---
title: Templating with knit_expand()
author: Yihui Xie
date: "`{r} Sys.Date()`"
output:
  litedown::html_format:
    meta:
      css: ["@default"]
---

<!--
%\VignetteEngine{litedown::vignette}
%\VignetteIndexEntry{Templating with knit_expand()}
-->

A few simple examples:

```{r}
library(knitr)
knit_expand(text = 'The value of pi is {{pi}}.')
knit_expand(text = 'The value of a is {{a}}, so a + 1 is {{a+1}}.', a = rnorm(1))
knit_expand(text = 'The area of a circle with radius {{r}} is {{pi*r^2}}', r = 5)
```

Any number of variables:

```{r}
knit_expand(text = 'a is {{a}} and b is {{b}}, with my own pi being {{pi}} instead of {{base::pi}}', a=1, b=2, pi=3)
```

Custom delimiter `<% %>`:

```{r}
knit_expand(text = 'I do not like curly braces, so use % with <> instead: a is <% a %>.', a = 8, delim = c("<%", "%>"))
```

The pyexpander delimiter:

```{r}
knit_expand(text = 'hello $(LETTERS[24]) and $(pi)!', delim = c("$(", ")"))
```

Arbitrary R code:

```{r}
knit_expand(text = 'you cannot see the value of x {{x=rnorm(1)}}but it is indeed created: x = {{x}}')
res = knit_expand(text = c(' x | x^2', '{{x=1:5;paste(sprintf("%2d | %3d", x, x^2), collapse = "\n")}}'))
cat(res)
```

The m4 example: <https://en.wikipedia.org/wiki/M4_(computer_language)>

```{r}
res = knit_expand(text = c('{{i=0;h2=function(x){i<<-i+1;sprintf("<h2>%d. %s</h2>", i, x)} }}<html>',
'{{h2("First Section")}}', '{{h2("Second Section")}}', '{{h2("Conclusion")}}', '</html>'))
cat(res)
```

Build regression models based on a template; loop through some variables in `mtcars`:

```{r, comment=''}
src = lapply(names(mtcars)[2:5], function(i) {
knit_expand(text=c("# Regression on {{i}}", '```{r lm-{{i}}}', 'lm(mpg~{{i}}, data=mtcars)', '```', ''))
})
# knit the source
litedown::fuse(unlist(src), 'markdown')
```
