r/man/Schema.Rd

% Generated by roxygen2: do not edit by hand
% Please edit documentation in R/schema.R
\docType{class}
\name{Schema}
\alias{Schema}
\alias{schema}
\title{Schema class}
\usage{
schema(...)
}
\arguments{
\item{...}{named list of \link[=data-type]{data types}}
}
\description{
A \code{Schema} is a list of \link{Field}s, which map names to
Arrow \link[=data-type]{data types}. Create a \code{Schema} when you
want to convert an R \code{data.frame} to Arrow but don't want to rely on the
default mapping of R types to Arrow types, such as when you want to choose a
specific numeric precision, or when creating a \link{Dataset} and you want to
ensure a specific schema rather than inferring it from the various files.

Many Arrow objects, including \link{Table} and \link{Dataset}, have a \verb{$schema} method
(active binding) that lets you access their schema.
}
\section{Methods}{

\itemize{
\item \verb{$ToString()}: convert to a string
\item \verb{$field(i)}: returns the field at index \code{i} (0-based)
\item \verb{$GetFieldByName(x)}: returns the field with name \code{x}
\item \verb{$WithMetadata(metadata)}: returns a new \code{Schema} with the key-value
\code{metadata} set. Note that all list elements in \code{metadata} will be coerced
to \code{character}.
}
}

\section{Active bindings}{

\itemize{
\item \verb{$names}: returns the field names (called in \code{names(Schema)})
\item \verb{$num_fields}: returns the number of fields (called in \code{length(Schema)})
\item \verb{$fields}: returns the list of \code{Field}s in the \code{Schema}, suitable for
iterating over
\item \verb{$HasMetadata}: logical: does this \code{Schema} have extra metadata?
\item \verb{$metadata}: returns the key-value metadata as a named list.
Modify or replace by assigning in (\code{sch$metadata <- new_metadata}).
All list elements are coerced to string.
}
}

\examples{
\donttest{
df <- data.frame(col1 = 2:4, col2 = c(0.1, 0.3, 0.5))
tab1 <- Table$create(df)
tab1$schema
tab2 <- Table$create(df, schema = schema(col1 = int8(), col2 = float32()))
tab2$schema
}
}