r/man/Expression.Rd

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/expression.R
\name{Expression}
\alias{Expression}
\title{Arrow expressions}
\description{
\code{Expression}s are used to define filter logic for passing to a \link{Dataset}
\link{Scanner}.

\code{Expression$scalar(x)} constructs an \code{Expression} which always evaluates to
the provided scalar (length-1) R value.

\code{Expression$field_ref(name)} is used to construct an \code{Expression} which
evaluates to the named column in the \code{Dataset} against which it is evaluated.

\code{Expression$compare(OP, e1, e2)} takes two \code{Expression} operands, constructing
an \code{Expression} which will evaluate these operands then compare them with the
relation specified by OP (e.g. "==", "!=", ">", etc.) For example, to filter
down to rows where the column named "alpha" is less than 5:
\code{Expression$compare("<", Expression$field_ref("alpha"), Expression$scalar(5))}

\code{Expression$and(e1, e2)}, \code{Expression$or(e1, e2)}, and \code{Expression$not(e1)}
construct an \code{Expression} combining their arguments with Boolean operators.

\code{Expression$is_valid(x)} is essentially (an inversion of) \code{is.na()} for \code{Expression}s.

\code{Expression$in_(x, set)} evaluates x and returns whether or not it is a member of the set.
}