Code Exercise with a Time-Traveling Debugger

In this video I do a simple roman numeral conversion code exercise to demonstrate what it's like to code with a time-traveling debugger.

Transcript

The following transcript was automatically generated by an algorithm.

00:00 : hi there uh this is Toby I am um
00:04 : preparing for a talk about time
00:07 : traveling debuggers in a few weeks um if
00:12 : you're interested if you're in the
00:13 : Atlanta area and you're interested in
00:16 : going to the conference where my talk
00:18 : will
00:19 : be
00:23 : uh that conference is called reactor.
00:27 : te uh you can find more information
00:30 : on this website okay so I thought I
00:33 : would uh
00:35 : demonstrate
00:37 : how uh what programming is like using a
00:40 : time traveling debugger by programming a
00:44 : little easy uh exercise on lead code.com
00:48 : um it's the convert an integer to a
00:51 : Roman numeral problem uh I have done
00:55 : this in the past uh you might have as
00:58 : well uh and
01:01 : um I've been just chucking along working
01:04 : through the simplest cases and uh I just
01:08 : encounter a bug so I thought that I
01:10 : would slow down and use the time
01:12 : traveling debugger to solve this um the
01:15 : time traveling debugger I'm
01:17 : using is py rewind uh which is a project
01:22 : that I worked on for a while maybe about
01:26 : a year uh so I'm going to use this to
01:28 : run the program
01:32 : numeral pi and currently I'm getting
01:35 : this assertion error and uh so let's go
01:40 : in with the
01:42 : debugger and we'll debug it um in order
01:45 : to invoke the debugger for p rewind I
01:48 : provide the rewind file which is a log
01:51 : file that py rewind generates and um
01:54 : here
01:56 : um it when possible the debugger starts
02:00 : at the end of the program or not when
02:03 : possible when there's a runtime
02:04 : exception which in this case there was
02:07 : because it was an assertion error it
02:09 : stops at the end of the program where
02:11 : the assertion error occurred and now I
02:13 : can actually walk backwards in time to
02:16 : figure out what
02:18 : happened and it stepped backwards to
02:22 : this line which I definitely wasn't
02:24 : expecting because I was expecting it to
02:26 : go to line 13 uh I can also tell that
02:29 : line 13 is red which means it has never
02:33 : encountered that line which is also
02:35 : suspicious
02:37 : so uh but this jumps me to the root
02:41 : cause right away it was going to the
02:43 : wrong uh wrong branch of the program
02:48 : that I meant for it to go
02:50 : to so uh mystery solved for that one um
02:54 : so I'm going to
02:56 : reorder these branches to to make the 10
03:00 : one go above the five
03:04 : one now run the program and now uh a lot
03:08 : of the assertions went through without
03:09 : errors okay
03:27 : cool test something like
03:34 : 44 should be
03:36 : like
03:40 : that okay this is a good chance to debug
03:45 : this um
03:51 : so all right so I'll go and go backwards
03:57 : and we are looking at the number
04:04 : 44
04:07 : 34 num is
04:11 : 34 num is 24 num is
04:15 : 14
04:18 : so uhuh so it trimmed
04:22 : 10 at a time four
04:26 : okay all right so
04:31 : it trimmed 10 at a time is what it
04:34 : did instead of trimming the 40 because
04:38 : you have to match exactly 40 before it
04:41 : will do this
04:42 : trimming so if it is so maybe what we
04:47 : need to
04:52 : do oh unless I could do
04:58 : this
05:00 : might be able to do the same trick
05:05 : here just order it after
05:15 : 15 like
05:26 : this that seems to have worked
05:32 : and then
05:34 : for
05:51 : 90
05:54 : XC
05:57 : that EXC
06:01 : it's okay so need to add I want to put
06:05 : if number is greater
06:16 : 90 good so I'm going to try to test
06:22 : 99 it should give me this I
06:27 : believe that didn't work okay
06:30 : let's use the
06:35 : debugger okay so we're here we're going
06:38 : to walk
06:39 : backwards and it says 99 is the
06:43 : number uh I'm going to walk
06:50 : backwards and this is I * num where num
06:57 : is1 what
07:00 : why is num1 I'm going to consult our
07:05 : timeline okay so actually the execution
07:09 : of this call isn't super complicated it
07:12 : was like 20 lines or
07:16 : something um when
07:19 : it hit this if the number is greater or
07:22 : equal to 90 then it hit this clause and
07:27 : then it it into Roman numus 100
07:36 : here and again here I guess the num is
07:40 : negative one yes the reason the num is
07:44 : negative one is because we have 99 minus
07:48 : 100 yeah that's the problem 99- 100
07:54 : is1 and uh that shouldn't happen so
08:00 : we should be subtracting uh 90 here
08:10 : instead that works
08:13 : okay um I think at this point I just
08:16 : need to add the Clauses for 500 or or
08:20 : 400 and 900 and then we're probably
08:26 : done to uh pick B in my
08:37 : solution plug it
08:42 : in run
08:49 : it okay submit
08:55 : it yay accept it
08:58 : cool
08:59 : so the solution I like this solution
09:02 : it's very simple I mean I could have
09:05 : made it smarter in a sense by doing
09:08 : digit by
09:10 : digit but this is simple enough I think
09:14 : that it doesn't warrant that we did get
09:17 : a few debugging sessions in there so I'm
09:21 : happy about that