[R] building a formula string bit by bit ..

[Marc Schwartz]

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