How to change fontface (bold/italics) for a cell in a kable table in rmarkdown?

Just generalising the question to include other font faces. Pandoc offers other ways to easily reformat text, and as explained in the RMarkdown Cheatsheet:

• italics can be achieved using *italics*
• bold can be achieved using **bold**
• strikethrough can be achieved using ~~strikethrough~~

This output will support the various output methods, including PDF, html and word:

Here is a basic example:

---
output: pdf_document: default
---

```{r}
knitr::kable(data.frame(char = c('*a*','**b**','~~c~~'),
                 num = c(1,2,3)))
```

Applying formatting with a function

To make it easier to select cells to reformat, I have put together the following function format_cells.

format_cells <- function(df, rows ,cols, value = c("italics", "bold", "strikethrough")){

  # select the correct markup
  # one * for italics, two ** for bold
  map <- setNames(c("*", "**", "~~"), c("italics", "bold", "strikethrough"))
  markup <- map[value]  

  for (r in rows){
    for(c in cols){

      # Make sure values are not factors
      df[[c]] <- as.character( df[[c]])

      # Update formatting
      df[r, c] <- paste0(markup, df[r, c], markup)
    }
  }

  return(df)
}

It allows the user to select the cell row and columns which need to be reformatted and select the styling to apply. Multiple cells can be selected at the same time if several values in a row/column need to be reformatted. Here is an example:

library(tidyverse)

df <- data.frame(char = c('a','b','c'),
                 num = c(1,2,3))

df %>%
  format_cells(1, 1, "italics") %>%
  format_cells(2, 2, "bold") %>%
  format_cells(3, 1:2, "strikethrough") %>%
  knitr::kable()

---

Further Reading: the kableExtra package has been written to offer a lot of extra controls of styling of tables. However, the advice is different for the different types of output, and therefore there are different approaches for HTML and LaTeX

This also works: the first argument is row number, so you can bold rows programmatically or columns using column_spec.

library(kableExtra)

library(tidyverse)


kable(data.frame(letter =c('a','b','c'),number =c(1,2,3)))%>%
  kable_styling()%>%
  row_spec(3,bold=T,hline_after = T)