Text representation of a list with gaps

Question

I have a list of integers that is sorted and contains no duplicates:

mylist =  [2, 5,6,7, 11,12, 19,20,21,22, 37,38, 40]

I want a summarized text representation that shows groups of adjacent integers in a compressed form as a hyphenated pair. To be specific: Adjacent implies magnitude differing by 1. So an integer i is considered to be adjacent to j if j = i ± 1. Recall that the list is sorted. That means that adjacent integers will appear in monotonically increasing series in the list.

So I want some elegant Python that will represent mylist as the string

"2, 5-7, 11-12, 19-22, 37-38, 40,"

That is,

• an isolated integer (example: 2, because the list contains neither 1 nor 3) is represented as 2,
• a group of adjacent integers (example: 19,20,21,22 because each member of the group differs from one other member by 1) is represented as ‹lowest›-‹highest›, that is 19-22,.

I can't believe this is a problem nobody has thought important enough to solve. Feel free to point me at a solution I have missed.

Answer 1

You can try this:

mylist = [2, 5, 6, 7, 11, 12, 19, 20, 21, 22, 37, 38, 40]
ans = []
ret = ''

# assuming mylist is sorted
for i in mylist:
    if len(ans) == 0 or ans[-1][1] < i - 1: #if the array is empty or we can't add current value to the last range
        ans.append([i, i]) # make a new range
    else:
        ans[-1][1] = i # add to last range

for i in ans:
    # formating
    if i != ans[0]: 
        ret = ret + ' '
    if i[0] == i[1]: 
        ret = f'{ret}{i[0]},'
    else:
        ret = f'{ret}{i[0]}-{i[1]},'
print(ret)

Hope this helps!