  Primitive types are fundamental components available in any programming
language, which serve as the building blocks of data manipulation.
Understanding the role of these types in source code is essential to write
software. Little work has been conducted on how often these variables are
documented in code comments and what types of knowledge the comments provide
about variables of primitive types. In this paper, we present an approach for
detecting primitive variables and their description in comments using lexical
matching and advanced matching. We evaluate our approaches by comparing the
lexical and advanced matching performance in terms of recall, precision, and
F-score, against 600 manually annotated variables from a sample of GitHub
projects. The performance of our advanced approach based on F-score was
superior compared to lexical matching, 0.986 and 0.942, respectively. We then
create a taxonomy of the types of knowledge contained in these comments about
variables of primitive types. Our study showed that developers usually
documented the variables' identifiers of a numeric data type with their
purpose~(69.16%) and concept~(72.75%) more than the variables' identifiers of
type String which were less documented with purpose~(61.14%) and
concept~(55.46%). Our findings characterise the current state of the practice
of documenting primitive variables and point at areas that are often not well
documented, such as the meaning of boolean variables or the purpose of fields
and local variables.
