allinteract.fun_function(x.mat){
  ## this function gives all main effects and all 2-way interactions
  ## for a set of X variables.  
  ## x.mat is a cbind() of the x vars.
  ## command for the discrim data: x.mat.allinteract <- allinteract.fun(cbind(senior,age,educ,exper))
  ## will produce a dataframe with the following columns:
  ##
  ##  > > names(x.mat.allinteract)
  ## "senior"  "age"  "educ"  "exper" "senior.age"  "senior.educ"
  ## "senior.exper" "age.educ"     "age.exper"    "educ.exper"  

  x.mat_data.frame(x.mat)
  interact.mat_NULL 
  interact.names_NULL
  for (i in 1:(dim(x.mat)[2]-1)){ 
    for (j in (i+1):(dim(x.mat)[2])){
      interact_x.mat[,i]*x.mat[,j]
      interact.mat_cbind(interact.mat,interact)
      interact.names_c(interact.names,paste(names(x.mat)[i],".",names(x.mat)[j],sep=""))
    }
  }
  interact.mat_data.frame(interact.mat)
  names(interact.mat)_interact.names
  return(cbind(x.mat,interact.mat))
}

### example commands on discrim dataset:

x.mat.allinteract <- allinteract.fun(cbind(senior,age,educ,exper))

## first 5 lines of resulting dataset
> x.mat.allinteract[1:5,]
  senior age educ exper senior.age senior.educ senior.exper age.educ age.exper 
1     96 329   15  14.0      31584        1440       1344.0     4935    4606.0
2     82 357   15  72.0      29274        1230       5904.0     5355   25704.0
3     67 315   15  35.5      21105        1005       2378.5     4725   11182.5
4     97 354   12  24.0      34338        1164       2328.0     4248    8496.0
5     66 351   12  56.0      23166         792       3696.0     4212   19656.0
  educ.exper 
1      210.0
2     1080.0
3      532.5
4      288.0
5      672.0