Applying winpdb_reborn

[Rich Shepard]

On Sun, 30 May 2021, Chris Angelico wrote:

> (Plus, there's not always an opportunity to use a debug harness. Sometimes
> you just have to put your prints into production and let it run for two
> weeks in the hope that the bug will show itself.)

ChrisA,

Please excuse my long-winded description of a FORTRAN IV bug in an lake
ecosystem energy model I wrote at the University of Illinois in the early
1970s. It is an example of what you wrote above.

The program fill two boxes of 80-column Hollerith cards punched on an IBM
29 (if I correctly recall the model) keypuch machine. The output was written
on green-barred wide paper on a line printer.

Each time I ran the model it would produce one of four different, but wrong,
answers for one variable. I closely examined the 1.5" thick fan-folded line
printer output many times. I never saw the error, even with diagnostic stubs
added. When I took the output to the computer center they looked at it for a
week and couldn't find the error, either.

Shortly after that, when I again examined the code, line-by-line, I saw the
error: a line that should have read
FOR I=1 to N
actually read
FOR I=I to N

All of us saw the uppercase I as a 1 because a) that's what we expected to
see and b) line printer output on green-barred paper was not that clear,
even with fresh ribbons.

That lesson has stuck with me ever since.

Regards,

Rich