list_comprehensions.pi

/* 

  List comprehensions (Rosetta code) in Picat.

  http://rosettacode.org/wiki/List_comprehensions
  """
  A list comprehension is a special syntax in some programming languages to 
  describe lists. It is similar to the way mathematicians describe sets, with 
  a set comprehension, hence the name.

  Some attributes of a list comprehension are that:

   - They should be distinct from (nested) for loops within the syntax 
     of the language.
   - They should return either a list or an iterator (something that 
     returns successive members of a collection, in order).
   - The syntax has parts corresponding to that of set-builder notation. 

  Write a list comprehension that builds the list of all Pythagorean triples with 
  elements between 1 and n. If the language has multiple ways for expressing such a 
  construct (for example, direct list comprehensions and generators), 
  write one example for each. 
  """

  This Picat model was created by Hakan Kjellerstrand, hakank@gmail.com
  See also my Picat page: http://www.hakank.org/picat/

*/


% import util.
% import cp.


main => go.


go =>

  N = 20,
  println(pyth1=pyth1(N)),
  println(pyth1b=pyth1b(N)),  
  println(pyth2=pyth2(N)),
  println(pyth3=pyth3(N)),
  
  nl.

pyth1(N) = [[A,B,C] : A in 1..N, B in A..N, C in B..N, A**2 + B**2 == C**2].
pyth1b(N) = {[A,B,C] : A in 1..N, B in A..N, C in B..N, A**2 + B**2 == C**2}.

% Using between and findall
pyth2(N) = findall([A,B,C], (between(1,N,A), between(A,N,B), between(B,N,C), A**2 + B**2 == C**2)).

% pyth2 with symmetry breaking (A < B)
pyth3(N) = findall([A,B,C], (member(A,1..N), member(B,1..N), member(C,1..N), A < B, A**2 + B**2 == C**2)).