For R Markdown, How do I display a matrix from R variable

Question

I have a rmd document where I have the following

```{r code_block, echo=FALSE}
A = matrix(c(1,3,0,1),2,2)
B = matrix(c(5,3,1,4),2,2)
```

$$
\\begin{bmatrix} 
1          


        

Answer 1

This is a perfect use case for knitr::knit_print().
You will find details about the knitr::knit_print() method in its dedicated vignette:

vignette('knit_print', package = 'knitr')

The goal is to provide a knit_print method for objects of class matrix. As other answers suggested, it could be useful to define operators.

You will find below an Rmd file that provides a solution to your problem. It also contains a proposal for operators.

The main feature of this answer is that you only have to write

`r A`

to output the matrix A in LaTeX inline mode (no $ to type) and write

```{r echo=FALSE}
A
```

to write in LaTeX display mode.

I also propose you to define a %times% operator. Therefore, you only have to write:

`r A %times% B`

This answer is quite generic and you should be able to extend it to other objects.

---
title: "R Markdown: Display a Matrix for R Variable"
author: "Romain Lesur"
output: 
  html_document:
    keep_md: true
---

```{r setup, include=FALSE}
# Define a generic method that transforms an object x in a LaTeX string
as_latex = function(x, ...) {
  UseMethod('as_latex', x)
}

# Define a class latex for LaTeX expressions
as_latex.character = function(x) {
  structure(
    paste(x, collapse = ' '), 
    class = c('latex', 'character')
  )
}

# A character string of class latex is rendered in display mode
# Define a knit_print() method for the latex class
knit_print.latex = function(x, ...) {
  knitr::asis_output(
    paste0('$$', x, '$$')
  )
} 

# Now, define a method as_latex for matrix
as_latex.matrix = function(x, ...) {
  as_latex(c(
    '\\begin{bmatrix}',
    paste(
      t(x),
      rep(c(rep('&', nrow(x) - 1), '\\\\'), ncol(x)),
      collapse = ''
    ),
    '\\end{bmatrix}'
  ))
}

# Indicate to knitr that matrix are rendered as latex
knit_print.matrix = function(x, ...) {
  knitr::knit_print(as_latex(x))
}

# Build a knitr inline hook to display inline latex in inline mode
default_inline_hook = knitr::knit_hooks$get('inline')
knitr::knit_hooks$set(inline = function(x) {
  x = paste(gsub('\\$\\$', '$', x))
  default_inline_hook(x)
})
```


```{r}
A = matrix(c(1,3,0,1),2,2)
B = matrix(c(5,3,1,4),2,2)
```


Now, matrix are rendered as LaTeX:

Matrix A in inline mode: `r A`

Matrix A in display mode:

```{r echo=FALSE}
A
```


### Operators

As other answers suggested, it could be useful to define operators.  
With the previous class, it is relatively straightforward:

```{r operators, include=FALSE}
`%times%` = function(x, y) {
  as_latex(sapply(list(x, '\\times', y), as_latex))  
}

`%add%` = function(x, y) {
  as_latex(sapply(list(x, '+', y), as_latex))  
}
```

Example in inline mode: `r A %add% A %times% B`

Display mode:
```{r echo=FALSE}
A %times% B
```