List comprehensions: Difference between revisions
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=={{header|Python}}==
# List comprehension:
# A Python generator expression (note the outer round brackets),
▲<syntaxhighlight 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]</syntaxhighlight>
# returns an iterator over the same result rather than an explicit list:
# A slower but more readable version: ▼
▲A Python generator expression (note the outer round brackets), returns an iterator over the same result rather than an explicit list:
# Or as an iterator:▼
▲<syntaxhighlight 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)</syntaxhighlight>
# Alternatively we shorten the initial list comprehension but this time without compromising on speed.
▲A slower but more readable version:
# First we introduce a generator which generates all triplets:
def triplets(n):
▲<syntaxhighlight lang="python">[(x, y, z) for (x, y, z) in itertools.product(xrange(1,n+1),repeat=3) if x**2 + y**2 == z**2 and x <= y <= z]</syntaxhighlight>
▲Or as an iterator:
▲<syntaxhighlight lang="python">((x, y, z) for (x, y, z) in itertools.product(xrange(1,n+1),repeat=3) if x**2 + y**2 == z**2 and x <= y <= z)</syntaxhighlight>
▲Alternatively we shorten the initial list comprehension but this time without compromising on speed. First we introduce a generator which generates all triplets:
▲<syntaxhighlight lang="python">def triplets(n):
for x in xrange(1, n + 1):
for y in xrange(x, n + 1):
for z in xrange(y, n + 1):
yield x, y, z
# Apply this to our list comprehension gives:
# Or as an iterator:▼
▲<syntaxhighlight lang="python">[(x, y, z) for (x, y, z) in triplets(n) if x**2 + y**2 == z**2]</syntaxhighlight>
# More generally, the list comprehension syntax can be understood as a concise syntactic sugaring
▲Or as an iterator:
# of a use of the list monad, in which non-matches are returned as empty lists, matches are wrapped
# as single-item lists, and concatenation flattens the output, eliminating the empty lists.
# The monadic 'bind' operator for lists is concatMap, traditionally used with its first two arguments flipped.
▲<syntaxhighlight lang="python">((x, y, z) for (x, y, z) in triplets(n) if x**2 + y**2 == z**2)</syntaxhighlight>
# The following three formulations of a '''pts''' (pythagorean triangles) function are equivalent:
▲The monadic 'bind' operator for lists is concatMap, traditionally used with its first two arguments flipped. The following three formulations of a '''pts''' (pythagorean triangles) function are equivalent:
▲<syntaxhighlight lang="python">from functools import (reduce)
from operator import (add)
# pts :: Int -> [(Int, Int, Int)]
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main()
</syntaxhighlight>
{{Out}}
<pre>[(3, 4, 5), (5, 12, 13), (6, 8, 10), (8, 15, 17), (9, 12, 15), (12, 16, 20)]
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