gistfile1.py

# given list [2, 3, 5, 78]
# return 1, 4, 6-77, 79-99
# the range is always <1, 100) 
def get_ranges(lst, N=100):
    s = set(lst)
    filtered = (i for i in xrange(1, N) if not i in s)
    output2 = []
    prev = None
    skipping = False
    for num in filtered:
        if prev and prev + 1 == num:
            if not skipping:
                output2.append('%d-' % prev)
                skipping = True
        else:
            if skipping:
                output2[-1] += str(prev)
                skipping = False
            else:
                output2.append(str(num))
                skipping = False
        prev = num

    if skipping:
        output2[-1] += str(N - 1)
    return ",".join(output2)

print "***get_ranges***"

n1 = get_ranges([2, 3, 5, 78, 99])
res1 = "1,4,6-77,79-98"
print n1, res1 , res1 == n1
n2 = get_ranges([1, 2, 5, 78])
res2 = "3,6-77,79-99"
print n2, res2, res2 == n2
n3 = get_ranges([2, 3, 4, 78, 99])
res3 = "1,5-77,79-98"
print n3, res3, res3 == n3

# better if N is big
def get_ranges2(lst, N=100):
    current = 1
    output = []
    for num in lst:
        if current == num:
            pass
        elif current + 1 == num:
            output.append(str(current))
        else:
            output.append("%d-%d" % (current, num - 1))
        current = num + 1

    if current + 1 == N:
        output.append(current)
    elif current + 1 < N:
        output.append("%d-%d" % (current, N - 1))

    return ",".join(output)

print "***get_ranges2***"

n1 = get_ranges2([2, 3, 5, 78, 99])
res1 = "1,4,6-77,79-98"
print n1, res1 , res1 == n1
n2 = get_ranges2([1, 2, 5, 78])
res2 = "3-4,6-77,79-99"
print n2, res2, res2 == n2
n3 = get_ranges2([2, 3, 4, 78, 99])
res3 = "1,5-77,79-98"
print n3, res3, res3 == n3

# python int.py
# ***get_ranges***
# 1,4,6,6-77,79-99 1,4,6-77,79-98 False
# 3,3-4,6-77,79-99 3,6-77,79-99 False
# 1,5,5-77,79-99 1,5-77,79-98 False
# ***get_ranges2***
# 1,4,6-77,79-98 1,4,6-77,79-98 True
# 3-4,6-77,79-99 3-4,6-77,79-99 True
# 1,5-77,79-98 1,5-77,79-98 True