do: Do arbitrary operations on a tbl.

In sctyner/dplyr050: A Grammar of Data Manipulation

Description

This is a general purpose complement to the specialised manipulation functions filter, select, mutate, summarise and arrange. You can use do to perform arbitrary computation, returning either a data frame or arbitrary objects which will be stored in a list. This is particularly useful when working with models: you can fit models per group with do and then flexibly extract components with either another do or summarise.

Usage

do(.data, ...)

do_(.data, ..., .dots)

## S3 method for class 'tbl_sql'
do_(.data, ..., .dots, .chunk_size = 10000L)

Arguments

.data	a tbl
...	Expressions to apply to each group. If named, results will be stored in a new column. If unnamed, should return a data frame. You can use . to refer to the current group. You can not mix named and unnamed arguments.
.dots	Used to work around non-standard evaluation. See vignette("nse") for details.
.chunk_size	The size of each chunk to pull into R. If this number is too big, the process will be slow because R has to allocate and free a lot of memory. If it's too small, it will be slow, because of the overhead of talking to the database.

Details

For an empty data frame, the expressions will be evaluated once, even in the presence of a grouping. This makes sure that the format of the resulting data frame is the same for both empty and non-empty input.

Value

do always returns a data frame. The first columns in the data frame will be the labels, the others will be computed from .... Named arguments become list-columns, with one element for each group; unnamed elements must be data frames and labels will be duplicated accordingly.

Groups are preserved for a single unnamed input. This is different to summarise because do generally does not reduce the complexity of the data, it just expresses it in a special way. For multiple named inputs, the output is grouped by row with rowwise. This allows other verbs to work in an intuitive way.

Connection to plyr

If you're familiar with plyr, do with named arguments is basically equivalent to dlply, and do with a single unnamed argument is basically equivalent to ldply. However, instead of storing labels in a separate attribute, the result is always a data frame. This means that summarise applied to the result of do can act like ldply.

Examples

by_cyl <- group_by(mtcars, cyl)
do(by_cyl, head(., 2))

models <- by_cyl %>% do(mod = lm(mpg ~ disp, data = .))
models

summarise(models, rsq = summary(mod)$r.squared)
models %>% do(data.frame(coef = coef(.$mod)))
models %>% do(data.frame(
  var = names(coef(.$mod)),
  coef(summary(.$mod)))
)

models <- by_cyl %>% do(
  mod_linear = lm(mpg ~ disp, data = .),
  mod_quad = lm(mpg ~ poly(disp, 2), data = .)
)
models
compare <- models %>% do(aov = anova(.$mod_linear, .$mod_quad))
# compare %>% summarise(p.value = aov$`Pr(>F)`)

if (require("nycflights13")) {
# You can use it to do any arbitrary computation, like fitting a linear
# model. Let's explore how carrier departure delays vary over the time
carriers <- group_by(flights, carrier)
group_size(carriers)

mods <- do(carriers, mod = lm(arr_delay ~ dep_time, data = .))
mods %>% do(as.data.frame(coef(.$mod)))
mods %>% summarise(rsq = summary(mod)$r.squared)

## Not run: 
# This longer example shows the progress bar in action
by_dest <- flights %>% group_by(dest) %>% filter(n() > 100)
library(mgcv)
by_dest %>% do(smooth = gam(arr_delay ~ s(dep_time) + month, data = .))

## End(Not run)
}

sctyner/dplyr050 documentation built on May 17, 2019, 2:22 p.m.