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How can you group successive dates together into a a shorter string format such as the following: 

date_list = ['2016-01-01', '2017-01-01', '2017-01-02', '2017-01-03', '2017-01-05']

I'd like to turn that date list into the following 

desired_output = "1/1/16,1/1/17-1/3/17,1/1/17"

I've tried to implement solutions from here by using a function that calculates the difference between dates with little luck: Identify groups of continuous numbers in a list

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Chris
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  • Possible duplicate of [Identify groups of continuous numbers in a list](http://stackoverflow.com/questions/2154249/identify-groups-of-continuous-numbers-in-a-list) – juanpa.arrivillaga Jan 20 '17 at 01:17
  • Thanks - I tried to apply those solutions, but the challenge I ran into is that I'm actually dealing with dates, so I've updated my question – Chris Jan 20 '17 at 01:27
  • Same principle applies only you'll have to use `datetime` objects, most-likely – juanpa.arrivillaga Jan 20 '17 at 01:28
  • Right - that's what I thought as well, but it's not producing the correct groups. I can post what I've tried as well – Chris Jan 20 '17 at 01:29
  • Duplication question: http://stackoverflow.com/questions/21768963/group-consecutive-dates-together-with-python – Jcloud Jan 20 '17 at 01:29
  • fair enough - that one does answer my question. I'll close. Thanks! – Chris Jan 20 '17 at 01:35
  • Possible duplicate of [Group consecutive dates together with Python](http://stackoverflow.com/questions/21768963/group-consecutive-dates-together-with-python) – Chris Jan 20 '17 at 01:35

1 Answers1

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Before writing a script in python to do the task, let's see how we'd do this manually. Let's follow the given algorithm and see how it works.

Procedure:

Step 0: Take an empty string, named output_string.

Step 1: Take an empty sequence_list.

Step 2: Take the element (number) at the start of number_list, see if it can be added to sequence_list, then add the number to the sequence_list and remove it from number_list, and repeat Step 2. Otherwise, go to Step 3.

Step 3: Compress sequence_list to a string, and add it to output.

Step 4: If number_list is not empty, go to Step 1 again. Otherwise, display the output_string.

Let's take one iteration over the list in question for example.

number_list = [1,2,3,4,5,9,10,11,12,18,19,40]

After Step 0:

output_string = ""

After Step 1:

sequence_list = []

After Step 2: (Iterating 5 times)

sequence_list = [1, 2, 3, 4, 5]
number_list = [9,10,11,12,18,19,40]

Since, our next element is 9, which we can't add to the sequence list, we move to Step 3

After Step 3

output_string = "1-5"
number_list = [9,10,11,12,18,19,40]

At Step 4

Since, our number_list is not empty, we return to Step 1, and the whole process repeats for 3 more times. After which we get following output_string

output_string = "1-5,9-12,18-19,40"

This is the output we require. So, we are done.

Python Script for the same:

def numlist_string(numlist):
    # If the length of list is 1 then we don't need to write it in the 
    # form x-y, because that increases the number characters in string
    if(len(numlist) == 1):
        return "{}".format(numlist[0])
    else:
        numlist_last = len(numlist) - 1
        return "{}-{}".format(numlist[0], numlist[numlist_last])

def numlist_compress(numlist):
    if(len(numlist) == 0):
        return []

    compressed_list = []
    for i in range(len(numlist)):
        if(numlist[i] != numlist[0] + i):
            # Sequence is broken

            # Splitting up list into sequence and remaining
            req_list = numlist[0 : i]
            rem_list = numlist[i : ]

            # Getting sequence list as a string
            numlist_s = numlist_string(req_list)

            # Adding sequence string to the list of strings
            compressed_list.append(numlist_s)

            # Compressing the remaining list and adding that to previously 
            # generated sequence string
            compressed_list.extend(numlist_compress(rem_list))

            return compressed_list

    # Whole list is a sequence
    numlist_s = numlist_string(numlist)
    compressed_list.append(numlist_s)

    return compressed_list

if __name__ == '__main__':
    number_list = [1,2,3,4,5,9,10,11,12,18,19,40]

    sequence_list = numlist_compress(number_list)

    sequence_string = ','.join(sequence_list) 

    print(sequence_string)
Nikhil H
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