Formula with dynamic number of variables

See ?as.formula, e.g.:

factors <- c("factor1", "factor2")
as.formula(paste("y~", paste(factors, collapse="+")))
# y ~ factor1 + factor2

where factors is a character vector containing the names of the factors you want to use in the model. This you can paste into an lm model, e.g.:

set.seed(0)
y <- rnorm(100)
factor1 <- rep(1:2, each=50)
factor2 <- rep(3:4, 50)
lm(as.formula(paste("y~", paste(factors, collapse="+"))))

# Call:
# lm(formula = as.formula(paste("y~", paste(factors, collapse = "+"))))

# Coefficients:
# (Intercept)      factor1      factor2  
#    0.542471    -0.002525    -0.147433

An oft forgotten function is reformulate. From ?reformulate:

reformulate creates a formula from a character vector.

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A simple example:

listoffactors <- c("factor1","factor2")
reformulate(termlabels = listoffactors, response = 'y')

will yield this formula:

y ~ factor1 + factor2

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Although not explicitly documented, you can also add interaction terms:

listofintfactors <- c("(factor3","factor4)^2")
reformulate(termlabels = c(listoffactors, listofintfactors), 
    response = 'y')

will yield:

y ~ factor1 + factor2 + (factor3 + factor4)^2