` are moved at the end of the
data frame, and the same thing was made with all columns starting with `"Pet"`.



# Useful to know

## Ignore the case

In every selection that uses variable names, we can ignore the case in the 
selection by applying `ignore_case = TRUE`.

```{r}
data_select(iris, c("sepal.length", "petal.width"), ignore_case = TRUE)

data_select(iris, ~ Sepal.length + petal.Width, ignore_case = TRUE)

data_select(iris, starts_with("sep", "peta"), ignore_case = TRUE)
```

## Formulas

It is also possible to use formulas to select variables:

```{r}
data_select(iris, ~ Sepal.Length + Petal.Width)
```

This made it easier to use selection in custom functions before `datawizard` 
0.6.0, and is kept available for backward compatibility. 

