10.13: Debugging Gri Programs

• If no data appear on an xy plot, insert `show columns statistics' or `show columns' after the `read columns' command. It may be that you have fixed your axes, and that the axes frame does not include the data.
• If you get an error message, rerun your Gri program using the `-trace' command-line option, to see which line is causing the problem. This often reveals logic errors (e.g. in (`if' statements). You may also turn tracing on or off at any point in your Gri program by setting the built-in variable `..trace..' to 1 or 0. Many Gri users have the Gri command aliased to be in trace mode by default.
• If Gri complains of a syntax error, consult the printed manual, one of the online manuals, or the online help facility (see Online Help).
• Check the version number (printed at startup) to see if a new version of Gri has been installed, and check the manual for known incompatabilities.
• Sprinkle `show' commands throughout your program, to see what's happening. Even when you are sure your program works, it is a good idea to embed `show' statements so are they executed if the `-debug' flag is set:

  if ..debug.. show "X=" .x. "and label is `\label'" end if

• If your `draw' commands don't draw anything, check to see whether you've fooled yourself by enforcing an improper scaling; remove explict scaling (`set x axis ...'), clipping (`set clip'), data selection windows (`set input data window x|y') and missing values (`set missing value'). Another trick is to read only a portion of the data set (`read columns 10 x y') and then print out all the values (`show columns').

If you determine that the bug is in Gri, not in your program, please report the bug, so that other users will not have the same hassle; (see Bugs).