readr (version 1.1.0)

callback: Callback classes

Description

These classes are used to define callback behaviors.

Details

ChunkCallback

Callback interface definition, all callback functions should inherit from this class.

SideEffectChunkCallback

Callback function that is used only for side effects, no results are returned.

DataFrameCallback

Callback function that combines each result together at the end.

See Also

Other chunked: read_delim_chunked, read_lines_chunked

Examples

## If given a regular function it is converted to a SideEffectChunkCallback

# view structure of each chunk
read_lines_chunked(readr_example("mtcars.csv"), str, chunk_size = 5)

# Print starting line of each chunk
f <- function(x, pos) print(pos)
read_lines_chunked(readr_example("mtcars.csv"), SideEffectChunkCallback$new(f), chunk_size = 5)

# If combined results are desired you can use the DataFrameCallback

# Cars with 3 gears
f <- function(x, pos) subset(x, gear == 3)
read_csv_chunked(readr_example("mtcars.csv"), DataFrameCallback$new(f), chunk_size = 5)

# The ListCallback can be used for more flexible output
f <- function(x, pos) x$mpg[x$hp > 100]
read_csv_chunked(readr_example("mtcars.csv"), ListCallback$new(f), chunk_size = 5)

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