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

by 
>>> from itertools import count, groupby
>>> L=[1, 2, 3, 4, 6, 7, 8, 9, 12, 13, 19, 20, 22, 23, 40, 44]
>>> G=(list(x) for _,x in groupby(L, lambda x,c=count(): next(c)-x))
>>> print ",".join("-".join(map(str,(g[0],g[-1])[:len(g)])) for g in G)
1-4,6-9,12-13,19-20,22-23,40,44

The idea here is to pair each element with count(). Then the difference between the value and count() is constant for consecutive values. groupby() does the rest of the work

As Jeff suggests, an alternative to count() is to use enumerate(). This adds some extra cruft that needs to be stripped out in the print statement

G=(list(x) for _,x in groupby(enumerate(L), lambda (i,x):i-x))
print ",".join("-".join(map(str,(g[0][1],g[-1][1])[:len(g)])) for g in G)

Update: for the sample list given here, the version with enumerate runs about 5% slower than the version using count() on my computer

More Related Contents:

  1. Concatenating two lists – difference between ‘+=’ and extend()
  2. Test if lists share any items in python
  3. What’s the most Pythonic way to identify consecutive duplicates in a list?
  4. Pythonic way to return list of every nth item in a larger list
  5. How to check if all of the following items are in a list?
  6. Extract list of attributes from list of objects in python
  7. Iterate over pairs in a list (circular fashion) in Python
  8. Python: find a list within members of another list(in order)
  9. Slicing a list into a list of sub-lists [duplicate]
  10. Why does this code for initializing a list of lists apparently link the lists together? [duplicate]
  11. Why is [] faster than list()?
  12. How can I multiply all items in a list together with Python?
  13. How to unzip a list of tuples into individual lists? [duplicate]
  14. Remove trailing newline from the elements of a string list
  15. How to loop through all but the last item of a list?
  16. How to create an Object that outputs a List (Not A Table)
  17. Writing a list to a file with Python, with newlines
  18. How can I convert each item in the list to string, for the purpose of joining them? [duplicate]
  19. How can I initialize a dictionary of distinct empty lists in Python?
  20. Extract elements of list at odd positions
  21. How to read an array of integers from single line of input in python3
  22. Unexpected IndexError while removing list items [duplicate]
  23. How to remove item from a python list in a loop? [duplicate]
  24. How to split an iterable into two lists with alternating elements
  25. How do I sort a list of objects based on an attribute of the objects?
  26. How can I match up permutations of a long list with a shorter list (according to the length of the shorter list)? [duplicate]
  27. Removing duplicates from a list in Haskell without elem
  28. list comprehension with multiple conditions (python)
  29. How to convert only numbers in a mixed list into float
  30. Grouping a list into lists of n elements in Haskell

Leave a Comment