From: Bob Gustafson <bobgus@...>
Date: 2001-09-28T07:30:03+09:00
Subject: [ruby-talk:21789] single line loop oddity

I am trying to debug a program with a loop which contains one executable
line (see below)

[user1@hoho0 cron]$ cat loop-prob.rb

tub = ["First phrase","Second phrase"]
  tub.each do |line|
    puts line
#   puts line.reverse
  end
  puts "*** Done with LOOP **"

When I run the program with the debugger, when I execute the one line
(puts line) inside the loop, instead of going just once around the loop
(and printing one line), it does not stop, but continues to the end of
the loop without stopping.

This is annoying - particularly when the loop goes around many times. (I
don't know if the program is wrong, or the debugger is wrong)

Note that the variable 'line' is printed with the debugger command p
before the s which goes to the end of the loop.  It contains just the
first phrase.

[user1@hoho0 cron]$ ruby -r debug loop-prob.rb
Debug.rb
Emacs support available.

loop-prob.rb:1:tub = ["First phrase","Second phrase"]
(rdb:1) s
loop-prob.rb:2:  tub.each do |line|
(rdb:1) s
loop-prob.rb:3:    puts line
(rdb:1) p line
"First phrase"
(rdb:1) s
First phrase
Second phrase
loop-prob.rb:6:  puts "*** Done with LOOP **"
(rdb:1) s
*** Done with LOOP **
[user1@hoho0 cron]$

----- If I take the comment # out of the program, so that there are two
statements within the loop, then the debugger behaves correctly (executing
only one line for each debugger 's' command - see transcript below).

[user1@hoho0 cron]$ ruby -r debug loop-prob.rb
Debug.rb
Emacs support available.

loop-prob.rb:1:tub = ["First phrase","Second phrase"]
(rdb:1) n
loop-prob.rb:2:  tub.each do |line|
(rdb:1) s
loop-prob.rb:3:    puts line
(rdb:1) s
First phrase
loop-prob.rb:4:    puts line.reverse
(rdb:1) s
esarhp tsriF
loop-prob.rb:3:    puts line
(rdb:1) s
Second phrase
loop-prob.rb:4:    puts line.reverse
(rdb:1) s
esarhp dnoceS
loop-prob.rb:6:  puts "*** Done with LOOP **"
(rdb:1) s
*** Done with LOOP **
[user1@hoho0 cron]$


There seems to be a problem in the interaction between the debugger and
program loops containing one line.