How do you pivot data from a list of data frames in R?

Let's say I have a list of data frames ldf:

``````df1 <- data.frame(date = c(1,2), value = c(4,5))
df2 <- data.frame(date = c(1,2), value = c(4,5))
ldf <- list(df1, df2)
``````

What is the best way to get the sum (or any other function) of values by date, i.e. some data frame like:

``````data.frame(date = c(1,2), value = c(8,10))
``````

You could use:

``````library(data.table)
dt1 <- rbindlist(ldf)
setkey(dt1,'date')
dt1[,list(value=sum(value)), by='date']
date value
1:    1     8
2:    2    10
``````

If these rows were all in the same data frame, you would use `aggregate` to do the sum. You can combine them with `rbind` so they are in the same data frame:

``````aggregate(value ~ date, data=do.call(rbind, ldf), FUN=sum)
date value
1    1     8
2    2    10
``````

If the `date` columns in all the data frames are identical, you can easily use `Reduce` to do the sum:

``````Reduce(function(x, y) data.frame(date=x\$date, value=x\$value+y\$value), ldf)
date value
1    1     8
2    2    10
``````

This should be a lot faster than `rbind`-ing the data together and aggregating.

Another option is to use `unnest` from "tidyr" in conjunction with the typical grouping and aggregation functions via "dplyr":

``````library(dplyr)
library(tidyr)

unnest(ldf) %>%
group_by(date) %>%
summarise(value = sum(value))
# Source: local data frame [2 x 2]
#
#   date value
# 1    1     8
# 2    2    10
``````
The answers/resolutions are collected from stackoverflow, are licensed under cc by-sa 2.5 , cc by-sa 3.0 and cc by-sa 4.0 .

# More Articles