# Python `Meshtal` Class Fails for `mct` < 0 on `PRDMP` Card

[gonsie2]

Python Meshtal Class Fails for mct < 0 on PRDMP Card

I'm performing validation of MCNP6.3 and thus have disabled printing of run-specific information in my output files. I'm using the mct option on the PRDMP card (mct = -1 or -2). Though the MCNP6.3 manual only mentions the disabled printing for MCTAL and output files, run-specific information is also removed from MESHTAL files. For example,

mcnp   version 6     ld=02/20/18  probid =  10/11/23 13:51:15 
 MY TITLE
 Number of histories used for normalizing tallies =         10000.00

 Mesh Tally Number         4
 ...

becomes

 MY TITLE
 Number of histories used for normalizing tallies =         10000.00

 Mesh Tally Number         4
 ...

When I try to read these MESHTAL files using the Meshtal class within MCNPTools' Python bindings, I get a generic RuntimeError: McnpToolsException. Re-enabling printing of run-specific info fixes the issue. I haven't had the same issue with the MctalTally and MctalKcode classes, so it appears to be limited to the Meshtal class.

I have very limited knowledge of C++, but I think the issue is in the source code for file Meshtal.cpp, lines 31-43 (and repeated on lines 422-434?):

  // read the header line
  cjsoft::stringops::getline(m_handle,line);
  std::vector<std::string> spline;
  cjsoft::stringops::tokenize<std::vector<std::string>,cjsoft::stringops::StringTrim>(line,spline);

  if( spline.size() == 8 ) {
    m_code = spline[0];
    m_version = spline[2];
    m_probid = spline[6] + " " + spline[7];
  }
  else {
    throw McnpToolsException("file " + fname + " does not look like an MCNP meshtal file");
  }

It looks like the code expects to find 8 substrings which make up the run-specific information in the header line. If not found, it throws an exception. In pseudocode, it seems like the fix would be:

read header line

if (header line starts with 'mcnp'): \\ Follow existing logic
	split line into substrings
	assign values to run-specific variables
	load next line

else if (header line starts with space): \\ Comment line seems to always start with space, even if blank
	assign blank values to run-specific variables
	\\ already on comment line, so don't load next line

else:
	throw exception

process comment line...