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## Re: [Bug-apl] Tree Traversal in gnu-apl

 From: Rowan Cannaday Subject: Re: [Bug-apl] Tree Traversal in gnu-apl Date: Wed, 9 Oct 2019 13:29:47 -0400

making progress...

foo ← {{(1+1↑⍵),(foo 1↓⍵)}⍣(~0=⍴⍵)⍵}
foo 1 2 3
2 3 4

On Wed, Oct 9, 2019 at 12:47 PM Rowan Cannaday <address@hidden> wrote:
this is a slightly better way of writing my (still broken) accumulator:
acc←{⍺{⍺ acc 1↑⍵}⍣(0=⍴⍵)⊢⍵}

On Wed, Oct 9, 2019 at 12:40 PM Rowan Cannaday <address@hidden> wrote:
Given a recursive factorial definition:
fact←{{⍵ × fact ⍵-1}⍣(⍵>2)⊢1⌈⍵}

[written by Kacper Gutowski in the 'Recursive Lambda' thread]

I am attempting to write a basic accumulator. This should take an empty vector as the left value argument, and a rank 1 array as the right value argument.
Every iteration it should drop a value from the right value, and append it to the left, until the right value is an empty vector.

I thought I'd be able to do something like the following:
acc←{⍺,{acc 1↑⍵}⍣(0=⍴⍵)⊢⍵}
⍬ acc 1 2 3

But modifying this to say, add a 1 to every number, still returns the input vector ⍵.

Thoughts?

On Fri, Sep 27, 2019 at 3:44 PM Rowan Cannaday <address@hidden> wrote:
Hello y'all.

I have been attempting to learn function composition & higher-order functions in gnu-apl, and how to use it to perform tree traversal.

https://en.wikipedia.org/wiki/Function_composition_(computer_science)#APL
https://en.wikipedia.org/wiki/Higher-order_function#APL
https://rosettacode.org/wiki/Tree_traversal#APL

Unfortunately a lot of the syntax used is dyalog & dfn specific, so working out some of the examples is a bit tricky for myself.
(the main inconsistencies are '∇' as a recursive function definition, ⍺⍺ & ⍵⍵ to refer to left and right operands, '@' as the 'at' operator, '⍣' operator differences, as well as possibly others).

Has anybody done 'idiomatic' tree traversal in gnu-apl? Does anybody use primitive composition functions in their code?

Trying to figure out what works and feels natural in the language. Any examples or guidance would be appreciated.

Examples:

Higher order fns in gnu-apl:
∇Z ← (L twice) B
Z ← L L B

∇Z ← plusthree B
Z ← B + 3

∇Z ← g B
Z ← plusthree twice B