List Commands Library for 50g

[John Keith]

The discussion of LMRPT reminds me of the recently added command LRPC\->. While it is the complement to the command LRPCT which run-length encodes a list, it can be very useful in its own right.

As an example, this simple program

Code:


\<< LSEQ DUP 2. \->LIST LRPC\->
\>>

returns the first T(n) terms of A002024, where T(n) is the nth triangular number. The above program is similar to the second Haskell program in the OEIS link. It is also quite fast: given an input of 100, it returns the first 5050 terms in less than 700 milliseconds.

A longer (and clunkier) example

Code:


\<< I\->R { 1 2 } \-> n b
  \<< b 1. n
    START DUP SIZE b OVER 2. / LMRPT 1. ROT SUB SWAP 2. \->LIST LRPC\->
    NEXT
  \>>
\>>

returns the nth iteration of the Kolakoski sequence. The program is longer and more involved than the Haskell program in the Kolakoski link because the infinite lists and "lazy" evaluation of Haskell make the explicit calculation of list lengths unnecessary.

(Edited to remove the CEIL command following DavidM's comments below)

These programs also demonstrate the similarity of LMRPT and LRPC\-> to the Haskell functions cycle and replicate respectively.