The mget() fuction is a multiple get() function

We use mget() to retrieve multiple objects by their names

I have found this helpful when I want to perform operations on many df (with similar names) without having to type out each name.

For example, I can create four data.frames. They all have similar name patterns. The only difference is the number.

df_1_pr <- data.frame(x = 1:4, y = letters[1:4])
df_2_pr <- data.frame(x = 5:8, y = letters[5:8])
df_3_pr <- data.frame(x = 9:12, y = letters[9:12])
df_4_pr <- data.frame(x = 13:16, y = letters[13:16])

Here is a quick look at the df1 data.frame

      x    y   
1    1    a      
2    2    b      
3    3    c      
4    4    d      

We can make a list of the four data.frames using mget()

df_list <- mget(c("df_1_pr", "df_2_pr", "df_3_pr", "df_4_pr))

Or we can alternatively add paste0() and feed in a sequence from 1:4 to write the code more quickly in mget() instead

df_list <- mget(paste0("df_", 1:4, "_pr"))

We can make a function to create a new column to each data frame in the list

df_list <- lapply(df_list, function(df) { 
   df$x_mult_2 <- df$x * 2  
   return(df) }) 

We can look at the fourth data.frame in the list,

df_list[4]

   x y x_mult_2
1 13 m       26
2 14 n       28
3 15 o       30
4 16 p       32

Before we combine all the data.frames, we can make an ID variable for each df with the following function:

add_id_variable <- function(df_list) {
  for (i in seq_along(df_list)) {
    df_list[[i]]$id <- i}
  return(df_list)}

Add a year variable

add_year_variable <- function(df_list) {
  years <- 2017:2020
  for (i in seq_along(df_list)) {
    df_list[[i]]$year <- rep(years[i], nrow(df_list[[i]]))}
 return(df_list)}

The rep(years[i], nrow(df_list[[i]])) repeats the i-th year from the years vector (years[i]) for nrow(df_list[[i]]) times.

nrow(df_list[[i]]) is the number of rows in the selected data frame.

We can run the function with the df_list

df_list <- add_id_variable(df_list)

Now we can convert the list into a data.frame with the do.call() function in R

all_df_pr <- do.call(rbind, df_list)

do.call() runs a function with a list of arguments we supply.

do.call(fun, args)

For us, we can feed in the rbind() function with all the data.frames in the df_list. It is quicker than writing out all the data.frames names into the rbind() function direction.