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Hi,
frankly I try to find out what it does...
4/2 2⍴⍳4
1 1 1 1 2 2 2 2
3 3 3 3 4 4 4 4
basically replicates the last column
4//2 2⍴⍳4
2 2 2 2 4 4 4 4
But how do I get to.
1 1 1 1 2 2 2 2
or
1 1 1 1 3 3 3 3
or
3 3 3 3 4 4 4 4
???
and why is (n*n-1) = ⍴1⊃n//(n,n)⍴⍳n*2 ? ⍝ n ← 2 ....
e.g.
n←5
n/(n,n)⍴⍳n*2
1 1 1 1 1 2 2 2 2 2 3 3 3 3 3 4 4 4 4 4 5 5 5 5 5
6 6 6 6 6 7 7 7 7 7 8 8 8 8 8 9 9 9 9 9 10 10 10 10 10
11 11 11 11 11 12 12 12 12 12 13 13 13 13 13 14 14 14 14 14 15 15 15 15 15
16 16 16 16 16 17 17 17 17 17 18 18 18 18 18 19 19 19 19 19 20 20 20 20 20
21 21 21 21 21 22 22 22 22 22 23 23 23 23 23 24 24 24 24 24 25 25 25 25 25
n//(n,n)⍴⍳n*2 ⍝ could be
5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 10 10 10 10 10 10
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 ....
but it is
625¨⍴5 10 15 20 25
Just asking ..:-)
Best Regards
Hans-Peter
Am 02.12.20 um 17:27 schrieb Dr. Jürgen Sauermann:
Hi,
in following up the recent email discussions with Jay Foad and
Hans-Peter Sorge,
I have changed the operator-to-operator binding in GNU APL. *SVN 1367*.
Before the change an expression with 2 adjacent operators,
for example 2 / / 3 4⍴5, would have given this:
* B←**3 4**⍴5
** 2 / / **B
** 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5·**
** 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5·**
** 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5·**
*
GNU APL would have bound the left / to the right / and evaluation
the expression with:
* the left / being function compress, and
* the right / being operator reduce with function compress as left
operand.
After the change the same expression now evaluates to:
* B←3 4⍴5**
** 2 / / B**
** 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 **
*
GNU APL now binds 2 to / as discussed earlier. This creates a derived
function,
say TWICE, like:
*∇ Z←A TWICE B**
** ⍝ A not used**
** Z←2 / B**
**∇**
*
and then reduces *TWICE* with *B*:*
**
** TWICE / B**
** 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 **
*
Interestingly:
* as seen above,*TWICE / B* and *2 / / B* now give the same result in
GNU APL,
** TWICE / B *gives the same result in GNU APL and in IBM APL2,
but: *2 / / B* gives a *DOMAIN ERROR* in IBM APL2, and
** TWICE / B* also give the same results in GNU APL and in Dyalog APL,
but Dyalog APL gives a different result (apparently the same as the old
GNU APL implementation) for *2 / / B* (from *tryapl.org)*:
* B←3 4 ⍴ 5 **
****TWICE←{⍺ ⊢ 2/⍵}**
** TWICE / B**
**┌───────────────┬───────────────┬───────────────┐**
**│5 5 5 5 5 5 5 5│5 5 5 5 5 5 5 5│5 5 5 5 5 5 5 5│**
**└───────────────┴───────────────┴───────────────┘**
****
** 2 / / B**
**┌─────────┬─────────┬─────────┐**
**│5 5 5 5 5│5 5 5 5 5│5 5 5 5 5│**
**├─────────┼─────────┼─────────┤**
**│5 5 5 5 5│5 5 5 5 5│5 5 5 5 5│**
**├─────────┼─────────┼─────────┤**
**│5 5 5 5 5│5 5 5 5 5│5 5 5 5 5│**
**└─────────┴─────────┴─────────┘*
That suggest that the binding of operators is not very portable
between interpreters and one should not rely on them in the first
place. It seems to be more reliable bind explicitly as shown above
as to avoid unexpected surprises later on.
Best Regards,
Jürgen
