RE Boss wrote:
> If I rebuild the example in
> ~/help/jforc/loopless_code_vii_sequential.htm#_Toc167755760
> with (5;s;m;0 _1 0 0) ;: 'qqq0x30x30x40x0x34a' J602abeta crashes.
Roger responded:
> Please provide a sequence of phrases that leads to a crash.
This is a slippery one. It'll crash if the offending line is displayed in the
session:
m =. a. e. '0x123456789abcdefABCDEF'
m =. m + a. e. '0x'
m =. m + a. e. '0'
s =. 1 4 2 $ 0 0 0 0 0 0 1 1
s =. s , 4 2 $ 0 0 0 0 2 0 0 0
s =. s , 4 2 $ 0 0 3 0 0 0 3 0
s =. s , 4 2 $ 0 3 3 0 0 3 3 0
1!:2&2 (5;s;m;0 _1 0 0) ;: 'qqq0x30x30x40x0x34a'
The final 1!:2&2 is superflous if these lines are entered in the IJX window
verbatim, instead of running from a script. Similarly if they're run from a
script through the Run>Window Display mechanism.
Interestingly, in immex, assigning the result and displaying it does not cause
a crash:
Q =: (5;s;m;0 _1 0 0) ;: 'qqq0x30x30x40x0x34a'
Q
0 _1 0 0 0 0
1 _1 0 0 0 0
2 _1 0 0 0 0
...
Nor will this crash if run from a script, UNLESS the script is run twice AND
the assignment to Q is global. That is, if the above text is saved into a
script and load 'd, it won't crash, until you load it again. If you
restart J and make the assignment to Q local, you may load the script
multiple times without error.
I attempted to to formalize these rules in J code, but when I do I get strange
behavior.
T =: 0 : 0
m =. a. e. '0x123456789abcdefABCDEF'
m =. m + a. e. '0x'
m =. m + a. e. '0'
s =. 1 4 2 $ 0 0 0 0 0 0 1 1
s =. s , 4 2 $ 0 0 0 0 2 0 0 0
s =. s , 4 2 $ 0 0 3 0 0 0 3 0
s =. s , 4 2 $ 0 3 3 0 0 3 3 0
Q =: (5;s;m;0 _1 0 0) ;: 'qqq0x30x30x40x0x34a'
)
3 : '0!:100 y' T NB. ok
erase'Q'
1
3 : '0!:100 y' T NB. Same sentence
|spelling error
|
Q
|value error: Q
T NB. Noun T has been trashed (shifted backwards in memory?).
?? m =. a. e. '0x123456789abcdefABCDEF'
m =. m + a. e. '0x'
m =. m + a. e. '0'
s =. 1 4 2 $ 0 0 0 0 0 0 1 1
s =. s , 4 2 $ 0 0 0 0 2 0 0 0
s =. s , 4 2 $ 0 0 3 0 0 0 3 0
s =. s , 4 2 $ 0 3 3 0 0 3 3 0
Q =: (5;s;m;0 _
T =: 'hello' NB. Including its header.
|read-only data
| T =:'hello'
These problems are not neccesarily reproducible, so I'm guessing somewhere
along the line J's memory was corrupted. But I can't pin down precisely when.
The bug may be sensitive to 0~:13!:11 '' (that is, whether an error has been
raised during the session), but that's not definite either.
-Dan
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