title: error message in r: no rows to aggregate
body: <p>I am running a program written in r language which is designed to compile many csv data files into one csv file and then generate an output file that contains the output of simple calculations on the few variables selected in the combined file. The later process is done by using the combined file as an input, selecting variables and specifying the kind of values that I need. Adding onto this explanation, data files are obtained on daily basis and these go through the r program to obtain some analysis, such as min, max, mean or sum of the data in selected time intervals. </p>
  
  <p>Moving on to the problem that I have with this program, the program is usually able to generate the combined csv file without much problem, but when it comes to the part of producing an output file, the program sometimes generate the aggregation error message: </p>
  
  <p><strong>Error in aggregate.date.frame (mf[1L], mf[-1L], FUN = FUN, ...) :
  no rows to aggregate</strong></p>
  
  <p>This error message only appears for some combined data files, and is making me even more puzzled since the data in the combined csv files that are causing such problem do not show any difference with the combined files that do not cause this problem. However, currently I am still suspecting that there should be something wrong with the combined data csv files instead of the program since the program generally works fine for many analysis cases.</p>
  
  <p>Basically, the program is using aggregate function to compute the output file.</p>
  
  <p>I am quite new to using r, and the program that I am using consists of several stages, so I apologize if my explanation was rather unclear.  </p>
  
  <p>At least, I would like to know what are the possible causes for such aggregation error. Thank you in advance.</p>