Roger,
>From the example below, it appears that the correct processing is being done,
>it is just when assigning a name to the results that your special code runs
>into trouble, OR the J system is doing some lazy evaluation.
"In certain cases, instead of explicitly doing }:y and }.y, the code constructs
headers which point to the same data, thereby saving time and space."
Does J do lazy evaluation, that is delaying a calculation until needed with the
potential of not doing a calculation if the result is not used?
Playing around with 2 infix below has given me a bit more insight into J.
1 load '~temp/3.ijs'
NB. Script showing infix2 problem and exploring what happens
NB. when you do nothing.
NB. Running from a script using
NB. 1 load 'script_file.ijs'
NB. can produce vastly different "wrong" values, and the second set
NB. of a, b and c are all not identical.
NB. Entering the sentence 'c =: 0 + b =: a' at the keyboard
NB. produced identical results for a, b and c
JVERSION
Engine: j701/2011-01-10/11:25
Library: 7.01.048
Platform: Win 32
Installer: j701a_win.exe
InstallPath: r:/j701
NB. simple expression which produces the error.
c =: 0 + b =: a=: 2 ]/\ i. n =: 8
a NB. wrong
23816912 23860120 16144024 16003072 16003072 17745504 16001920
b NB. wrong
23816912 23860120 16144024 16003072 16003072 17745504 16001920
c NB. correct
1 2 3 4 5 6 7
NB. adding 0 produces the correct answer
NB. But if we use the value a without the infix calculation
c =: 0 + b =: a
a NB. wrong
23816912 23860120 16144024 16003072 16003072 17745504 16001920
b NB. same wrong
23816912 23860120 16144024 16003072 16003072 17745504 16001920
c NB. same wrong @ keyboard, maybe different wrong from script
23816912 23860120 16144024 16003072 16003072 17745504 16001920
NB. My guess is "c" values are normalised floating point values
NB. and "a" and "b" are un-normalised floats.
NB. My conclusion is that it is the assignment and not the calculation
NB. introducing the error. J 'knows' what the answer should be so
NB. it uses Roger's quick headers, but carries the calculated value
NB. along and when asked to add 0 it produces the correct value
NB. for the calculated value to put into "c"
NB. instead of adding 0 try identity verb between "a" and "b"
c =: 0 + b =: ] a=: 2 ]/\ i. n
a NB. wrong
23816912 23860120 16144024 16003072 16003072 17745504 16001920
b NB. wrong
23816912 23860120 16144024 16003072 16003072 17745504 16001920
c NB. correct
1 2 3 4 5 6 7
NB. Doing identity (ie nothing) makes no difference.
NB. Same result when doing nothing a finite number of times
c =: 0 + b =: ]^:25 a=: 2 ]/\ i. n
a NB. wrong
23816912 23860120 16144024 16003072 16003072 17728352 16001920
b NB. wrong
23816912 23860120 16144024 16003072 16003072 17728352 16001920
c NB. correct
1 2 3 4 5 6 7
c =: 0 + b =: ]^:1e6 a=: 2 ]/\ i. n
a NB. wrong
23816912 23860120 16144024 16003072 16003072 17745504 16001920
b NB. wrong
23816912 23860120 16144024 16003072 16003072 17745504 16001920
c NB. correct
1 2 3 4 5 6 7
NB. or even doing the inverse of nothing
c =: 0 + b =: ]^:_1 a=: 2 ]/\ i. n
a NB. wrong
23816912 23860120 16144024 16003072 16003072 17728352 16001920
b NB. wrong
23816912 23860120 16144024 16003072 16003072 17728352 16001920
c NB. correct
1 2 3 4 5 6 7
NB. however doing something zero time gives different results
c =: 0 + b =: *~^:0 a=: 2 ]/\ i. n
a NB. wrong
23860120 16144024 16003072 16139800 23817168 16003136 16144024
b NB. correct
1 2 3 4 5 6 7
c NB. correct
1 2 3 4 5 6 7
NB. as does doing nothing an infinite number of times
c =: 0 + b =: ]^:_ a=: 2 ]/\ i. n
a NB. wrong
23816912 23860120 16144024 16003072 16003072 17728352 16001920
b NB. correct
1 2 3 4 5 6 7
c NB. correct
1 2 3 4 5 6 7
NB. or the inverse of nothing an infinite number of times
c =: 0 + b =: ]^:__ a=: 2 ]/\ i. n
a NB. wrong
23816912 23860120 16144024 16003072 16003072 17726304 16001920
b NB. correct
1 2 3 4 5 6 7
c NB. correct
1 2 3 4 5 6 7
NB. Neville Holmes in the days of the Sydney APL user group
NB. gave a very entertaining talk on "nothing" in APL
NB. ( Where are you now Neville! )
NB. demonstrating how well array languages (that is APL)
NB. handled arrays of zero rank and zero length.
NB. J's power conjunction provides me with more delights.
NB. Further exploring this shows that the inverse of the
NB. "do nothing verb" applied more than once does affect
NB. the value transferred.
NB. Hence the conclusion that (for current J system) there
NB. is somehow a difference between doing nothing and
NB. doing the inverse of nothing when applied more than once!
NB. Some defined verbs for doing nothing will do exactly that
NB. - maintaining the same state - and
NB. others return the correct value which shows they are
NB. collecting the calculation and not being effectively bypassed.
Ian
Ian Shannon
Landscape Modelling & Decision Support
Scientific Services
Office of Environment and Heritage
Department of Premier and Cabinet
PO Box A290
Sydney South
NSW 1232
T: +61 2 99 955 490
E: Ian.Shannon Environment nsw gov au (you add in 'dot's and 'at')
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