Monday, January 05, 2015

6 Coding Exercises to Reshape Your Programming Brain

The ProgramingPraxis Christmas / New Year's Exercise couldn't have come at a better time. Given my latest Forth on Android kick, it served as the perfect refresher course for this language I love in theory, yet can't code in practice.

The challenge was to reproduce 6 'ancient' algorithms, which range from implementing basic multiplication to calculating prime numbers.

The exercises were ideal because they forced me to deal with a number of core language constructs, such as recursion vs. iteration, floating point math, working with arrays and dealing with name clashes. They also got me thinking about bigger topics like code reuse, modularization and debugging. None of this is Forth specific, which means that this set of algorithms would be ideal for learning any new language.

I solved the exercise over a number of short coding sessions. What started as an agonizing struggle to implement basic multiplication (oh the stack makes my brain hurt!) finished with a Sieve of Eratosthenes implementation that begins to show a little beauty and elegance (at least in my head, it does). Mind you, any seasoned Forth programmer would look at my code below and shake his or her head. But for me, I was excited to see some of the nuances of Forth work for me (oh yeah, functions don't have to 'return' a single value!) rather than just serve as stumbling blocks.

Whether it's forth, haskell, C++ or any other language; this challenge really is ideal for busting you out of your comfort zone. Make sure you implement all 6, though. Had I given up after the first couple, I would have never got that Whoa! so that's what Forth programmers see when they see the world; remarkable!, feeling. And trust me, that feeling is awesome.

Here's the code. Try not to laugh too hard.

\ http://programmingpraxis.com/2014/12/23/ancient-algorithms/
\ -----------------------------------------------------

: odd? ( n -- b )
 1 and 0<> ;
 
: even? ( n -- b )
 odd? 0= ;

: odd-value ( w1 -- w2 )
 dup even? if
  drop 0
 endif ;

: 3rd ( w1 w2 w3 - w1 w2 w3 w1 )
 2 pick ;


: pstep { lhs rhs sum -- new-lhs new-rhs sum }
  lhs 2 /
  rhs 2 * 
  lhs 2 / odd? if
    rhs 2 * sum +
  else
    sum
  endif ;

: pinit { lhs rhs -- sum }
 lhs rhs
 lhs odd? if
   rhs
 else
   0
 endif ;

: product { lhs rhs -- prod }
 lhs rhs pinit
 begin
  pstep
  3rd 1 > while 
 repeat
 2nip ;

: .mul { lhs rhs -- prod1 prod2 }
 lhs rhs product
 lhs rhs *
 .s clearstack ;
 
\ -----------------------------------------------------
 
0.00000001e0 fconstant fsqrt-epsilon

: fwithin? { F: x F: y F: error }
 x y f- fabs
 error f< ;

: fsqrt-needs-refining? { F: x F: n F: x' -- x n x' continue? }
 x' n x'
 x x' fsqrt-epsilon fwithin? IF
  false
 else
  true
 endif ;

: fsqrt-step { F: n F: x -- n x' }
 n
 x n x f/ f+ 2e0 f/ ;


: fsqrt { F: n -- x }
 n 1e0 ftuck
 begin
  fsqrt-step
  fsqrt-needs-refining? while
 repeat
 fnip fnip ; 
 
\ -----------------------------------------------------

: ^2 ( n1 -- n2 )
 dup * ;

: *3 ( n1 n2 n3 -- n4 )
 * * ;
 
: triple { m n -- a b c }
 m ^2  n ^2  -
 2 m n *3
 m ^2 n ^2 + ; 
 
\ -----------------------------------------------------

: gcd-step { m n -- m' n' }
 n 0= if
  m 0
 else
  n m n mod
 endif ;
 
: gcd ( m n -- d )
 begin
  gcd-step
  dup 0<> while
 repeat
 drop ;

\ -----------------------------------------------------

: fpi-init ( -- outer inner )
 3e0 3e0 fsqrt f*
 fdup 2e0 f/ ;
 
: finv ( x -- 1/x )
 1e0 fswap f/ ;
 
: fpi-step { F: outer F: inner -- outer' inner' }
 2e0 outer finv inner finv f+ f/
 fdup inner f* fsqrt ;

: fpi { n -- approx }
 fpi-init
 n 1 u+do
  fpi-step
 loop fdrop ;

\ -----------------------------------------------------

: siv-addr ( s index -- addr )
 1 - cells + ;

: siv-upper ( s -- n )
 @ ;
 
: siv! ( value s index -- )
 siv-addr ! ;
  
: siv@ ( s index -- value )
 siv-addr @ ; 
  
: siv-init { s upper -- s }
 upper s !
 upper 1 + 2 u+do
  true s i siv!
 loop s ;

: siv-range { s start-index -- to from }
 s siv-upper 1 +
 start-index ;
 
: siv.s { s -- }
 s 2 siv-range u+do
  cr i . ." :" s i siv@ .
 loop cr ;

: new-siv { upper -- s }
 here upper cells allot
 upper siv-init ;


: siv-mark { s index -- }
 true s index siv!
 s index 2 * siv-range u+do
  false s i siv!
 index +loop ;
 
: siv-primes { s -- s }
 s 2 siv-range u+do
  s i siv@ if
   s i siv-mark
  endif
 loop s ;

: prime? ( n -- b )
 dup
 new-siv siv-primes
 swap siv@ ;
 
: primes { lower upper -- p1 p2 ... }
 upper new-siv siv-primes { s }
 upper 1+  lower u+do
  s i siv@ if
   i
  endif
 loop ; 

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