Question

I have two lists which have been created by .csv files. The first consists of a branch id number and a list of corresponding flows. The second is the order in which I wish to have the branche ids and their corrsponding flows sorted. They are as follows:

branch_flows = [['1234-2321-1', [55, 76, 3, 55, 6]],
                ['1546-2645-1', [4, 6, 56, 3, 4]],
                // ...
                ['4123-1234-1', [6, 12, -4, 7, 9]]
               ]

and

ordered_branches = ['1234-2321-1',
                    '1234-4123-1',
                    // ...
                    '1546-2645-1']

I am wondering how to sort branch_flows the same way ordered_branches is ordered, but for the flows to stay related to the same ids after sorting? The main difficulty being that some of the branch ids in branch_flows first two parts are reversed, but I need them to be sorted as if they were not.

e.g. looking to the lists above, desired output would be having branch_flows sorted in a way that the final list in branch_flows was placed second in the sorted list (as 1234-4123-1 in ordered_branches can equal both 1234-4123-1 AND 4123-1234-1 in branch_list, as the order in branch_flows can sometimes be the reverse of that in ordered_branches).

I originally tried to use dictionarys as look up tables but ran into trouble with the reading reverse order part. Help much appreciated!

Answer 1

You need to construct an appropriate key function to the Python sort function.

Ignoring the reversed-order issue, it’s quite easy:

def key(branch):
    id, flows = branch
    return ordered_branches.index(id)

Considering the reversed-order issue, we can use:

def key(branch):
    id, flows = branch
    try:
        return ordered_branches.index(id)
    except ValueError:
        parts = id.split('-')
        id = '-'.join((parts[1], parts[0], parts[2]))
        return ordered_branches.index(id)

Now you can sort branch_flows as sorted(branch_flows, key=key).

---

You can speed this up by turning ordered_branches into a dictionary:

order_dict = dict((x, i) for i, x in enumerate(ordered_branches))

and instead of ordered_branches.index(id) use order_dict[id] (also change ValueError to KeyError).

---

As a time-space tradeoff you could construct the reversed-order ids in the dict:

def reverse_id(id):
    parts = id.split('-')
    return '-'.join((parts[1], parts[0], parts[2]))
order_dict = dict((x, i) for i, x in enumerate(ordered_branches))
order_dict.update((reverse_id(x), i) for x, i in order_dict.items())

Now your key function just looks like:

def key(branch):
    id, flows = branch
    return order_dict[id]