Transform (shuffle) just 2 Fields in a Dataframe

Question

I have a data frame with firstname, lastname and I want to permutate them but ONLY for the rows that have values. There are many null fields and I don't want reorder them so that there is ever a firstname value without a lastname value. Ex:

number<- c(1,2,3,4,5)
firstname<- c('','Eddie','Edward','','Edurardo')
lastname <- c('','Vedder', 'Van Halen', '', 'Norton')
permtest <- data.frame(number,firstname,lastname)
permtest
  number firstname  lastname
1      1                    
2      2     Eddie    Vedder
3      3    Edward Van Halen
4      4                    
5      5  Eduardo    Norton

Expected results would be:

• Eddie Norton
• Edward Vedder
• Eduardo Van Halen

But not:

• Eddie _____

or:

• ______ Van Halen

I tried the transform function but it didn't work:

permtest2 <- transform( permtest2, firstname = sample(firstname,lastname) )

Answer 1

What helps is shuffling only the nonempty entries:

permtest$lastname[permtest$lastname != ''] <- sample(permtest$lastname[permtest$lastname != ''])
permtest
#   number firstname  lastname
# 1      1                    
# 2      2     Eddie Van Halen
# 3      3    Edward    Vedder
# 4      4                    
# 5      5  Edurardo    Norton