Fit gam with dynamic df and choice of 2 rabbit variables -- example of using substitute and eval

gam_fun_rabbits

gam_fun <- function(data, rabbit_var = c("rabbitwarrens", "warrenspretreatment"), df = integer(length = 1)){
  if(rabbit_var == "rabbitwarrens"){
    f <- substitute(gam(peren_chen_pc_cover ~ s(winterrain, df) + s(otherrain, df) + year + site + s(warrenspretreatment, df) + s(regionalroos, df) + landsystem, data=data, family=gaussian))
  } else{
    f <- substitute(gam(peren_chen_pc_cover ~ s(winterrain, df) + s(otherrain, df) + year + site + s(rabbitwarrens, df) + s(regionalroos, df) + landsystem, data=data, family=gaussian))
  }
  eval(f)
}