[R] convert text to exprission good for lm arguments

From: Vadim Ogranovich <vogranovich_at_jumptrading.com>
Date: Thu, 03 May 2007 07:57:45 -0500 (CDT)


Hi,

I ran into a problem of converting a text representation of an expression into parsed expression to be further evaluated inside lm ().

> n <- 100
> data <- data.frame(x= rnorm (n), y= rnorm (n))
> data. lm <- lm (y ~ x, data=data)
>
> ## this works
> update(data. lm , subset=x<0)

Call:
lm (formula = y ~ x, data = data, subset = x < 0)

Coefficients:
(Intercept) x
-0.07864094193322170023 -0.14596982635007796358

>
> ## this doesn't work
> ## text representation of subset
> subset <- "x<0"
> update(data. lm , subset=parse(text=subset))
Error in `[.data.frame`(list(y = c(-0.601925958140825, -0.111931189071517, : invalid subscript type

What is the correct way to convert "x<0" into a valid subset argument?

Thanks,
Vadim

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