A common task in financial analyses is to perform a rolling
calculation. This might be a single value like a rolling mean or
standard deviation, or it might be more complicated like a rolling
linear regression. To account for this flexibility,
tibbletime has the rollify() function. This
function allows you to turn any function into a rolling version
of itself.
In the tidyverse, this type of function is known as an
adverb because it modifies an existing function, which
are typically given verb names.
library(tibbletime)
library(dplyr)
library(tidyr)
# Facebook stock prices.
data(FB)
# Only a few columns
FB <- select(FB, symbol, date, open, close, adjusted)To calculate a rolling average, picture a column in a data frame where you take the average of the values in rows 1-5, then in rows 2-6, then in 3-7, and so on until you reach the end of the dataset. This type of 5-period moving window is a rolling calculation, and is often used to smooth out noise in a dataset.
Let’s see how to do this with rollify().
# The function to use at each step is `mean`.
# The window size is 5
rolling_mean <- rollify(mean, window = 5)
rolling_mean## function (...)
## {
## roller(..., .f = .f, window = window, unlist = unlist, na_value = na_value)
## }
## <bytecode: 0x7fa00f377cf0>
## <environment: 0x7fa00f3776d0>We now have a rolling version of the function, mean().
You use it in a similar way to how you might use
mean().
mutate(FB, mean_5 = rolling_mean(adjusted))## # A tibble: 1,008 × 6
## symbol date open close adjusted mean_5
## <chr> <date> <dbl> <dbl> <dbl> <dbl>
## 1 FB 2013-01-02 27.4 28 28 NA
## 2 FB 2013-01-03 27.9 27.8 27.8 NA
## 3 FB 2013-01-04 28.0 28.8 28.8 NA
## 4 FB 2013-01-07 28.7 29.4 29.4 NA
## 5 FB 2013-01-08 29.5 29.1 29.1 28.6
## 6 FB 2013-01-09 29.7 30.6 30.6 29.1
## 7 FB 2013-01-10 30.6 31.3 31.3 29.8
## 8 FB 2013-01-11 31.3 31.7 31.7 30.4
## 9 FB 2013-01-14 32.1 31.0 31.0 30.7
## 10 FB 2013-01-15 30.6 30.1 30.1 30.9
## # … with 998 more rowsYou can create multiple versions of the rolling function if you need to calculate the mean at multiple window lengths.
rolling_mean_2 <- rollify(mean, window = 2)
rolling_mean_3 <- rollify(mean, window = 3)
rolling_mean_4 <- rollify(mean, window = 4)
FB %>% mutate(
rm10 = rolling_mean_2(adjusted),
rm20 = rolling_mean_3(adjusted),
rm30 = rolling_mean_4(adjusted)
)## # A tibble: 1,008 × 8
## symbol date open close adjusted rm10 rm20 rm30
## <chr> <date> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
## 1 FB 2013-01-02 27.4 28 28 NA NA NA
## 2 FB 2013-01-03 27.9 27.8 27.8 27.9 NA NA
## 3 FB 2013-01-04 28.0 28.8 28.8 28.3 28.2 NA
## 4 FB 2013-01-07 28.7 29.4 29.4 29.1 28.6 28.5
## 5 FB 2013-01-08 29.5 29.1 29.1 29.2 29.1 28.8
## 6 FB 2013-01-09 29.7 30.6 30.6 29.8 29.7 29.5
## 7 FB 2013-01-10 30.6 31.3 31.3 30.9 30.3 30.1
## 8 FB 2013-01-11 31.3 31.7 31.7 31.5 31.2 30.7
## 9 FB 2013-01-14 32.1 31.0 31.0 31.3 31.3 31.1
## 10 FB 2013-01-15 30.6 30.1 30.1 30.5 30.9 31.0
## # … with 998 more rowsrollify() is built using pieces from the
purrr package. One of those is the ability to accept an
anonymous function using the ~ function syntax.
The documentation, ?rollify, gives a thorough
walkthrough of the different forms you can pass to
rollify(), but let’s see a few more examples.
# Rolling mean, but with function syntax
rolling_mean <- rollify(.f = ~mean(.x), window = 5)
mutate(FB, mean_5 = rolling_mean(adjusted))## # A tibble: 1,008 × 6
## symbol date open close adjusted mean_5
## <chr> <date> <dbl> <dbl> <dbl> <dbl>
## 1 FB 2013-01-02 27.4 28 28 NA
## 2 FB 2013-01-03 27.9 27.8 27.8 NA
## 3 FB 2013-01-04 28.0 28.8 28.8 NA
## 4 FB 2013-01-07 28.7 29.4 29.4 NA
## 5 FB 2013-01-08 29.5 29.1 29.1 28.6
## 6 FB 2013-01-09 29.7 30.6 30.6 29.1
## 7 FB 2013-01-10 30.6 31.3 31.3 29.8
## 8 FB 2013-01-11 31.3 31.7 31.7 30.4
## 9 FB 2013-01-14 32.1 31.0 31.0 30.7
## 10 FB 2013-01-15 30.6 30.1 30.1 30.9
## # … with 998 more rowsYou can create anonymous functions (functions without a name) on the fly.
# 5 period average of 2 columns (open and close)
rolling_avg_sum <- rollify(~ mean(.x + .y), window = 5)
mutate(FB, avg_sum = rolling_avg_sum(open, close))## # A tibble: 1,008 × 6
## symbol date open close adjusted avg_sum
## <chr> <date> <dbl> <dbl> <dbl> <dbl>
## 1 FB 2013-01-02 27.4 28 28 NA
## 2 FB 2013-01-03 27.9 27.8 27.8 NA
## 3 FB 2013-01-04 28.0 28.8 28.8 NA
## 4 FB 2013-01-07 28.7 29.4 29.4 NA
## 5 FB 2013-01-08 29.5 29.1 29.1 56.9
## 6 FB 2013-01-09 29.7 30.6 30.6 57.9
## 7 FB 2013-01-10 30.6 31.3 31.3 59.1
## 8 FB 2013-01-11 31.3 31.7 31.7 60.4
## 9 FB 2013-01-14 32.1 31.0 31.0 61.4
## 10 FB 2013-01-15 30.6 30.1 30.1 61.8
## # … with 998 more rowsTo pass optional arguments (not .x or .y)
to your rolling function, they must be specified in the non-rolling form
in the call to rollify().
For instance, say our dataset had NA values, but we
still wanted to calculate an average. We need to specify
na.rm = TRUE as an argument to mean().
FB$adjusted[1] <- NA
# Do this
rolling_mean_na <- rollify(~mean(.x, na.rm = TRUE), window = 5)
FB %>% mutate(mean_na = rolling_mean_na(adjusted))## # A tibble: 1,008 × 6
## symbol date open close adjusted mean_na
## <chr> <date> <dbl> <dbl> <dbl> <dbl>
## 1 FB 2013-01-02 27.4 28 NA NA
## 2 FB 2013-01-03 27.9 27.8 27.8 NA
## 3 FB 2013-01-04 28.0 28.8 28.8 NA
## 4 FB 2013-01-07 28.7 29.4 29.4 NA
## 5 FB 2013-01-08 29.5 29.1 29.1 28.8
## 6 FB 2013-01-09 29.7 30.6 30.6 29.1
## 7 FB 2013-01-10 30.6 31.3 31.3 29.8
## 8 FB 2013-01-11 31.3 31.7 31.7 30.4
## 9 FB 2013-01-14 32.1 31.0 31.0 30.7
## 10 FB 2013-01-15 30.6 30.1 30.1 30.9
## # … with 998 more rows# Don't try this!
# rolling_mean_na <- rollify(~mean(.x), window = 5)
# FB %>% mutate(mean_na = rolling_mean_na(adjusted, na.rm = TRUE))
# Reset FB
data(FB)
FB <- select(FB, symbol, date, adjusted)Say our rolling function returned a call to a custom
summary_df() function. This function calculates a 5 number
number summary and returns it as a tidy data frame.
We won’t be able to use the rolling version of this out of the box.
dplyr::mutate() will complain that an incorrect number of
values were returned since rollify() attempts to unlist at
each call. Essentially, each call would be returning 5 values instead of
1. What we need is to be able to create a list-column. To do this,
specify unlist = FALSE in the call to
rollify().
# Our data frame summary
summary_df <- function(x) {
data.frame(
rolled_summary_type = c("mean", "sd", "min", "max", "median"),
rolled_summary_val = c(mean(x), sd(x), min(x), max(x), median(x))
)
}
# A rolling version, with unlist = FALSE
rolling_summary <- rollify(~summary_df(.x), window = 5,
unlist = FALSE)
FB_summarised <- mutate(FB, summary_list_col = rolling_summary(adjusted))
FB_summarised## # A tibble: 1,008 × 4
## symbol date adjusted summary_list_col
## <chr> <date> <dbl> <list>
## 1 FB 2013-01-02 28 <lgl [1]>
## 2 FB 2013-01-03 27.8 <lgl [1]>
## 3 FB 2013-01-04 28.8 <lgl [1]>
## 4 FB 2013-01-07 29.4 <lgl [1]>
## 5 FB 2013-01-08 29.1 <df [5 × 2]>
## 6 FB 2013-01-09 30.6 <df [5 × 2]>
## 7 FB 2013-01-10 31.3 <df [5 × 2]>
## 8 FB 2013-01-11 31.7 <df [5 × 2]>
## 9 FB 2013-01-14 31.0 <df [5 × 2]>
## 10 FB 2013-01-15 30.1 <df [5 × 2]>
## # … with 998 more rowsThe neat thing is that after removing the NA values at
the beginning, the list-column can be unnested using
tidyr::unnest() giving us a nice tidy 5-period rolling
summary.
FB_summarised %>%
filter(!is.na(summary_list_col)) %>%
unnest(cols = summary_list_col)## # A tibble: 5,020 × 5
## symbol date adjusted rolled_summary_type rolled_summary_val
## <chr> <date> <dbl> <chr> <dbl>
## 1 FB 2013-01-08 29.1 mean 28.6
## 2 FB 2013-01-08 29.1 sd 0.700
## 3 FB 2013-01-08 29.1 min 27.8
## 4 FB 2013-01-08 29.1 max 29.4
## 5 FB 2013-01-08 29.1 median 28.8
## 6 FB 2013-01-09 30.6 mean 29.1
## 7 FB 2013-01-09 30.6 sd 1.03
## 8 FB 2013-01-09 30.6 min 27.8
## 9 FB 2013-01-09 30.6 max 30.6
## 10 FB 2013-01-09 30.6 median 29.1
## # … with 5,010 more rowsThe last example was a little clunky because to unnest we had to
remove the first few missing rows manually. If those missing values were
empty data frames then unnest() would have known how to
handle them. Luckily, the na_value argument will allow us
to specify a value to fill the NA spots at the beginning of
the roll.
rolling_summary <- rollify(~summary_df(.x), window = 5,
unlist = FALSE, na_value = data.frame())
FB_summarised <- mutate(FB, summary_list_col = rolling_summary(adjusted))
FB_summarised## # A tibble: 1,008 × 4
## symbol date adjusted summary_list_col
## <chr> <date> <dbl> <list>
## 1 FB 2013-01-02 28 <df [0 × 0]>
## 2 FB 2013-01-03 27.8 <df [0 × 0]>
## 3 FB 2013-01-04 28.8 <df [0 × 0]>
## 4 FB 2013-01-07 29.4 <df [0 × 0]>
## 5 FB 2013-01-08 29.1 <df [5 × 2]>
## 6 FB 2013-01-09 30.6 <df [5 × 2]>
## 7 FB 2013-01-10 31.3 <df [5 × 2]>
## 8 FB 2013-01-11 31.7 <df [5 × 2]>
## 9 FB 2013-01-14 31.0 <df [5 × 2]>
## 10 FB 2013-01-15 30.1 <df [5 × 2]>
## # … with 998 more rowsNow unnesting directly:
FB_summarised %>%
unnest(cols = summary_list_col)## # A tibble: 5,020 × 5
## symbol date adjusted rolled_summary_type rolled_summary_val
## <chr> <date> <dbl> <chr> <dbl>
## 1 FB 2013-01-08 29.1 mean 28.6
## 2 FB 2013-01-08 29.1 sd 0.700
## 3 FB 2013-01-08 29.1 min 27.8
## 4 FB 2013-01-08 29.1 max 29.4
## 5 FB 2013-01-08 29.1 median 28.8
## 6 FB 2013-01-09 30.6 mean 29.1
## 7 FB 2013-01-09 30.6 sd 1.03
## 8 FB 2013-01-09 30.6 min 27.8
## 9 FB 2013-01-09 30.6 max 30.6
## 10 FB 2013-01-09 30.6 median 29.1
## # … with 5,010 more rowsFinally, if you want to actually keep those first few NA rows in the unnest, you can pass a data frame that is initialized with the same column names as the rest of the values.
rolling_summary <- rollify(~summary_df(.x), window = 5,
unlist = FALSE,
na_value = data.frame(rolled_summary_type = NA,
rolled_summary_val = NA))
FB_summarised <- mutate(FB, summary_list_col = rolling_summary(adjusted))
FB_summarised %>% unnest(cols = summary_list_col)## # A tibble: 5,024 × 5
## symbol date adjusted rolled_summary_type rolled_summary_val
## <chr> <date> <dbl> <chr> <dbl>
## 1 FB 2013-01-02 28 <NA> NA
## 2 FB 2013-01-03 27.8 <NA> NA
## 3 FB 2013-01-04 28.8 <NA> NA
## 4 FB 2013-01-07 29.4 <NA> NA
## 5 FB 2013-01-08 29.1 mean 28.6
## 6 FB 2013-01-08 29.1 sd 0.700
## 7 FB 2013-01-08 29.1 min 27.8
## 8 FB 2013-01-08 29.1 max 29.4
## 9 FB 2013-01-08 29.1 median 28.8
## 10 FB 2013-01-09 30.6 mean 29.1
## # … with 5,014 more rowsA final use of this flexible function is to calculate rolling regressions.
A very ficticious example is to perform a rolling regression on the
FB dataset of the form
close ~ high + low + volume. Notice that we have 4 columns
to pass here. This is more complicated than a .x and
.y example, but have no fear. The arguments can be
specified in order as ..1, ..2, … for as far
as is required, or you can pass a freshly created anonymous function.
The latter is what we will do so we can preserve the names of the
variables in the regression.
Again, since this returns a linear model object, we will specify
unlist = FALSE. Unfortunately there is no easy default NA
value to pass here.
# Reset FB
data(FB)
rolling_lm <- rollify(.f = function(close, high, low, volume) {
lm(close ~ high + low + volume)
},
window = 5,
unlist = FALSE)
FB_reg <- mutate(FB, roll_lm = rolling_lm(close, high, low, volume))
FB_reg## # A tibble: 1,008 × 9
## symbol date open high low close volume adjusted roll_lm
## <chr> <date> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <list>
## 1 FB 2013-01-02 27.4 28.2 27.4 28 69846400 28 <lgl [1]>
## 2 FB 2013-01-03 27.9 28.5 27.6 27.8 63140600 27.8 <lgl [1]>
## 3 FB 2013-01-04 28.0 28.9 27.8 28.8 72715400 28.8 <lgl [1]>
## 4 FB 2013-01-07 28.7 29.8 28.6 29.4 83781800 29.4 <lgl [1]>
## 5 FB 2013-01-08 29.5 29.6 28.9 29.1 45871300 29.1 <lm>
## 6 FB 2013-01-09 29.7 30.6 29.5 30.6 104787700 30.6 <lm>
## 7 FB 2013-01-10 30.6 31.5 30.3 31.3 95316400 31.3 <lm>
## 8 FB 2013-01-11 31.3 32.0 31.1 31.7 89598000 31.7 <lm>
## 9 FB 2013-01-14 32.1 32.2 30.6 31.0 98892800 31.0 <lm>
## 10 FB 2013-01-15 30.6 31.7 29.9 30.1 173242600 30.1 <lm>
## # … with 998 more rowsTo get some useful information about the regressions, we will use
broom::tidy() and apply it to each regression using a
mutate() + map() combination.
FB_reg %>%
filter(!is.na(roll_lm)) %>%
mutate(tidied = purrr::map(roll_lm, broom::tidy)) %>%
unnest(tidied) %>%
select(symbol, date, term, estimate, std.error, statistic, p.value)## # A tibble: 4,016 × 7
## symbol date term estimate std.error statistic p.value
## <chr> <date> <chr> <dbl> <dbl> <dbl> <dbl>
## 1 FB 2013-01-08 (Intercept) -2.84e- 1 10.2 -0.0279 0.982
## 2 FB 2013-01-08 high 7.09e- 1 1.95 0.364 0.778
## 3 FB 2013-01-08 low 2.70e- 1 2.16 0.125 0.921
## 4 FB 2013-01-08 volume 1.12e- 8 0.0000000266 0.422 0.746
## 5 FB 2013-01-09 (Intercept) -5.95e+ 0 7.48 -0.796 0.572
## 6 FB 2013-01-09 high 2.08e+ 0 1.88 1.10 0.468
## 7 FB 2013-01-09 low -9.20e- 1 1.75 -0.526 0.692
## 8 FB 2013-01-09 volume -1.45e-10 0.0000000168 -0.00861 0.995
## 9 FB 2013-01-10 (Intercept) 9.55e- 1 4.46 0.214 0.866
## 10 FB 2013-01-10 high 7.17e- 1 1.30 0.553 0.679
## # … with 4,006 more rows