A missing value representation for Julia for databases and statistics
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The package is registered in METADATA.jl and so can be installed with Pkg.add.
julia> Pkg.add("Missings")The package is tested against the current Julia 0.6 release and nightly on Linux, OS X, and Windows.
Contributions are very welcome, as are feature requests and suggestions. Please open an issue if you encounter any problems or would just like to ask a question.
Missings.jl provides a single type Missing with a single instance missing which represents a missing value in data. missing values behave essentially like NULL in SQL or NA in R. missing differs from nothing (the object returned by Julia functions and blocks which do not return any value) in that it can be passed to many operators and functions, prompting them to return missing. Where appropriate, packages should provide methods propagating missing for the functions they define.
The package defines standard operators and functions which propagate missing values: for example 1 + missing and cos(missing) both return missing. In particular, note that comparison operators ==, <, >, <= and => (but not isequal nor isless) also propagate missing, so that 1 == missing and missing == missing both return missing. Use ismissing to test whether a value is missing. Logical operators &, |, ⊻/xor implement three-valued logic: they return missing only when the result cannot be determined. For example, true & missing returns missing but true | missing returns true.
In many cases, missing values will have to be skipped or replaced with a valid value. For example, sum([1, missing]) returns missing due to the behavior of +. Use sum(Missings.skip([1, missing]) to ignore missing values. sum(Missings.replace([1, missing], 0)) would have the same effect. Missings.fail throws an error if any value is found while iterating over the data. These three functions return an iterator and therefore do not need to allocate a copy of the data. Finally, the Missings.coalesce function is a more complex and powerful version of Missings.replace.