Expressions

Expressions are a tree structure that can be traversed and evaluated.

Methods

evaluate

evaluate takes a namespace argument for looking up column references, a selection argument containing the valid row numbers and a length argument that gives the allocation size of the output It returns a Column object containing the computed data for all selected rows.

alias

Returns a default name for the column in case the user does not specify one.

Classes

These are the expression classes:

ConstExpr

ConstExpr represents a single constant value. When evaluated, it returns a Column containing length copies of the value.

RefExpr

RefExpr represents a column reference. When evaluated, it just returns the named Column from the namespace. If the column does not exist, it will throw a ReferenceError.

FuncExpr

FuncExpr represents a function call. It has a function and an Array of arguments (args). When evaluated, it first evaluates its arguments and then applies the function to each argument set.

CaseExpr

CaseExpr evaluate sql-style CASE statements:

CASE [<case expression>]
WHEN <when expression 1> THEN <then expression 1>
WHEN <when expression 2> THEN <then expression 2>
[ELSE <else expression>]
END

If there is no case expression, it defaults to the constant value true. If there is no else expression, it defaults to null.

The cases are tested in order, and the output expressions are only evaluated if the case matches. This can be useful for avoiding expcetions like dividing by zero.