Shoelace formula for polygonal area: Difference between revisions

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 .0
 >>> </lang>
+Or, defined in terms of '''reduce''' and '''cycle''':
+{{Trans|Haskell}}
+{{Works with|Python|3}}
+<lang python>'''Shoelace formula for polygonal area'''
+from itertools import (cycle, islice)
+from functools import (reduce)
+# shoelaceArea :: [(Float, Float)] -> Float
+def shoelaceArea(xys):
+    '''Area of polygon with vertices
+       at (x, y) points in xys.'''
+    def go(a, tpl):
+        (x, y), (dx, dy) = tpl
+        l, r = a
+        return (l + x * dy, r + dx * y)
+    (nl, nr) = reduce(
+        go,
+        zip(xys, tail(cycle(xys))),
+        (0, 0)
+    )
+    return abs(nl - nr) / 2
+# TEST -------------------------------------------------
+# main :: IO()
+def main():
+    '''Test'''
+    print(
+        shoelaceArea(
+            [(3, 4), (5, 11), (12, 8), (9, 5), (5, 6)]
+        )
+    )
+# GENERIC -------------------------------------------------
+# tail :: [a] -> [a]
+# tail :: Gen [a] -> [a]
+def tail(xs):
+    '''The elements following the head of a
+       (non-empty) list or generator stream.'''
+    if isinstance(xs, list):
+        return xs[1:]
+    else:
+        list(islice(xs, 1))  # First item dropped.
+        return xs
+if __name__ == '__main__':
+    main()</lang>
+{{Out}}
+<pre>30.0</pre>
 =={{header|Racket}}==