arg_index: Check Index Argument
In arg: Clean and Simple Argument Checking

arg_index

R Documentation

Check Index Argument

Description

Checks whether an argument is a valid column index (arg_index()) of a data set or a vector thereof (arg_indices()).

Usage

arg_index(
  x,
  data,
  .arg = rlang::caller_arg(x),
  .arg_data = rlang::caller_arg(data),
  .msg = NULL,
  .call
)

arg_indices(
  x,
  data,
  .arg = rlang::caller_arg(x),
  .arg_data = rlang::caller_arg(data),
  .msg = NULL,
  .call
)

Arguments

`x`	the argument to be checked
`data`	a data set (i.e., a matrix or data frame)
`.arg`	the name of the argument supplied to `x` to appear in error messages. The default is to extract the argument's name using `rlang::caller_arg()`. Ignored if `.msg` is supplied.
`.arg_data`	the name of the argument supplied to `data` to appear in error messages. The default is to extract the argument's name using `rlang::caller_arg()`. Ignored if `.msg` is supplied.
`.msg`	an optional alternative message to display if an error is thrown instead of the default message.
`.call`	the execution environment of a currently running function, e.g. `.call = rlang::current_env()`. The corresponding function call is retrieved and mentioned in error messages as the source of the error. Passed to `err()`. Set to `NULL` to omit call information. The default is to search along the call stack for the first user-facing function in another package, if any.

Details

For arg_indices(), an error will be thrown unless one of the following are true:

x is a vector of counts (see arg_counts()) less than or equal to ncol(data)
x is a character vector with values a subset of colnames(data)

For arg_index(), x additionally must have length equal to 1. Passing arg_index() ensures that data[, x] (if data is a matrix) or data[[x]] (if x is a data frame) evaluate correctly.

If data has no column names, an error will be thrown if x is a character vector.

Value

Returns NULL invisibly if an error is not thrown.

Examples

dat <- data.frame(col1 = 1:5,
                  col2 = 6:10)

f <- function(z) {
  arg_index(z, dat)
}

try(f(1))         # No error
try(f(3))         # Error: not a valid index
try(f("col1"))    # No error
try(f("bad_col")) # Error: not a valid index
try(f(1:2))       # Error: arg_index() requires scalar

mat <- matrix(1:9, ncol = 3)

g <- function(z) {
  arg_indices(z, mat)
}

try(g(1))     # No error
try(g(1:3))   # No error
try(g("col")) # Error: `mat` has no names

arg documentation built on April 9, 2026, 5:09 p.m.