Using group_by() function on multiple data frames? - r

Question

I have data that were collected from a year but are broken up by months. For my code, I labeled them df1-df12 for each corresponding month. I am trying to group these data using the group_by function to group all the dataframes similarly. When I do the following code- it works fine alone:
df <- df %>%
group_by(date,id) %>%
slice(n()) %>%
ungroup()
However, I would like to streamline this code so that I can use this function for all 12 dataframes without having to copy/paste 12 times, since there is a lot of data to go through. Here is what I have tried to do to that end:
func1<-function(df)
{
df <- df %>%
group_by(date,id) %>%
slice(n()) %>%
ungroup()
}
yr19<-c(df1, df2, df3, df4, df5, df6, df7, df8, df9, df10, df11, df12)
map(yr19, func1)
However, i get the following error message: Error in UseMethod("group_by") :
no applicable method for 'group_by' applied to an object of class "character". As stated above- i don't get this error message if I go through and do it individually, but there are many months and many years to be analyzed and from a time perspective I don't think doing this code manually is feasible. Thanks for your help

Answer 1

Two ways you can approach this, first using the approach suggested by #ktiu:
## Create example data
library(dplyr) # for pipe and group_by()
set.seed(914)
df1 <- tibble(
date = sample(1:30, 50, replace = T),
id = sample(1:10, 50, replace = T),
var1 = rnorm(50, mean = 10, sd = 3)
)
df2 <- tibble(
date = sample(1:30, 50, replace = T),
id = sample(1:10, 50, replace = T),
var1 = rnorm(50, mean = 10, sd = 3)
)
Modifying your function to address error
func1<-function(df)
{
df <- df %>%
group_by(date,id) %>%
slice(n()) %>%
ungroup()
df
}
## And using list rather than c to combine data frames.
yr19 <- list(df1, df2)
yr19_data <- lapply(yr19, func1)
# This will return a list of data frames you can access with `yr19_data[[1]]`
Alternative approach is to add variable for your source data frames, then collapse it all into a single data frame and manipulate from there. Which approach makes more sense will depend on what else you want to do later.
func2 <- function(df.name){
mutate(get(df.name), source = df.name)
}
# This is set up to get objects given their names, so we'll use a character vector
# of names to iterate off of.
yr19 = c("df1", "df2")
df.list <- lapply(yr19, func2)
df.long <- do.call(bind_rows, df.list)
df.long
# # A tibble: 100 x 4
# date id var1 source
# <int> <int> <dbl> <chr>
# 1 27 9 9.31 df1
# 2 5 3 16.5 df1
# 3 28 3 2.67 df1
# 4 24 4 8.94 df1
# 5 13 3 1.68 df1
At this point you can manipulate one data frame in your original pipe:
df <- df.long %>%
group_by(source, date,id) %>%
slice(n()) %>%
ungroup()
df
# # A tibble: 93 x 4
# date id var1 source
# <int> <int> <dbl> <chr>
# 1 1 8 9.89 df1
# 2 2 4 10.9 df1
# 3 4 3 8.45 df1
# 4 5 3 16.5 df1
# 5 5 7 10.6 df1