Schema class

Description

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.

Usage

schema(...)

Arguments

Methods

• $ToString(): convert to a string

• $field(i): returns the field at index i (0-based)

• $GetFieldByName(x): returns the field with name x

• $WithMetadata(metadata): returns a new Schema with the key-value metadata set. Note that all list elements in metadata will be coerced to character.

Active bindings

• $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?

• $metadata: returns the key-value metadata as a named list. Modify or replace by assigning in (sch$metadata <- new_metadata). All list elements are coerced to string.

Examples

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