readr (version 1.3.1)

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.

AccumulateCallBack

Callback function that accumulates a single result. Requires the parameter acc to specify the initial value of the accumulator. The parameter acc is NULL by default.

See Also

Other chunked: melt_delim_chunked, read_delim_chunked, read_lines_chunked

Examples

# NOT RUN {
## 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)

# The AccumulateCallback accumulates results from each chunk
f <- function(x, pos, acc) sum(x$mpg) + acc
read_csv_chunked(readr_example("mtcars.csv"), AccumulateCallback$new(f, acc = 0), chunk_size = 5)
# }