[R] formula () problems

[Marc Schwartz]

On Dec 9, 2009, at 2:22 PM, Dr R.K.S. Hankin wrote:

> Hi.
>
> I am having difficulty creating a formula for use with glm()
>
> I have a matrix of an unknown number of columns and wish to estimate a
> coefficient for each column, and one for the each product of a column
> with another column.
>
> In the case of a five-column matrix this would be:
>
>> x <- matrix(rnorm(100),ncol=5)
>> colnames(x) <- letters[1:5]
>> z <- rnorm(20)
>> lm(z~ -1+(a+b+c+d+e)^2,data=data.frame(x))
>
> Call:
> lm(formula = z ~ -1 + (a + b + c + d + e)^2, data = data.frame(x))
>
> Coefficients:
>      a b c d e a:b a:c a:d -0.30021 -0.21465 0.12208 0.06308 0.28806  
> 0.34482 -1.00072 0.48218
>    a:e       b:c       b:d       b:e       c:d       c:e       d:e   
> 0.28786  -0.46306   0.39844   0.04436   0.32236  -0.09210  -1.06625
>
> This is what I want: five single terms (a-e) and 5*(5-1)/2=10 (a:b to
> d:e) for the cross terms.  If there were 6 columns I would want
> (a+b+c+d+e+f)^2 and have 21 (=6+15) terms.
>
> How do I create a formula that does this for an arbitrary number of  
> columns?
>
>
> thanks
>
> Robin


Robin,

Try this:

   lm(z ~ (.)^2 - 1, data = data.frame(x))

See the Details section of ?formula, which describes the use of '.' to  
refer to all columns not otherwise already in the formula.

HTH,

Marc Schwartz