Pythonic way to convert a list of integers into a string of comma-separated ranges

Question

I have a list of integers which I need to parse into a string of ranges.

For example:

 [0, 1, 2, 3] -> \"0-3\"
 [0, 1, 2, 4, 8] -> \"0-2,4,8\"
         


        

Answer 1

To concatenate strings you should use ','.join. This removes the 2nd loop.

def createRangeString(zones):
        rangeIdx = 0
        ranges   = [[zones[0], zones[0]]]
        for zone in list(zones):
            if ranges[rangeIdx][1] in (zone, zone-1):
                ranges[rangeIdx][1] = zone
            else:
                ranges.append([zone, zone])
                rangeIdx += 1

       return ','.join(
                map(
                  lambda p: '%s-%s'%tuple(p) if p[0] != p[1] else str(p[0]),
                  ranges
                )
              )

Although I prefer a more generic approach:

from itertools import groupby

# auxiliary functor to allow groupby to compare by adjacent elements.
class cmp_to_groupby_key(object):
  def __init__(self, f):
    self.f = f
    self.uninitialized = True
  def __call__(self, newv):
    if self.uninitialized or not self.f(self.oldv, newv):
      self.curkey = newv
      self.uninitialized = False
    self.oldv = newv
    return self.curkey

# returns the first and last element of an iterable with O(1) memory.
def first_and_last(iterable):
  first = next(iterable)
  last = first
  for i in iterable:
    last = i
  return (first, last)

# convert groups into list of range strings
def create_range_string_from_groups(groups):
  for _, g in groups:
    first, last = first_and_last(g)
    if first != last:
      yield "{0}-{1}".format(first, last)
    else:
      yield str(first)

def create_range_string(zones):
  groups = groupby(zones, cmp_to_groupby_key(lambda a,b: b-a<=1))
  return ','.join(create_range_string_from_groups(groups))

assert create_range_string([0,1,2,3]) == '0-3'
assert create_range_string([0, 1, 2, 4, 8]) == '0-2,4,8'
assert create_range_string([1,2,3,4,6,7,8,9,12,13,19,20,22,22,22,23,40,44]) == '1-4,6-9,12-13,19-20,22-23,40,44'