R data.table：如何使用字符串中的名称创建新列？

Question

I have a vector of strings, and I need to create one new column in my data.table for each of them. Like so:

dt <- data.table(a = c(1,2,3), b = c(4,5,6))
column_names <- c("x", "y", "z")

I want to do something like this:

for (column_name in column_names) {
    dt[, column_name := paste0(column_name, a, b)]
}

This should result in something like this:

a | b |   x |   y |   z
-----------------------
1 | 4 | x14 | y14 | z14
2 | 5 | x25 | y25 | z25
3 | 6 | x36 | y36 | z36

But instead, it tries to create a column with the name "column_name" 3 times. How do I get around this?

Answer 1

You may want to use .SD instead of a loop. Replace log with whatever function you want to apply to the columns.

mtcars <- as.data.table(mtcars)
columnstolog <- c('mpg', 'cyl', 'disp', 'hp')

mtcars[, (columnstolog) := lapply(.SD, log), .SDcols = columnstolog]

Answer 2

Here's a quick and dirty way of doing this:

CODE

library(data.table)
dt <- as.data.table(mtcars)
col_names <- c("col1", "col2", "col3")
for(i in 1:length(col_names)){
  dt[, paste(col_names[i]) := i]
}

OUTPUT

> head(dt)
    mpg cyl disp  hp drat    wt  qsec vs am gear carb col1 col2 col3
1: 21.0   6  160 110 3.90 2.620 16.46  0  1    4    4    1    2    3
2: 21.0   6  160 110 3.90 2.875 17.02  0  1    4    4    1    2    3
3: 22.8   4  108  93 3.85 2.320 18.61  1  1    4    1    1    2    3
4: 21.4   6  258 110 3.08 3.215 19.44  1  0    3    1    1    2    3
5: 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2    1    2    3
6: 18.1   6  225 105 2.76 3.460 20.22  1  0    3    1    1    2    3

I'm sure there're more elegant ways of doing this.

Answer 3

Try wrapping column_name in parenthesis. For example:

mtcars <- as.data.table(mtcars)

for (col in names(mtcars)) {
mtcars[, (col) := 1]
}

So as long as you can retrieve the assigned value in the for-loop, that should get you what you want.