Write ranges of numbers with dashes

Question

I didn't quite know how to ask this question, or even search for the answer on Google, but I will write it out here. I have a sorted list of integers, that correspond to line numbers in a file. I would like to convert them into strings, but for the numbers that are sequential, I want the string to have the first number of the sequence, a dash, and then the last number. Here is an example:

line_nums = [ 1, 2, 3, 5, 7, 8, 9, 10 ]

I want to turn that list into:

[ '1-3', '5', '7', '8-10' ]

I wrote some code that works for the most part. On some sequences, it will put the same number in a string twice. On a recent execution of this code, the input was:

[ 10007, 10008, 10009, 10010, 10011, 10013, 10015, 10016, 10017, 10018, 10019 ]

But what I got back was:

[ '10007-10011', '10013-10013', '10015-10019' ]

---

Here is my code:

def get_line_numbers_concat(line_nums):
    seq = []
    final = []
    last = 0

    for index, val in enumerate(line_nums):

        if last + 1 == val or index == 0:
            seq.append(val)
            last = val
        else:
            final.append(str(seq[0]) + '-' + str(seq[len(seq)-1]))
            seq = []
            seq.append(val)
            last = val

        if index == len(line_nums) - 1:
            if len(seq) > 1:
                final.append(str(seq[0]) + '-' + str(seq[len(seq)-1]))
            else:
                final.append(str(seq[0]))

    final_str = ', '.join(map(str, final))
    return final_str

Answer 1

You're almost there except in the case when seq[0] is actually the same element as seq[len(seq)-1] which you then simplify to the case of len(seq)==1 or as shown below if len(seq) > 1 then you perform your normal processing, otherwise JUST add the first element.

def get_line_numbers_concat(line_nums):
    seq = []
    final = []
    last = 0

    for index, val in enumerate(line_nums):

        if last + 1 == val or index == 0:
            seq.append(val)
            last = val
        else:
            if len(seq) > 1:
               final.append(str(seq[0]) + '-' + str(seq[len(seq)-1]))
            else:
               final.append(str(seq[0]))
            seq = []
            seq.append(val)
            last = val

        if index == len(line_nums) - 1:
            if len(seq) > 1:
                final.append(str(seq[0]) + '-' + str(seq[len(seq)-1]))
            else:
                final.append(str(seq[0]))

    final_str = ', '.join(map(str, final))
    return final_str