This function extracts the variable part from a character vector of
predicate names. Each element of x is expected to follow the pattern
<varname>=<value>, where <varname> is a variable name and <value> is
the associated value.
Value
A character vector containing the <varname> parts of predicate
names in x. If an element does not contain =, the entire string is
returned as is. If x is NULL, the function returns NULL. If x has
length zero (character(0)), the function returns character(0).
Details
If an element does not contain an equal sign (=), the entire string is
returned unchanged.
This function is the counterpart to values(), which extracts the value
part of predicates. Together, var_names() and values() provide a
convenient way to split predicate strings into their variable and value
components.
