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
accto specify the initial value of the accumulator. The parameteraccisNULLby default.
See also
Other chunked:
melt_delim_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)
#> chr [1:5] "\"mpg\",\"cyl\",\"disp\",\"hp\",\"drat\",\"wt\",\"qsec\",\"vs\",\"am\",\"gear\",\"carb\"" ...
#> chr [1:5] "18.7,8,360,175,3.15,3.44,17.02,0,0,3,2" ...
#> chr [1:5] "19.2,6,167.6,123,3.92,3.44,18.3,1,0,4,4" ...
#> chr [1:5] "10.4,8,472,205,2.93,5.25,17.98,0,0,3,4" ...
#> chr [1:5] "33.9,4,71.1,65,4.22,1.835,19.9,1,1,4,1" ...
#> chr [1:5] "19.2,8,400,175,3.08,3.845,17.05,0,0,3,2" ...
#> chr [1:3] "19.7,6,145,175,3.62,2.77,15.5,0,1,5,6" ...
#> NULL
# Print starting line of each chunk
f <- function(x, pos) print(pos)
read_lines_chunked(readr_example("mtcars.csv"), SideEffectChunkCallback$new(f), chunk_size = 5)
#> [1] 1
#> [1] 6
#> [1] 11
#> [1] 16
#> [1] 21
#> [1] 26
#> [1] 31
#> NULL
# If combined results are desired you can use the DataFrameCallback
# Cars with 3 gears
f <- function(x, pos) subset(x, gear == 3)
read_csv_chunked(readr_example("mtcars.csv"), DataFrameCallback$new(f), chunk_size = 5)
#>
#> ── Column specification ──────────────────────────────────────────────────
#> cols(
#> mpg = col_double(),
#> cyl = col_double(),
#> disp = col_double(),
#> hp = col_double(),
#> drat = col_double(),
#> wt = col_double(),
#> qsec = col_double(),
#> vs = col_double(),
#> am = col_double(),
#> gear = col_double(),
#> carb = col_double()
#> )
#> # A tibble: 15 × 11
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1 21.4 6 258 110 3.08 3.22 19.4 1 0 3 1
#> 2 18.7 8 360 175 3.15 3.44 17.0 0 0 3 2
#> 3 18.1 6 225 105 2.76 3.46 20.2 1 0 3 1
#> 4 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4
#> 5 16.4 8 276. 180 3.07 4.07 17.4 0 0 3 3
#> 6 17.3 8 276. 180 3.07 3.73 17.6 0 0 3 3
#> 7 15.2 8 276. 180 3.07 3.78 18 0 0 3 3
#> 8 10.4 8 472 205 2.93 5.25 18.0 0 0 3 4
#> 9 10.4 8 460 215 3 5.42 17.8 0 0 3 4
#> 10 14.7 8 440 230 3.23 5.34 17.4 0 0 3 4
#> 11 21.5 4 120. 97 3.7 2.46 20.0 1 0 3 1
#> 12 15.5 8 318 150 2.76 3.52 16.9 0 0 3 2
#> 13 15.2 8 304 150 3.15 3.44 17.3 0 0 3 2
#> 14 13.3 8 350 245 3.73 3.84 15.4 0 0 3 4
#> 15 19.2 8 400 175 3.08 3.84 17.0 0 0 3 2
# 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$new(f), chunk_size = 5)
#>
#> ── Column specification ──────────────────────────────────────────────────
#> cols(
#> mpg = col_double(),
#> cyl = col_double(),
#> disp = col_double(),
#> hp = col_double(),
#> drat = col_double(),
#> wt = col_double(),
#> qsec = col_double(),
#> vs = col_double(),
#> am = col_double(),
#> gear = col_double(),
#> carb = col_double()
#> )
#> [[1]]
#> [1] 21.0 21.0 21.4 18.7
#>
#> [[2]]
#> [1] 18.1 14.3 19.2
#>
#> [[3]]
#> [1] 17.8 16.4 17.3 15.2 10.4
#>
#> [[4]]
#> [1] 10.4 14.7
#>
#> [[5]]
#> [1] 15.5 15.2 13.3 19.2
#>
#> [[6]]
#> [1] 30.4 15.8 19.7
#>
#> [[7]]
#> [1] 15.0 21.4
#>
# The AccumulateCallback accumulates results from each chunk
f <- function(x, pos, acc) sum(x$mpg) + acc
read_csv_chunked(readr_example("mtcars.csv"), AccumulateCallback$new(f, acc = 0), chunk_size = 5)
#>
#> ── Column specification ──────────────────────────────────────────────────
#> cols(
#> mpg = col_double(),
#> cyl = col_double(),
#> disp = col_double(),
#> hp = col_double(),
#> drat = col_double(),
#> wt = col_double(),
#> qsec = col_double(),
#> vs = col_double(),
#> am = col_double(),
#> gear = col_double(),
#> carb = col_double()
#> )
#> [1] 642.9