R help - Aug 2017 - Problem in retrieving list parameters

Hello,
I am having some problem in retrieving the list elements

Code
Mean_of_mc is a list as described below

        mean_mc_1 mean_mc_2 mean_mc_3
mean_mc 0.9577246 2.000516  2.962521
mean_mc 7.957725  9.000516  9.962521

optim(par=c(lVal=2, mu=Mean_of_mc),
                 fn=xnorm,
      method = "BFGS", control=list(fnscale=-1))


xnorm= function(par)
{
  print(par)
  x=par['parl']
  l=par['lVal']
  val=par['mu']
  #some logic
}
 get NA for val,
 how can i get all the values of Mean_of_mc

Thanks
Niharika

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