List comprehensions: Difference between revisions
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List comprehension: |
List comprehension: |
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<python>[(x,y,z) for x in xrange(1,n+1) for y in xrange(x,n+1) for z in xrange(y,n+1) if x**2 + y**2 == z**2]</ |
<lang python>[(x,y,z) for x in xrange(1,n+1) for y in xrange(x,n+1) for z in xrange(y,n+1) if x**2 + y**2 == z**2]</lang> |
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Generator comprehension (note the outer round brackets): |
Generator comprehension (note the outer round brackets): |
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<python>((x,y,z) for x in xrange(1,n+1) for y in xrange(x,n+1) for z in xrange(y,n+1) if x**2 + y**2 == z**2)</ |
<lang python>((x,y,z) for x in xrange(1,n+1) for y in xrange(x,n+1) for z in xrange(y,n+1) if x**2 + y**2 == z**2)</lang> |
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Generator function: |
Generator function: |
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<python>def gentriples(n): |
<lang python>def gentriples(n): |
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for x in xrange(1,n+1): |
for x in xrange(1,n+1): |
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for y in xrange(x,n+1): |
for y in xrange(x,n+1): |
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for z in xrange(y,n+1): |
for z in xrange(y,n+1): |
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if x**2 + y**2 == z**2: |
if x**2 + y**2 == z**2: |
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yield (x,y,z)</ |
yield (x,y,z)</lang> |
Revision as of 15:38, 3 February 2009
List comprehensions
You are encouraged to solve this task according to the task description, using any language you may know.
You are encouraged to solve this task according to the task description, using any language you may know.
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.
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.
Clojure
(for [x (range 1 21) y (range x 21) z (range y 21) :when (= (+ (* x x) (* y y)) (* z z))] [x y z])
E
pragma.enable("accumulator") # considered experimental accum [] for x in 1..n { for y in x..n { for z in y..n { if (x**2 + y**2 <=> z**2) { _.with([x,y,z]) } } } }
Erlang
pythag(N) -> [ {A,B,C} || A <- lists:seq(1,N), B <- lists:seq(1,N), C <- lists:seq(1,N), A+B+C =< N, A*A+B*B == C*C ].
Haskell
pyth n = [(x,y,z) | x <- [1..n], y <- [x..n], z <- [y..n], x^2 + y^2 == z^2]
Since lists are Monads, one can alternatively also use the do-notation (which is practical if the comprehension is large):
import Control.Monad pyth n = do x <- [1..n] y <- [x..n] z <- [y..n] guard $ x^2 + y^2 == z^2 return (x,y,z)
Mathematica
Select[Tuples[Range[n], 3], #1[[1]]^2 + #1[[2]]^2 == #1[[3]]^2 &]
Pop11
lvars n = 10, i, j, k; [% for i from 1 to n do for j from 1 to n do for k from 1 to n do if i*i + j*j = k*k then [^i ^j ^k]; endif; endfor; endfor; endfor %] =>
Python
List comprehension:
<lang python>[(x,y,z) for x in xrange(1,n+1) for y in xrange(x,n+1) for z in xrange(y,n+1) if x**2 + y**2 == z**2]</lang>
Generator comprehension (note the outer round brackets):
<lang python>((x,y,z) for x in xrange(1,n+1) for y in xrange(x,n+1) for z in xrange(y,n+1) if x**2 + y**2 == z**2)</lang>
Generator function:
<lang python>def gentriples(n): for x in xrange(1,n+1): for y in xrange(x,n+1): for z in xrange(y,n+1): if x**2 + y**2 == z**2: yield (x,y,z)</lang>