How do I find the numbers that are not listed or missing?

Question

I'll have a list of numbers, each on its own line (say 0 -100). How do I find the numbers that are not listed or missing?

Answer 1

Add them all to a set. Then subtract from a set filled with 1-100. Here's an example for 0-9:

>>> set(range(10)) - set([1, 4, 5, 6, 8, 2])
set([0, 9, 3, 7])
>>> 

I had [1, 4, 5, 6, 8, 2] listed. To find out which numbers in range 0-9 are missing, I created a set with all of 0-9 and then subtracted the set with [1, 4, 5, 6, 8, 2] from it. And found out that [0, 9, 3, 7] were missing.

Sets are fairly efficient for this. As an added benefit, duplicates will be handled gracefully.

Answer 2

If L is the list of numbers, then

set(L).difference(xrange(101))

saves creating a set from the xrange

In [1]: L=[1, 4, 5, 6, 8, 2]

In [2]: timeit set(range(101)) - set(L)
10000 loops, best of 3: 21.7 µs per loop

In [3]: timeit set(L).symmetric_difference(range(101))
100000 loops, best of 3: 14.2 µs per loop

In [4]: timeit set(L).difference(range(101))
100000 loops, best of 3: 9.73 µs per loop