df_ridge <- function(lambda,X){ 
  #
  # R code to compute the degress of freedom for ridge regression.
  # This is formula 3.50 in the book.
  #
  # Input:
  #   lambda: the penalty term coefficent 
  #   X the data matrix 
  #
  # Output:
  #
  # dof: the degrees of freedom.
  #
  # Written by:
  # -- 
  # John L. Weatherwax                2009-04-21
  # 
  # email: wax@alum.mit.edu
  # 
  # Please send comments and especially bug reports to the
  # above email address.
  # 
  #-----

  library(MASS) # needed for ginv

  XTX = t(X) %*% X
  pprime = dim(XTX)[1] 

  Hlambda = X %*% ginv( XTX + lambda * diag(pprime) ) %*% t(X)
    
  dof = sum( diag( Hlambda ) )

  return(dof)
  
}