mlliarm/oldtimer_APL_code.md

## oldtimer_APL_code.md

      
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              oldtimer_APL_code.md
            
          
    An oldtimer's APL code

What

Some pretty cool code from the 70s'-80s' that a Reddit user shared at this question.
Here's what u/snarkuzoid wrote:

5x5 Knight's Tour in APL
It puts an A in the middle, then randomly does a knight's tour,
marking each cell with the next letter until it runs out of moves.
Yes, it's terrible code by modern standards.
For some reason we were obsessed with very short programs.

Here's the printout:

There's more to the story of the above code snippet:

Alas, I don't know of any other code fragments I might have laying around.
I only had this one because it was published as winner of the "Nice Knight" contest and I had a copy of the announcement.
Thanks again. This really made my day.

So apparently with this code the Reddit user had won a Leetcode competition of their time.
Pretty, cool, right?
Code

In modern Dyalog APL, v.18.0:
⎕IO←1 ⍝ If it's set to 0 instead, there's no 'A' character in the center and thus the code behaves incorrectly!
∇Z←EX2
  V←13+I←0×⍴A←25⍴'0'
  Z← 5 5 ⍴A,0⍴A[C←1↑V]←'ABCDEFGHIJKLMNOPQRSTUVWXY'[I←I+1]
  R←⍉(2,⍴V)⍴(⌊0.8+V÷5),1+5|¯1+V←(A='0')/⍳25
  T←(⌽2,⍴V)⍴(⌊0.8+C÷5),1+5|C-1
  →2×0≠⍴V←(?8)⌽((10⊥|⍉T-R)∊ 21 12)/V
∇
Results

Here are the results when run the above tradfn a couple times, from RIDE IDE on a Lubuntu 18.04, when I had ⎕IO←0:
     EX2
0JG0M
00DKH
0FIB0
LC0E0
00000
      EX2
00G00
00DIF
0H0BM
JCLE0
O00NK
      EX2
0JG00
000EH
0FIB0
0000D
00C00
      EX2
0RMF0
0EHQL
0NSBG
PIDOT
K0UJC

The above results are incorrect.
Here are some results with ⎕IO←1:
      EX2
00ELG
DMHQ0
IRAFK
NCJSP
0TOB0
      EX2
00000
0E0C0
00AF0
00D0B
0000G
      EX2
GBO00
N0FIP
CHALE
0MDQJ
0RK00
      EX2
GBK00
L0F0J
CHA0E
0MDI0
000N0

The above results are correct.
Correctness of the results

There's no Alpha in the middle. Does the code work correctly with Dyalog APL? (this was because IO had the value zero: ⎕IO←0).
Results with ⎕IO←1 look correct. There's the 'A' char in the middle.
Try it out online

Issue

As is with the tradfn it doesn't work on the tryapl.org site for some reason:
∇
SYNTAX ERROR
      ∇
      ∧

It worked fine on the RIDE IDE of Dyalog APL though.
It seems that tryapl.org doesn't support tradfns.
Fix

According to Adám from Dyalog, now tryapl.org supports tradfns. But the code cannot be copy pasted as is. Instead:

Either paste it line by line, pressing Alt+Enter after each line (except the last), or begin each line (except for the first) with a tab character (not the tab key).

I've found in the docs of Dyalog APL that the Tab key has to be defined as ⎕UCS 9.
So the following code should work on tryapl.org:
⎕IO←1 ⍝ Without this there's no 'A' character in the center!
Tab←⎕UCS 9 ⍝ Defining the tab character ('\t' in C).

∇Z←EX2_online
Tab V←13+I←0×⍴A←25⍴'0'
Tab Z← 5 5 ⍴A,0⍴A[C←1↑V]←'ABCDEFGHIJKLMNOPQRSTUVWXY'[I←I+1]
Tab R←⍉(2,⍴V)⍴(⌊0.8+V÷5),1+5|¯1+V←(A='0')/⍳25
Tab T←(⌽2,⍴V)⍴(⌊0.8+C÷5),1+5|C-1
Tab →2×0≠⍴V←(?8)⌽((10⊥|⍉T-R)∊ 21 12)/V
∇
Update: the above code still fails in tryapl.org.
Future work


Make sure that the tradfn implementation is correct for Dyalog APL. All good.
Rewrite the tradfn as a dfn so that it runs on the  online interpreter...First understand the program. From what the folks in the APLjk discord server say, the line →2×0≠⍴V←(?8)⌽((10⊥|⍉T-R)∊21 12)/V doesn't work in dfns because the right arrow is only for tradfns and tradfns aren't supported in tryapl.org.

Thanks


To the user u/snarkuzoid who trusted me with their code.
To bubbler from APLjk discord server that found the IO bug.
To Adám for trying to debug the tradfn error that appeared in tryapl.org.