R - Add columns to dataframes in list by looping through elements in a vector

问题

I am working with several datasets that measure the same variables over many years. I am trying to add a year variable to each dataset, but more generally I want to loop through elements in a vector and add each as a new column in a list of dataframes. This question was similar to mine but I want to iteratively add each element in a vector to the corresponding dataframe as a new column: R - New variables over several data frames in a loop

Here's sample data:

year <- c(1:3)
data1 <- data.frame(var1 = c(1:5))
data2 <- data.frame(var1 = c(11:15))
data3 <- data.frame(var1 = c(21:25))
data_list <- list(data1 = data1, data2 = data2, data3 = data3)

I want to do this but think there's probably some way to loop (or lapply) that I haven't been able to figure out yet:

data1$year <- year[1]
data2$year <- year[2]
data3$year <- year[3]

Since I have many years and datasets to work with, it'd be great to have a more efficient solution. Thanks!

回答1:

The full answer based on @@thelatemail comment:

The function Map(cbind, data_list, year=year) should do the job. Step by step:

• Map function indicates a loop through elements in a list
• cbind attaches newly created column
• year = years creates new column named year based on elements in a vector years

---

Having your dummy example:

# vector of values you wish to add
years <- c(1:3)     # changed to plural to indicate vector of values rather than single value

# make dummy list of dataframes
data1 <- data.frame(var1 = c(1:5))
data2 <- data.frame(var1 = c(11:15))
data3 <- data.frame(var1 = c(21:25))
data_list <- list(data1 = data1, data2 = data2, data3 = data3)

# Loop through list of dataframes and to each dataframe add a new column
Map(cbind, data_list, year=years)

The output you wish:

$`data1`
  var1 year
1    1    1
2    2    1
3    3    1
4    4    1
5    5    1

$data2
  var1 year
1   11    2
2   12    2
3   13    2
4   14    2
5   15    2

$data3
  var1 year
1   21    3
2   22    3
3   23    3
4   24    3
5   25    3

来源：https://stackoverflow.com/questions/40251202/r-add-columns-to-dataframes-in-list-by-looping-through-elements-in-a-vector