Dev102.com programming challenge #10

Programming challenge #10 has been posted at dev102.com.

If all the numbers 1..n were present, then they would sum to n(n+1)/2. If some x is absent, then they will sum to n(n+1)/2 - x. So to determine which number is missing, subtract the sum of the series from the expected sum.

This python code finds the missing number in the series named values:

def test(values) :
	max_val = len(values) + 1
	expected_sum = max_val*(max_val + 1) / 2
	return expected_sum - sum(values)

Here is a complete test:

import random
def test(values) :
	max_val = len(values) + 1
	expected_sum = max_val*(max_val + 1) / 2
	return expected_sum - sum(values)

test_range = 200
for i in range(1,test_range) :
	a = range(1, i) + range(i+1, test_range+1)
	random.shuffle(a)
	x = test(a)
	if x != i :
		print "Failed at ", i
		print a
		print sum(a)
		break