A Schema is a list of Fields, which map names to
Arrow data types. Create a Schema when you
want to convert an R 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 Dataset and you want to
ensure a specific schema rather than inferring it from the various files.
Many Arrow objects, including Table and Dataset, have a $schema method
(active binding) that lets you access their schema.
schema(...)named list of data types
$ToString(): convert to a string
$field(i): returns the field at index i (0-based)
$GetFieldByName(x): returns the field with name x
$names: returns the field names (called in names(Schema))
$num_fields: returns the number of fields (called in length(Schema))
$fields: returns the list of Fields in the Schema, suitable for
iterating over
$HasMetadata: logical: does this Schema have extra metadata?
$metadata: returns the extra metadata, if present, else NULL
# NOT RUN {
df <- data.frame(col1 = 2:4, col2 = c(0.1, 0.3, 0.5))
tab1 <- Table$create(df)
tab1$schema
tab2 <- Table$create(df, schema = schema(col1 = int8(), col2 = float32()))
tab2$schema
# }
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