utility function to identify variables with one value only.
util_check_one_unique_value(x)logical(1): TRUE, if -- except NA -- exactly only one value
is observed in x,
FALSE otherwise
vector with values
Other robustness_functions:
util_as_valid_missing_codes(),
util_correct_variable_use(),
util_empty(),
util_ensure_character(),
util_ensure_in(),
util_ensure_suggested(),
util_expect_scalar(),
util_fix_rstudio_bugs(),
util_is_integer(),
util_is_numeric_in(),
util_is_valid_missing_codes(),
util_match_arg(),
util_observations_in_subgroups(),
util_only_NAs(),
util_stop_if_not(),
util_warn_unordered()