86 lines
2.9 KiB
Plaintext
86 lines
2.9 KiB
Plaintext
|
---
|
||
|
|
title: "Custom knitr language engines"
|
||
|
|
output: rmarkdown::html_vignette
|
||
|
|
vignette: >
|
||
|
|
%\VignetteIndexEntry{Custom knitr language engines}
|
||
|
|
%\VignetteEngine{knitr::rmarkdown}
|
||
|
|
%\VignetteEncoding{UTF-8}
|
||
|
|
---
|
||
|
|
|
||
|
|
```{r setup, include=FALSE}
|
||
|
|
knitr::opts_chunk$set(echo = TRUE)
|
||
|
|
library(glue)
|
||
|
|
```
|
||
|
|
|
||
|
|
Glue provides a few [custom language engines](https://bookdown.org/yihui/rmarkdown-cookbook/custom-engine.html#custom-engine) for knitr, which allows you to use glue directly in knitr chunks.
|
||
|
|
|
||
|
|
## `glue` engine
|
||
|
|
|
||
|
|
The first engine is the `glue` engine, which evaluates the chunk contents as a glue template.
|
||
|
|
|
||
|
|
```{glue}
|
||
|
|
1 + 1 = {1 + 1}
|
||
|
|
```
|
||
|
|
|
||
|
|
Maybe the most useful use of the `glue` engine is to set the knitr option `results = 'asis'` and output markdown or HTML directly into the document.
|
||
|
|
|
||
|
|
````markdown
|
||
|
|
`r '' ````{glue, results = 'asis', echo = FALSE}
|
||
|
|
#### mtcars has **{nrow(mtcars)} rows** and _{ncol(mtcars)} columns_.
|
||
|
|
```
|
||
|
|
````
|
||
|
|
|
||
|
|
```{glue, results = 'asis', echo = FALSE}
|
||
|
|
#### mtcars has **{nrow(mtcars)} rows** and _{ncol(mtcars)} columns_.
|
||
|
|
```
|
||
|
|
|
||
|
|
If you want to pass additional arguments into the glue call, simply include them as chunk options.
|
||
|
|
|
||
|
|
````markdown
|
||
|
|
`r '' ````{glue, .open = "<<", .close = ">>", results = 'asis', echo = FALSE}
|
||
|
|
The **median waiting time** between eruptions is <<median(faithful$waiting)>>.
|
||
|
|
```
|
||
|
|
````
|
||
|
|
|
||
|
|
```{glue, .open = "<<", .close = ">>", results = 'asis', echo = FALSE}
|
||
|
|
The **median waiting time** between eruptions is <<median(faithful$waiting)>>.
|
||
|
|
```
|
||
|
|
|
||
|
|
## `glue_sql` engine
|
||
|
|
|
||
|
|
The second engine is `glue_sql`, which will use `glue::glue_sql()` to generate a SQL query and then run the query using the [sql engine](https://bookdown.org/yihui/rmarkdown/language-engines.html#sql).
|
||
|
|
|
||
|
|
First we create a new connection to an in-memory SQLite database, and write a new table to it.
|
||
|
|
```{r}
|
||
|
|
con <- DBI::dbConnect(RSQLite::SQLite(), ":memory:")
|
||
|
|
mtcars$model <- rownames(mtcars)
|
||
|
|
DBI::dbWriteTable(con, "mtcars", mtcars)
|
||
|
|
```
|
||
|
|
|
||
|
|
Next define some variables we that we can use with glue to interpolate.
|
||
|
|
```{r}
|
||
|
|
var <- "mpg"
|
||
|
|
tbl <- "mtcars"
|
||
|
|
num <- 150
|
||
|
|
```
|
||
|
|
|
||
|
|
Then we can use `glue_sql` to construct and run a query using those variables into that database. *Note* you need to provide the connection object as a `connection` chunk option.
|
||
|
|
|
||
|
|
In this example there are two type of quotes. The first is a bare backtick, these are passed directly to the SQL engine unchanged.
|
||
|
|
The second is backticks inside of braces, which are specially interpreted to do the proper quoting for the given SQL engine by glue.
|
||
|
|
In this example we use the `sqlite` engine, which uses backticks for quoting, but you would use the same backticks inside brace syntax for postgreSQL, and `glue_sql()` would automatically use double quotes for quoting instead.
|
||
|
|
|
||
|
|
````markdown
|
||
|
|
`r '' ````{glue_sql, connection = con}
|
||
|
|
SELECT `model`, `hp`, {`var`}
|
||
|
|
FROM {`tbl`}
|
||
|
|
WHERE {`tbl`}.`hp` > {num}
|
||
|
|
```
|
||
|
|
````
|
||
|
|
|
||
|
|
```{glue_sql, connection = con}
|
||
|
|
SELECT `model`, `hp`, {`var`}
|
||
|
|
FROM {`tbl`}
|
||
|
|
WHERE {`tbl`}.`hp` > {num}
|
||
|
|
```
|