Concatenate variable-length-strings as table

问题:

I am fetching an array of data from SQL, and then concatenating them as strings for display. The function looks like this:

function FetchTopStats( Conn, iLimit )
  local sToReturn = "\tS.No. \t UserName \t Score\n\t"
  SQLQuery = assert( Conn:execute( string.format( [[SELECT username, totalcount FROM chatstat ORDER BY totalcount DESC LIMIT %d]], iLimit ) ) )
  DataArray = SQLQuery:fetch ({}, "a")
  i = 1
  while DataArray do
    sToReturn = sToReturn..tostring( i ).."\t"..DataArray.username.." \t "..DataArray.totalcount.."\n\t"
    DataArray = SQLQuery:fetch ({}, "a")
    i = i + 1
  end
  return sToReturn
end

This gives me an output like:

    S.No.    UserName    Score
    1   aim      6641
    2   time     5021
    3   Shepard      4977

and so on. I am thinking of using a string.format function, to have a display as follows:

    S.No.    UserName    Score
    1       aim          6641
    2       time         5021
    3       Shepard      4977

But, I am totally out of ideas on how to have this. The only option coming to my mind is checking string length of username, and then applying \t accordingly. That, I want to use at the very last.

回答1:

Well, you need to either find out the maximum length of username thus making the algorithm 2-pass, or limit it to some arbitrary (but reasonable) size and unconditionally chop off the tails of too long strings. Once you have the column width, you can use left or right aligned format strings:

> print(string.format("|%-10d|%-20s|%10d|", 1, "Shepard", 9000))
|1         |Shepard             |      9000|

Also, for large tables consider using table.concat for building the final output: it's considerably faster than repeatedly appending strings (refer to the Chapter 11.6 of PIL for explanation).