Sort a sublist of elements in a list leaving the rest in place

Question

Say I have a sorted list of strings as in:

[\'A\', \'B\' , \'B1\', \'B11\', \'B2\', \'B21\', \'B22\', \'C\', \'C1\', \'C11\', \'C2\']

Now I wan

Answer 1

If you want to sort an arbitrary subset of elements while leaving other elements in place, it can be useful to design a view over the original list. The idea of a view in general is that it's like a lens over the original list, but modifying it will manipulate the underlying original list. Consider this helper class:

class SubList:
    def __init__(self, items, predicate):
        self.items = items
        self.indexes = [i for i in range(len(items)) if predicate(items[i])]

    @property
    def values(self):
        return [self.items[i] for i in self.indexes]

    def sort(self, key):
        for i, v in zip(self.indexes, sorted(self.values, key=key)):
            self.items[i] = v

The constructor saves the original list in self.items, and the original indexes in self.indexes, as determined by predicate. In your examples, the predicate function can be this:

def predicate(item):
    return item.startswith('B')

Then, the values property is the lens over the original list, returning a list of values picked from the original list by the original indexes.

Finally, the sort function uses self.values to sort, and then modifies the original list.

Consider this demo with doctests:

def demo(values):
    """
    >>> demo(['X', 'b3', 'a', 'b1', 'b2'])
    ['X', 'b1', 'a', 'b2', 'b3']

    """
    def predicate(item):
        return item.startswith('b')
    sub = SubList(values, predicate)

    def key(item):
        return int(item[1:])
    sub.sort(key)

    return values

Notice how SubList is used only as a tool through which to manipulate the input values. After the sub.sort call, values is modified, with elements to sort selected by the predicate function, and sorted according to the key function, and all other elements never moved.

Using this SubList helper with appropriate predicate and key functions, you can sort arbitrary selection of elements of a list.