EPI: uniform random number (py)

Author: jsgoller1

elements-of-programming-interviews/problem_mapping.js

@@ -150,7 +150,7 @@ problem_mapping = {
                 "total": 7
             },
             "Python: uniform_random_number.py": {
-                "passed": 0,
+                "passed": 7,
                 "total": 7
             }
         },

elements-of-programming-interviews/python/uniform_random_number.py

@@ -11,9 +11,36 @@ def zero_one_random():
     return random.randrange(2)
 
 
+def uniform_random_wrong(lower_bound: int, upper_bound: int) -> int:
+    # How about a binary search where hi vs lo is chosen randomly?
+    # This doesn't work, because numbers towards the middle of the array
+    # have a higher chance of being chosen than ones closer to the edge (there)
+    # are more paths leading to them.
+    while lower_bound <= upper_bound:
+        mid = (lower_bound + upper_bound) // 2
+        if zero_one_random():
+            lower_bound = mid + 1
+        else:
+            upper_bound = mid - 1
+    return lower_bound
+
+
 def uniform_random(lower_bound: int, upper_bound: int) -> int:
-    # TODO - you fill in here.
-    return 0
+    """
+    We want to randomly generate a value in [lo, hi]. Note that if we can generate a value
+    from [0, hi-lo], we can just add it to lo, which will range from lo (if the number is 0)
+    to hi (if the number is hi-lo, since hi-lo+lo = hi). To generate a number from 0 to hi-lo, 
+    this function uses a dumb strategy of determining how many bits are in hi-lo, then selecting
+    a value for each of those bits via zero_one_random(). If we overshoot hi-lo, we just retry.
+    This might not work (e.g. suppose hi = 2^30 + c and lo = 1, so hi-lo is 30 bits long; we have
+    potentially 1073741824 30-bit values that exceed 2^30, so we could be retrying for a long time.
+    """
+    k = upper_bound-lower_bound
+    bits = len(bin(k)[2:])
+    val = k+1
+    while val > k:
+        val = int(''.join([str(zero_one_random()) for i in range(bits)]), 2)
+    return val+lower_bound
 
 
 @enable_executor_hook
@@ -33,7 +60,10 @@ def uniform_random_runner(executor, lower_bound, upper_bound):
 
 
 if __name__ == '__main__':
+    # print(uniform_random(0, 10))
+    # """
     exit(
         generic_test.generic_test_main('uniform_random_number.py',
                                        'uniform_random_number.tsv',
                                        uniform_random_wrapper))
+    # """