Merge branch 'sieve' into working

sieve.py

@@ -0,0 +1,42 @@
+# Sieve of Eratosthenes, in python
+
+from numpy import array, zeros, int8
+from time import process_time
+
+def sieve(to):
+    """Construct a sieve of Eratosthenes up to the value of 'to'; return all
+    primes below this number.
+    """
+    zarr = zeros(to, int8)
+    zarr[0:2] = 1
+    for i in range(2, to):
+        if zarr[i] == 1:
+            continue
+        for x in range(i, ((to - 1) // i) + 1):
+            zarr[i * x] = 1
+    avals = array(range(to))
+    return avals[zarr[range(to)] == 0]
+
+def sieval(to):
+    """Run sieve algorithm up to 'to'; return process time, number of primes,
+    and last prime calculated (in that order).
+
+    'to' must be greater than 2.
+    """
+    t1 = process_time()
+    sh = sieve(to)
+    te = process_time() - t1
+    ls = len(sh)
+    return te, ls, sh[ls - 1]
+
+def sievemsg(to):
+    """Format sieval output for sieve up to 'to'
+
+    'to' must be greater than 2.
+    """
+    t, n, l = sieval(to)
+    rs = ("There are {1} prime numbers smaller than {3}; the largest is {2}. This "
+        "calculation took {0:.2f} seconds via the sieve of "
+        "Eratosthenes.".format(t, n, l, to))
+    return rs
+