About size of data frames

Dear R-list users,

let me ask you a very general question about performance of big data frames.

I deal with semi-hourly meteorological data of about 70 sensors during 28 winter seasons.


It means that for each sensor I have 48 data for each day, 181 days for each winter season (182 in case of leap year): 48 * 181 * 28 = 234,576

234,576 * 70 = 16420320


 From the computational point of view it is better to deal with a single data frame of approximately 16.5 M rows and 3 columns (one for data, one for sensor code and one for value), with a single data frame of approximately 235,000 rows and 141 rows or 70 different data frames of approximately 235,000 rows and 3 rows? Or it doesn't make any difference?

I personally would prefer the first choice, because it would be easier for me to deal with a single data frame and few columns.


Thank you for your usual help

Stefano


          (oo)
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Stefano Sofia MSc, PhD
Civil Protection Department - Marche Region - Italy
Meteo Section
Snow Section
Via Colle Ameno 5
60126 Torrette di Ancona, Ancona (AN)
Uff: +39 071 806 7743
E-mail: stefano.sofia at regione.marche.it
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Hello,

First of all, 48 * 181 * 28 = 243,264, not 234,576.
And 243264 * 70 = 17,028,480.

As for the question, why don't you try it with smaller data sets?
In the test bellow I have tested with the sizes you have posted and the 
many columns (wide format) is fastest. Then the df's list, then the 4 
columns (long format).
4 columns because it's sensor, day, season and data.
And the wide format df is only 72 columns wide, one for day, one for 
season and one for each sensor.

The test computes mean values aggregated by day and season. When the 
data is in the long format it must also include the sensor, so there is 
an extra aggregation column.

The test is very simple, real results probably depend on the functions 
you want to apply to the data.



# create the test data
makeDataLong <- function(sensor, x) {
   x[["data"]] <- rnorm(nrow(df1))
   cbind.data.frame(sensor, x)
}

makeDataWide <- function(sensor, x) {
   x[[sensor]] <- rnorm(nrow(x))
   x
}

set.seed(2025)

n_per_day <- 48
n_days <- 181
n_seasons <- 28
n_sensors <- 70

day <- rep(1:n_days, each = n_per_day)
season <- 1:n_seasons
sensor_names <- sprintf("sensor_%02d", 1:n_sensors)
df1 <- expand.grid(day = day, season = season, KEEP.OUT.ATTRS = FALSE)

df_list <- lapply(1:n_sensors, makeDataLong, x = df1)
names(df_list) <- sensor_names
df_long <- lapply(1:n_sensors, makeDataLong, x = df1) |> do.call(rbind, 
args = _)
df_wide <- df1
for(s in sensor_names) {
   df_wide <- makeDataWide(s, df_wide)
}


# test functions
f <- function(x) aggregate(data ~ season + day, data = x, mean)
g <- function(x) aggregate(data ~ sensor + season + day, data = x, mean)
h <- function(x) aggregate(. ~ season + day, x, mean)

# timings
bench::mark(
   list_base = lapply(df_list, f),
   long_base = g(df_long),
   wide_base = h(df_wide),
   check = FALSE
)



Hope this helps,

Rui Barradas