Selectors

How to use the i, j, and part arguments to target specific rows, columns, and parts of a flextable.

Selectors are the mechanism flextable uses to target specific cells when applying formatting, styling, or content operations. Nearly every formatting function — bold(), color(), bg(), padding(), align(), hline(), and more — accepts three selector arguments: i, j, and part.

When all three default to NULL (rows) or their default string, the operation applies to the entire part.

The part argument

The part argument determines which section of the table is targeted.

part = "body"    # data rows (default for most functions)
part = "header"  # header rows
part = "footer"  # footer rows
part = "all"     # all three parts

ft <- flextable(head(mtcars)) |>
  bold(part = "header") |>        # bold every cell in the header
  bg(bg = "#FFFDE7", part = "all") # yellow tint across header, body, footer

When part = "all", the operation is applied to each part independently. Formula row selectors (i = ~ condition) cannot be used with part = "all", "header", or "footer" — these parts store only character representations of values, so numeric conditions cannot be evaluated on them.

The i argument — row selector

The i argument selects which rows within a part to target. When i = NULL (the default), all rows in the part are selected.

Formula (body only)

Use a one-sided formula to filter rows by a condition evaluated against the body dataset. The expression uses column names from the original data.frame.

# Highlight rows where mpg is less than 22
ft <- flextable(head(mtcars, 10)) |>
  highlight(i = ~ mpg < 22, j = "mpg", color = "#ffe842")

# Color rows where a column equals a specific value
ft <- flextable(head(airquality, 20)) |>
  color(i = ~ Temp > 80, color = "red")

Formula selectors evaluate conditions using the actual data types from the dataset (numeric, logical, etc.). They only work with part = "body".

Integer index

Pass an integer vector to select rows by position.

# Bold only the first and third body rows
ft <- flextable(head(mtcars)) |>
  bold(i = c(1, 3))

# Target the second header row
ft <- bold(ft, i = 2, part = "header")

Integer selection works with all parts, including header and footer.

Logical vector

Pass a logical vector of the same length as the number of rows in the part.

data <- head(mtcars, 6)
ft <- flextable(data) |>
  bg(i = c(TRUE, FALSE, TRUE, FALSE, TRUE, FALSE), bg = "#f0f0f0")

Targeting the last row with nrow_part()

nrow_part() returns the number of rows in a given part. It is the recommended way to programmatically select the last row:

ft <- flextable(iris[48:52, ])

# Bold the last body row
ft <- bold(ft, i = nrow_part(ft, part = "body"), part = "body")

# Add a footnote to the last row
ft <- footnote(
  ft,
  i = nrow_part(ft, part = "body"), j = 1:4,
  value = as_paragraph("Calculated mean")
)

The j argument — column selector

The j argument selects which columns to target. When j = NULL (the default), all columns are selected.

Character names

The recommended approach — explicit column names are readable and resilient to column reordering:

# Color a single column
ft <- flextable(head(mtcars)) |>
  color(j = "mpg", color = "blue")

# Target multiple columns
ft <- flextable(head(mtcars)) |>
  bold(j = c("mpg", "cyl", "hp"))

Formula

A formula lists column names with +. This is useful for selecting or excluding multiple columns concisely:

# Select specific columns
ft <- flextable(head(mtcars)) |>
  bold(j = ~ vs + am + gear)

# Exclude a column using . - colname
ft <- flextable(head(mtcars)) |>
  bg(j = ~ . - mpg, bg = "#f5f5f5")

The formula is parsed as a model terms expression. Column names are extracted from the term labels.

Integer index

Select columns by position:

# First and third columns
ft <- flextable(head(mtcars)) |>
  bold(j = c(1, 3))

Negative integers exclude columns:

# All columns except the first
ft <- flextable(head(mtcars)) |>
  bold(j = -1)

Logical vector

Pass a logical vector of the same length as ncol_keys(ft):

ncols <- ncol_keys(ft)
# Apply alternating background
ft <- bg(ft, j = rep(c(TRUE, FALSE), length.out = ncols), bg = "#f0f0f0")

Prefer character vectors (j = c("col1", "col2")) over integer positions. Character names are unambiguous when columns are reordered or when sharing code with others.

Combining i, j, and part

Selectors can be combined freely. The operation applies to the intersection of selected rows and selected columns in the targeted part:

ft <- flextable(head(mtcars, 10)) |>
  # Red text in body rows where mpg < 22, only in the mpg column
  color(i = ~ mpg < 22, j = "mpg", color = "red", part = "body") |>
  # Bold all header cells
  bold(part = "header") |>
  # Grey background for the last body row, all columns
  bg(
    i = nrow_part(ft, "body"),
    bg = "#eeeeee",
    part = "body"
  )

Selectors and merged cells

Selectors work with merged cells — they target the individual cells by row and column index, not by the visual span. When you format a merged cell, apply formatting to the top-left cell of the merged region (the anchor cell); other cells in the span are hidden and their formatting has no visual effect.

ft <- flextable(head(mtcars)) |>
  merge_h(i = 1, part = "body") |>   # merge all cells in row 1
  bold(i = 1, j = 1, part = "body")  # bold the anchor (top-left) cell

Best practices

Rows

• Use i = ~ condition for conditional body formatting.
• Use nrow_part(ft, "body") to target the last row without hard-coding a number.
• Use integers or logical vectors to target header and footer rows.

Columns

• Prefer character names (j = c("col1", "col2")) for maintainability.
• Use formula syntax (j = ~ col1 + col2) when excluding columns is cleaner.
• Avoid integer positions when column order may change.

Quick reference

Selector	Type	Example	Notes
i	Formula	i = ~ mpg < 22	Body only; uses original data types
i	Integer	i = c(1, 3)	Any part; positional
i	Logical	i = c(TRUE, FALSE, ...)	Length must match part row count
i	NULL	i = NULL	All rows (default)
j	Character	j = c("mpg", "cyl")	Recommended; column names
j	Formula	j = ~ mpg + cyl	Column list or exclusion
j	Integer	j = 1:3	Positional; negative excludes
j	Logical	j = c(TRUE, FALSE, ...)	Length must match ncol_keys(ft)
j	NULL	j = NULL	All columns (default)
part	String	part = "body"	"body", "header", "footer", "all"