在 R 中的 data.table 环境中创建公式 [英] create a formula in a data.table environment in R
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问题描述
I would like to run a regression within a data.table
. The formula
needs to be constructed dynamically. I have tried the following method:
x = data.table(a=1:20, b=20:1, id=1:5)
> x[,as.list(coef(lm(as.formula("a ~ b")))),by=id]
Error in eval(expr, envir, enclos) : object 'a' not found
How does one specify the environment to be that of the actual data.table where the evaluation occurs?
EDIT: I realize I can do lm(a ~ b). I need the formula to be dynamic so it's built up as a character string. By dynamically I mean the formula can be paste0(var_1, "~", var_2)
where var_1 = a
and var_2 = b
Here is one solution thought I think we can do better:
txt = parse(text="as.list(coef(lm(a ~ b)))")
> x[,eval(txt),by=id]
id (Intercept) b
1: 1 21 -1
2: 2 21 -1
3: 3 21 -1
4: 4 21 -1
5: 5 21 -1
解决方案
lm
can accept a character string as the formula so combine that with .SD
like this:
> x[, as.list(coef(lm("a ~ b", .SD))), by = id]
id (Intercept) b
1: 1 21 -1
2: 2 21 -1
3: 3 21 -1
4: 4 21 -1
5: 5 21 -1
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