the use of eval and parse

["Colin Maurice" <colinsplus@hotmail.com>]

Dear members,

I was wondering why so many people use the "eval" and "parse" commands 
together along with the "paste" command, even when there doesn't appear to 
be any obvious reason why. For instance, I found the following code in an 
Splus Library:
table.to.data.frame <- function(TheTable, DataName, TheList)
{
#
#  convert a table to a data.frame containing the same information
#
#  TheTable contains the table
#  DataName is a string to head the data column
#  TheList is a list of strings being the headings for the dimensions
#     of the table
#
  TheData <- as.vector(TheTable)
  N <- length(TheData)
  eval(parse(text = paste("TheDataFrame <- data.frame(", DataName,
     " = TheData, row.names = 1:N)", sep = "")))
  DN <- dimnames(TheTable)
  K <- length(DN)
  if(K != length(TheList))
  {
     print("List is wrong length")
     return(NA)
  }
  r1 <- 1
  r2 <- N
  for(k in 1:K)
  {
     len <- length(DN[[k]])
     r2 <- r2/len
     eval(parse(text = paste("TheDataFrame$", TheList[k],
        " <- rep(rep(DN[[k]], rep(r1, len)), r2)", sep = "")))
     r1 <- r1 * len
  }
  TheDataFrame
}

But I don't know why

eval(parse(text = paste("TheDataFrame <- data.frame(", DataName,
     " = TheData, row.names = 1:N)", sep = "")))

couldn't have been replaced with

TheDataFrame <- data.frame(DataName= TheData, row.names = 1:N)

Can someone explain why it is sometimes better to use "eval" and "parse" 
rather than the standard why of assigning objects.
Any help is much appreciated.

Colin

_________________________________________________________________
Express yourself instantly with MSN Messenger! Download today - it's FREE! 
http://messenger.msn.click-url.com/go/onm00200471ave/direct/01/