# sweepex.r                    J F Monahan September 2007
 #
 # sweep operator -- function and example
 #
  A <- matrix( c(3,1,0,1,4,2,0,2,6), 3, 3)
 #  write it out
  A
 #  define function
   regsweep <- function(A,k) {
              d <- A[k,k]               # sweep out row k, col k
              A[k,] <- A[k,]/d
              b <- A[,k]
              b[k] <- 0                 # don't change row k here
              A <- A - outer(b,A[k,])   # main operation
              A[,k] <- -b/d             # fix col k
              A[k,k] <- 1/d             # diagonal element & done
              regsweep <- A     }
 # 
 #  write it out
  A <- regsweep(A,2)                    # sweep second row/col
  A
  A <- regsweep(A,3)                    # sweep third row/col
  A
  A <- regsweep(A,2)                    # sweep second row/col
  A
  A <- regsweep(A,1)                    # sweep first row/col
  A
 rm(list=ls())                # clean up