Calculate average daily value from large data set with R standard format date/times?

Question

I have a dataframe of approximately 10 million rows spanning about 570 days. After using striptime to convert the dates and times, the data looks like this:

          date          X1   
1 2004-01-01 07:43:00 1.2587 
2 2004-01-01 07:47:52 1.2585
3 2004-01-01 17:46:14 1.2586 
4 2004-01-01 17:56:08 1.2585
5 2004-01-01 17:56:15 1.2585 

I would like to compute the average value on each day (as in days of the year, not days of the week) and then plot them. Eg. Get all rows which have day "2004-01-01", compute average price, then do the same for "2004-01-2" and so on.

Similarly I would be interested in finding the average monthly value, or hourly price, but I imagine I can work these out once I know how to get average daily price.

My biggest difficulty here is extracting the day of the year from the date variable automatically. How can I cycle through all 365 days and compute the average value for each day, storing it in a list?

I was able to find the average value for day of the week using the weekdays() function, but I couldn't find anything similar for this.

Answer 1

day of year in lubridate is

yday, as in

lubridate::yday(Sys.time())

because the size of data is big I recommend a data.table approach

library(lubridate)
library(data.table)
df$ydate=yday(df$date)
df=data.table(df)
df[,mean(X1),ydate]

if you want different days for different years as in 1Jan2004 and 1Jan2005

library(lubridate)
library(data.table)
df$ydate=ymd(df$date)
df=data.table(df)
df[,mean(X1),ydate]

Note -instead of using striptime to convert dates you could just use ymd_hms function from lubridate

Answer 2

Just to contribute, here is the solution to do it for multiple columns in your data frame. It consists of the same method as George, so a little more is added an using summarise:

new_df <- df %>% mutate(date = hour(date)) %>%
group_by(date) %>%
summarise(across(.cols = where(is.numeric), .fns = ~mean(.x, na.rm = TRUE))

In this case, in ".cols" it is specified that the operation be applied to all columns with numeric format (you can modify it for specific columns). In the ".fns" section you can put the operation you want to perform (mean, sd, etc.) and you can apply na.rm.

Greetings!