Create List of Lists in Between Two Numbers --- Python

Question

Suppose I have the following list of lists:

intervals = [[3, 5], [11,25], [39,40], [45, 48]]

Beginning with the start number, 0 for this example, all the way to the end number, 50 for this example, my goal is to create a list of lists with ranges of every number not in the current list of list. For example, the list I want to produce would like like this:

nonIntervals = [[0, 3], [5, 11], [25, 39], [40, 45], [48, 50]]

I have already been working on an implementation to do just this but it has issues with certain test cases:

for index in range(len(intervals)):
    nonIntervals.append([start, intervals[index][0]])
    start = intervals[index][1]
    
if not(nonIntervals[index][1] == end):
    nonIntervals.append([intervals[index][1], end])

With this example I run into some problems where it produces intervals I do not need. For example, if I used the following interval:

intervals = [[0, 5], [11,25], [39,40], [45, 48]]

I get this interval when running the loop: nonIntervals = [[0, 0], [5, 11], [25, 39], [40, 45], [48, 50]]

Does anyone know how I can avoid the nonIntervals list containing the [0,0] list? For some reason, it include the start of the array when I want to ignore it and begin with [5,11]. The same thing applies for when the end is at the end of the list.

For example: intervals = [[0, 5], [11,25], [39,40], [45, 50]]

When running my program it returns, nonIntervals = [[0, 0], [5, 11], [25, 39], [40, 45], [50, 50]]

I am expecting the last list in this list of lists to be [40, 45]. If anyone knows how to correct my loop or proper if-statements I need to implement I'd greatly appreciate it.

You almost got it. Just add an if condition before every non-intervals.append() statement, and do not append if the items that you're gonna append are the same — prakasht
– prakasht, Commented Jun 10, 2021 at 3:08
Okay, I think I corrected ! So for the last if statement that I have in my current code, do you think there is a way to implement that into the first for loop that I have or do you think it is a necessary statement to have? — africanxmamba
– africanxmamba, Commented Jun 10, 2021 at 3:34

Guybrush · Accepted Answer · 2021-06-10 10:03:18Z

Someone has already commented on your question by providing an answer to make the code work. But you might be interested in portion, a library I wrote to deal with intervals. It's on PyPI, so you can install it as usual: pip install portion.

Applied on your example:

>>> items = [[3, 5], [11,25], [39,40], [45, 48]]
>>> import portion as P
>>> interval = P.Interval(*[P.closed(x, y) for x, y in items])
>>> interval
[3,5] | [11,25] | [39,40] | [45,48]

Now, in order to get all ranges that are not in your list, it suffices to take the complement (either using ~ or .complement), and to restrict it to the range you're interested in (either using & or .intersection with the desired range):

>>> lower, upper = 0, 50
>>> ~interval & P.closed(lower, upper)
[0,3) | (5,11) | (25,39) | (40,45) | (48,50]

Since portion automatically simplifies intervals (including empty ones), you won't have any issue with [0,0].

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