some math scripts

Cryptography/tools/quick_select.py

@@ -0,0 +1,35 @@
+def quickSelect(seq, k):
+    # this part is the same as quick sort
+    len_seq = len(seq)
+    if len_seq < 2: return seq
+
+    ipivot = len_seq // 2
+    pivot = seq[ipivot]
+
+    smallerList = [x for i,x in enumerate(seq) if x <= pivot and  i != ipivot]
+    largerList = [x for i,x in enumerate(seq) if x > pivot and  i != ipivot]
+
+    # here starts the different part
+    m = len(smallerList)
+    if k == m:
+        return pivot
+    elif k < m:
+        return quickSelect(smallerList, k)
+    else:
+        return quickSelect(largerList, k-m)
+
+
+
+
+if __name__ == '__main__':
+    # Checking the Answer
+    seq = [10, 60, 100, 50, 60, 75, 31, 50, 30, 20, 120, 170, 200]
+
+    # we want the middle element
+    k = len(seq) // 2
+
+    # Note that this only work for odd arrays, since median in
+    # even arrays is the mean of the two middle elements
+    print(quickSelect(seq, k))
+    import numpy
+    print numpy.median(seq)