Author: Stefano Meschiari
Date: 2015-11-30

# Using latex2exp

#### Stefano Meschiari

#### 2015-11-30

# latex2exp

latex2exp is an R package that parses and converts LaTeX math formulas to R’s plotmath expressions. Plotmath expressions are used to enter mathematical formulas and symbols to be rendered as text, axis labels, etc. throughout R’s plotting system. I find plotmath expressions to be quite opaque, while LaTeX is a de-facto standard for mathematical expressions, so this package might be useful to others as well.

## Installation

Install this package from CRAN:

    install.packages('latex2exp')

You can also install from GitHub using devtools:

    devtools::install_github('stefano-meschiari/latex2exp')

## Usage

    library(latex2exp)

The `TeX` function takes a LaTeX string and returns a plotmath expression suitable for use in plotting, e.g.,

    TeX('$\\alpha^\\beta$')

(note it is always necessary to escape the backslash within a string, hence the double backslash).

The return value of `TeX()` can be used anywhere a plotmath expression is accepted, including plot labels, legends, and text.

The following example shows plotting in base graphics:

    x <- seq(0, 4, length.out=100)
    alpha <- 1:5

    plot(x, xlim=c(0, 4), ylim=c(0, 10),
         xlab='x', ylab=TeX('$\\alpha  x^\\alpha$, where $\\alpha \\in 1\\ldots 5$'),
         type='n', main=TeX('Using $\\LaTeX$ for plotting in base graphics!'))

    invisible(sapply(alpha, function(a) lines(x, a*x^a, col=a)))

    legend('topleft', legend=TeX(sprintf("$\\alpha = %d$", alpha)),
           lwd=1, col=alpha)

This example shows plotting in ggplot2:

    library(plyr)
    library(ggplot2)
    x <- seq(0, 4, length.out=100)
    alpha <- 1:5
    data <- mdply(alpha, function(a, x) data.frame(v=a*x^a, x=x), x)

    p <- ggplot(data, aes(x=x, y=v, color=X1)) +
        geom_line() +
        ylab(TeX('$\\alpha  x^\\alpha$, where $\\alpha \\in 1\\ldots 5$')) +
        ggtitle(TeX('Using $\\LaTeX$ for plotting in ggplot2. I $\\heartsuit$ ggplot!')) +
        coord_cartesian(ylim=c(-1, 10)) +
        guides(color=guide_legend(title=NULL)) +
        scale_color_discrete(labels=lapply(sprintf('$\\alpha = %d$', alpha), TeX))
        # Note that ggplot2 legend labels must be lists of expressions, not vectors of expressions

    print(p)

You can quickly test out what a translated LaTeX string would look like by using `plot`:

    plot(TeX("A $\\LaTeX$ formula: $\\frac{2hc^2}{\\lambda^5} \\,
                   \\frac{1}{e^{\\frac{hc}{\\lambda k_B T}} - 1}$"), cex=2)

## Syntax

Use

    TeX('latexString')

to build a plotmath expression, ready for use in plots. If the parser cannot build a correct plotmath expression, it will `stop()` and show the invalid plotmath expression built.

    TeX('latexString', output=c('expression', 'character', 'ast'))

If the `output` option is equal to `character`, it will return the string representation of the expression (which could be converted into an expression using `parse(text=)`).

If the `output` option is equal to `ast`, it will return the tree built by the parser (this is only useful for debugging).

    latex2exp_examples()

will show a demo of the supported LaTeX syntax.

    latex2exp_supported(plot=FALSE)

returns a list of supported LaTeX. If `plot=TRUE`, a table of symbols will be plotted.

## “Supported” LaTeX

Formulas should go between dollar characters ($).

Only a subset of LaTeX is supported, and not 100% correctly. Greek symbols (\alpha, \beta, etc.) and the usual operators (+, -, etc.) are supported.

In addition, the following should be supported:

    latex2exp_supported(plot=TRUE)

Their rendering depends on R’s interpretation of the plotmath expression.

A few examples:

    latex2exp_examples()

    ## [1] TRUE

## Changes

### 0.4.0 [08/29/2015]

- Deprecated the `latex2exp()` function; use `TeX()` instead.
- Added `\lbrack` and `\rbrack` to type left and right square brackets.

### 0.3.3 [08/11/2015]

Fixes bug #4 (“fix parsing of numbers”), where certain numbers inside formulas where not parsed correctly.

### 0.3.2 [07/28/2015]

Fixes bug #3 (“subscript and superscript style”). `latex2exp` now renders combined subscripts and superscripts correctly.

### 0.3.1 [07/02/2015]

Fixes bug #2 (white space causes unexpected behaviour). `latex2exp` should now be a bit more compliant with how LaTeX handles whitespace.

### 0.3.0 [06/30/2015]

`latex2exp` is now a proper package.

### 0.2.0 [06/29/2015]

Formulas must now be enclosed between dollar characters ($), as in LaTeX proper. Text does not need to be enclosed in \text tags anymore.