(49g 50g) Fast Pascal's triangle and its relatives
08-11-2018, 04:58 PM (This post was last modified: 08-11-2018 05:01 PM by John Keith.)
Post: #1
 John Keith Senior Member Posts: 365 Joined: Dec 2013
(49g 50g) Fast Pascal's triangle and its relatives
Given an integer on the stack, this program will return the corresponding row of Pascal's triangle. Its speed comes from taking advantage of the symmetry of Pascal's triangle and only calculating explicit values for the left half of the row. Values are calculated by an efficient algorithm that does not require COMB (binomial coefficients). This algorithm is explained in the seventh comment of the OEIS link below.

This program requires the LSEQ command from the ListExt library. The program can be easily modified to use the built-in SEQ command if necessary. The variation below also requires GoferLists.

Code:
 %%HP: T(3)A(R)F(.); \<< DUP I\->R   IF DUP 1. >   THEN @ special cases for rows less than 4     IF DUP 3. >     THEN DUP 2. / IP SWAP 1. + 2. / IP     ELSE 0.     END ROT @ list of numbers 1..n and its reverse  LSEQ DUP REVLIST \-> m t a b @ main part of program:     \<< a 1. GET b 1. GET 2. m       FOR j DUP b j GET * a j GET /       NEXT m 1. + \->LIST DUP 1. t SUB REVLIST +     \>>   ELSE @fake it! 1. SAME { DUP 2. \->LIST } { DROP { 1 } } IFTE   END \>>

A simple variation will return rows of the Narayana triangle. More information here. This program should be run in exact mode since numbers can quickly grow to larger than 12 digits.

Code:
 %%HP: T(3)A(R)F(.); \<< 1 - DUP I\->R   IF DUP 1. >   THEN     IF DUP 3. >     THEN DUP 2. / IP SWAP 1. + 2. / IP     ELSE 0.     END ROT LSEQ     \<< +     \>> Scanl1 DUP REV \-> m t a b     \<< a 1. GET b 1. GET 2. m       FOR j DUP b j GET * a j GET /       NEXT m 1. + \->LIST DUP 1. t SUB REVLIST +     \>>   ELSE 1. SAME { DUP 2. \->LIST } { DROP { 1 } } IFTE   END \>>

The only changes are the 1 - in the first line since the Narayana triangle begins with row 1 rather than row 0, and the addition of << + >> Scanl1 in lines 8 and 9 which creates a list of triangular numbers 1 through n. Many other variations can be had by simple changes as above.
08-12-2018, 12:41 AM (This post was last modified: 08-12-2018 12:47 AM by Joe Horn.)
Post: #2 Joe Horn Senior Member Posts: 1,423 Joined: Dec 2013
RE: (49g 50g) Fast Pascal's triangle and its relatives
It's probably not very fast, but here's a very short RPL program (48G/GX or later) that returns the Nth row of Pascal's triangle:

Code:
<< IDN ->DIAG NEG PCOEF >>
BYTES: 26.0 #2587h

EDIT: Here's an even smaller one that's a tad faster:
Code:
<< 1. ->LIST -1. CON PCOEF >>
BYTES: 25.5 #E698h

X<> c
-Joe-
08-12-2018, 02:12 PM
Post: #3
 John Keith Senior Member Posts: 365 Joined: Dec 2013
RE: (49g 50g) Fast Pascal's triangle and its relatives
Very neat! Slower than mine but less than 1/10 the size!
08-19-2018, 02:14 PM (This post was last modified: 08-19-2018 07:10 PM by John Keith.)
Post: #4
 John Keith Senior Member Posts: 365 Joined: Dec 2013
RE: (49g 50g) Fast Pascal's triangle and its relatives
By definition, each entry in Pascal's triangle is the sum of the two numbers above it. This leads to a very fast method of computing a row of Pascal's triangle given the previous row (as a list) on the stack:

Code:
 \<< 2.   \<< +   \>> DOSUBS 1 SWAP + 1 + \>>

The following program returns rows 0 through n of Pascal's triangle as a list of lists. It calculates the first 100 rows (a 63K byte object) in less than 20 seconds. No external libraries are required.

Code:
 \<< I\->R \-> n   \<< { 1 } { 1 1 } 2. n     FOR k k 1. + DUP 1. + 2. / IP SWAP 2. / IP \-> c f       \<< DUP 1. c SUB 2.         \<< +         \>> DOSUBS 1 SWAP + DUP 1. f SUB REVLIST +       \>>     NEXT n 1. + \->LIST   \>> \>>

While generating Pascal's triangle is not very useful in general, the entries of Pascal's triangle are the binomial coefficients, which can be useful themselves. As an example, here is a program, based on the one above, which returns the ordered Bell numbers from 0 through n:

Code:
 \<< I\->R \-> n   \<< 1 DUP DUP2 2. \->LIST 2. n     FOR k k 1. + DUP 1. + 2. / IP SWAP 2. / IP \-> c f       \<< k 1. + ROLLD k DUPN k \->LIST REVLIST k 2. + ROLL 1. c SUB 2.         \<< +         \>> DOSUBS 1 SWAP + DUP 1. f SUB REVLIST + DUP TAIL SWAP UNROT * \GSLIST SWAP       \>>     NEXT DROP n 1. + \->LIST   \>> \>>

This program should be used in exact mode as the numbers involved are very large. However, all three programs in this post will work on the HP48g/gx.
08-20-2018, 12:43 AM
Post: #5 Joe Horn Senior Member Posts: 1,423 Joined: Dec 2013
RE: (49g 50g) Fast Pascal's triangle and its relatives
(08-19-2018 02:14 PM)John Keith Wrote:  By definition, each entry in Pascal's triangle is the sum of the two numbers above it. This leads to a very fast method of computing a row of Pascal's triangle given the previous row (as a list) on the stack:

Code:
 \<< 2.   \<< +   \>> DOSUBS 1 SWAP + 1 + \>>

Smaller but a tad slower (just thinking of alternative methods for fun):

Code:
\<< 0 + LASTARG SWAP + ADD \>>

X<> c
-Joe-
08-20-2018, 12:15 PM
Post: #6
 John Keith Senior Member Posts: 365 Joined: Dec 2013
RE: (49g 50g) Fast Pascal's triangle and its relatives
(08-20-2018 12:43 AM)Joe Horn Wrote:  Smaller but a tad slower (just thinking of alternative methods for fun):

Code:
\<< 0 + LASTARG SWAP + ADD \>>

Very clever! Alternative methods always appreciated. 03-02-2019, 07:12 PM
Post: #7
 Thomas Klemm Senior Member Posts: 1,449 Joined: Dec 2013
RE: (49g 50g) Fast Pascal's triangle and its relatives
(08-20-2018 12:43 AM)Joe Horn Wrote:  Smaller but a tad slower (just thinking of alternative methods for fun):

Code:
\<< 0 + LASTARG SWAP + ADD \>>

Similar idea without using LASTARG:
Code:
\<< 0 OVER + SWAP 0 + ADD \>>

BTW: Both variants work with { 1 } while John Keith's solutions starts working with { 1 1 }.
However they can't use the symmetry and do only half of the work when Pascal's triangle is generated.

Instead we'd have to use two programs say ODD and EVEN that we call alternatively.

ODD
Code:
\<< 0 OVER + REVLIST TAIL REVLIST ADD \>>

EVEN
Code:
\<< 0 OVER + SWAP DUP REVLIST HEAD + ADD \>>

Examples:

{ 1 }
ODD
{ 1 }
EVEN
{ 1 2 }
ODD
{ 1 3 }
EVEN
{ 1 4 6 }
ODD
{ 1 5 10 }
EVEN
{ 1 6 15 20 }
ODD
{ 1 7 21 35 }

Cheers
Thomas
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