# Re: [R] building a formula string bit by bit ..

From: Marc Schwartz <marc_schwartz_at_comcast.net>
Date: Tue, 06 May 2008 14:18:34 -0500

Esmail Bonakdarian wrote:
> Hello,
>
> Still a newbie with R, though I have learned a lot from reading
> this list. I'm hoping someone can help with this question:
>
> I have two vectors, one for variables, and one for bits.
>
> I want to build a string (really a formula) based on the values in my
> vector of 1s and 0s in bits. If I have a one, I want to include the
> corresponding entry in the vars vector, otherwise ignore it. Of course
> the "bits" vector will not stay the same.
>
> So for example:
>
> vars=c('X.1', 'X.2', 'X.3', 'X.4', 'X.5')
>
> bits=c(1, 0, 1, 1, 0)
> ones=which(bits==1)
>
> should yield:
>
> "X.1 + X.3 + X.4"
>
> where as
>
> bits=c(1, 1, 0, 0, 0)
>
> would yield
>
> "X.1 + X.2"
>
> the "which" operator gives me the index values, is there an easy and
> *efficient* way to build this string so that I can pass it on to glm?
> I can probably hack some ugly code together to do this, but it won't
> be efficient, and it won't be elegant :-)
>
> Can anyone help?
>
> Thanks!

Depending upon other factors, there may be a better and more general approach to what you are trying to do relative to selecting subsets of IV's. However, given the information you have provided:

vars <- c('X.1', 'X.2', 'X.3', 'X.4', 'X.5')    bits <- c(1, 0, 1, 1, 0)

> paste(vars[bits == 1], collapse = " + ")    [1] "X.1 + X.3 + X.4"

bits <- c(1, 1, 0, 0, 0)

> paste(vars[bits == 1], collapse = " + ")    [1] "X.1 + X.2"

You will then need to use as.formula() to coerce the resultant vectors before passing them to the model function.

HTH, Marc Schwartz

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