Convert List of Numbers to String Ranges

Question

I'd like to know if there is a simple (or already created) way of doing the opposite of this: Generate List of Numbers from Hyphenated.... This link could be used to do: ``` >> list(hyphen_range('1-9,12,15-20,23')) [1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 15, 16, 17, 18, 19, 20, 23]: ``` I'm looking to do the opposite (note that 10 and 21 are included so it would be compatible with the range function, where range(1,10)=[1,2,3,4,5,6,7,8,9]): ``` >> list_to_ranges([1, 2, 3, 4, 5, 6, 7, 8, 9, 12, 15, 16, 17, 18, 19, 20, 23]) '1-10,12,15-21,23' ``` Eventually, I would like to have the output also incorporate a step where the last number of the output indicates the step: ``` >> list_to_ranges([1, 3, 5, 7, 8, 9, 10, 11]) '1-13:2,8,10' ``` Essentially, this would end up being kind of like an "inverse" range function ``` >> tmp = list_to_ranges([1, 3, 5]) >> print tmp '1-7:2' >> range(1, 7, 2) [1, 3, 5] ``` My guess is that there is no really easy/simple way to do this, but I thought I would ask on he…

Accepted Answer

One approach could be "eating" piece by piece the input sequence and store the partial range results untill you've got them all:

```
def formatter(start, end, step):
    return '{}-{}:{}'.format(start, end, step)
    # return '{}-{}:{}'.format(start, end + step, step)

def helper(lst):
    if len(lst) == 1:
        return str(lst[0]), []
    if len(lst) == 2:
        return ','.join(map(str,lst)), []

step = lst[1] - lst[0]
    for i,x,y in zip(itertools.count(1), lst[1:], lst[2:]):
        if y-x != step:
            if i > 1:
                return formatter(lst[0], lst[i], step), lst[i+1:]
            else:
                return str(lst[0]), lst[1:]
    return formatter(lst[0], lst[-1], step), []

def re_range(lst):
    result = []
    while lst:
        partial,lst = helper(lst)
        result.append(partial)
    return ','.join(result)
```

I test it with a bunch of unit tests and it passed them all, it can handle negative numbers too, but they'll look kind of ugly (it's really anybody's fault).

Example:

```
>>> re_range([1,  4,5,6, 10, 15,16,17,18, 22, 25,26,27,28])
'1,4-6:1,10,15-18:1,22,25-28:1'
>>> re_range([1, 3, 5, 7, 8, 9, 10, 11, 13, 15, 17])
'1-7:2,8-11:1,13-17:2'
```

Note: I wrote the code for Python 3.

Performance

I didn't put any performance effort in the solution above. In particular, every time a list get re-builded with slicing, it might take some time if the input list has a particular shape. So, the first simple improvement would be using `itertools.islice()` where possible.

Anyway here's another implementation of the same algorithm, that scan through the input list with a `scan` index instead of slicing:

```
def re_range(lst):
    n = len(lst)
    result = []
    scan = 0
    while n - scan > 2:
        step = lst[scan + 1] - lst[scan]
        if lst[scan + 2] - lst[scan + 1] != step:
            result.append(str(lst[scan]))
            scan += 1
            continue

for j in range(scan+2, n-1):
            if lst[j+1] - lst[j] != step:
                result.append(formatter(lst[scan], lst[j], step))
                scan = j+1
                break
        else:
            result.append(formatter(lst[scan], lst[-1], step))
            return ','.join(result)

if n - scan == 1:
        result.append(str(lst[scan]))
    elif n - scan == 2:
        result.append(','.join(map(str, lst[scan:])))

return ','.join(result)
```

I stopped working on it once it got ~65% faster than the previous top solution, it seemed enough :)

Anyway I'd say that there might still be room for improvement (expecially in the middle for-loop).

Convert List of Numbers to String Ranges

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