Get columns by string from data.table [duplicate]

问题

raw is a data.table and the following code works:

raw[,r_responseTime] #Returns the whole column
raw[,c_filesetSize]  #Same as above, returns column
plot(raw[,r_responseTime]~raw[,c_filesetSize]) #draws something

Now I want to specify these columns from a string, so for example:

col1="r_reponseTime"
col2="c_filesetSize"

How can I now achieve the same as above while referencing the columns by the string?

raw[,col1] #Returns the whole column
raw[,col2]  #Same as above, returns column
plot(raw[,col1]~raw[,col2]) #draws something

Does not work, of course because I need some kind of "dereferencation". I didn't know what to search in the help and the internet, so sorry for the dumb question.

回答1:

It would be nice if you had provided a reproducible example, or at the very least shown what the column names of raw are and what r_responseTime and c_filesetSize contain. This being said, get is your function for dereferencing so give these a try:

raw[, get(col1)]
raw[, get(col2)]
plot(raw[, get(col1)] ~ raw[, get(col2)])

回答2:

A modern approach is to use ..:

raw[ , ..col1]

.. "looks up a level" to find col1.

An older, less preferred alternative is to use the match() function or %in% operator.

raw[, match(col1, names(raw)),with=FALSE]

回答3:

If you have a vector of strings, you can use mget

cols = c("r_reponseTime", "c_filesetSize")
raw[, mget(cols)]

回答4:

Unfortunately "get" can be problematic! See example below:

m = 100
x1 = sample(c("cat", "dog", "horse"), m, replace=TRUE)
y1 = rnorm(m)
fill1 = sample(c("me", "myself", "dude"), m, replace=TRUE)
df = data.frame("x"=x1, "y"=y1, "fill"=fill1)
dt = data.table(df)

get does not work!

y = "y"
dt[ , get(y)]

get works!

yCol = "y"
dt[ , get(yCol)]

works always, but it's not pretty!

eval(parse(text = paste0("values = dt[ ,",  y, "]")))
eval(parse(text = paste0("values = dt[ ,",  yCol, "]")))

来源：https://stackoverflow.com/questions/9864055/get-columns-by-string-from-data-table

标签

data.table