Some output documents may need appropriate header information. For example, for LaTeX output, we need to write \usepackage{tikz} into the preamble if we use tikz graphics; this function sets the header information to be written into the output.
set_header(...)The header vector in opts_knit is set.
Header components; currently possible components are
highlight, tikz and framed, which contain the
necessary commands to be used in the HTML header or LaTeX preamble. Note that
HTML output does not use the tikz and framed components, since
they do not make sense in the context of HTML.
By default, knitr will set up the header automatically. For example, if
the tikz device is used, knitr will add \usepackage{tikz} to
the LaTeX preamble, and this is done by setting the header component
tikz to be a character string: set_header(tikz =
'\usepackage{tikz}'). Similary, when we highlight R code using the
highlight package (i.e. the chunk option highlight = TRUE),
knitr will set the highlight component of the header vector
automatically; if the output type is HTML, this component will be different
-- instead of LaTeX commands, it contains CSS definitions.
For power users, all the components can be modified to adapt to a customized
type of output. For instance, we can change highlight to LaTeX
definitions of the listings package (and modify the output hooks
accordingly), so we can decorate R code using the listings package.
set_header(tikz = "\\usepackage{tikz}")
opts_knit$get("header")
Run the code above in your browser using DataLab