在元组列表中查找并移动元组

Question

What is the most efficient way of finding a certain tuple based on eg the second element of that tuple in a list and move that tuple to the top of the list Something of the form:

LL=[('a','a'),('a','b'),('a','c'),('a','d')]
LL.insert(0,LL.pop(LL.index( ... )))

where I would like something in index() that would give me the position of the tuple that has 'c' as second element.

Is there a classic python 1-line approach to do that?

Answer 1

To find position you can:

positions = [i for i, tup in enumerate(LL) if tup[1] == 'c']

You can now take the index of the desired element, pop it and push to the beginning of the list

pos = positions[0]
LL.insert(0, LL.pop(pos))

But you can also sort your list using the item in the tuple as key:

sorted(LL, key=lambda tup: tup[1] == 'c', reverse=True)

if you don't care about order of the other elements

Answer 2

>>> LL.insert(0,LL.pop([x for x, y in enumerate(LL) if y[1] == 'c'][0]))
>>> LL
[('a', 'c'), ('a', 'a'), ('a', 'b'), ('a', 'd')]
>>> 

Answer 3

2 lines, however 1 line solutions are all inefficient

>>> LL=[('a','a'),('a','b'),('a','c'),('a','d')]
>>> i = next((i for i, (x, y) in enumerate(LL) if y == 'c'), 0) # 0 default index
>>> LL[0], LL[i] = LL[i], LL[0]
>>> LL
[('a', 'c'), ('a', 'b'), ('a', 'a'), ('a', 'd')]

This does nothing if the index is not found

>>> LL=[('a','a'),('a','b'),('a','c'),('a','d')]
>>> i = next((i for i, (x, y) in enumerate(LL) if y == 'e'), 0) # 0 default index
>>> LL[0], LL[i] = LL[i], LL[0]
>>> LL
[('a', 'a'), ('a', 'b'), ('a', 'c'), ('a', 'd')]

Answer 4

The problem with terse, pythonic, 'fancy schmancy' solutions is the code might not be easily maintained and/or reused in other closely aligned contexts.

It seems best to just use 'boiler plate' code to do the search, and then continue with the application specific requirements.

So here is an example of easy to understand search code that can be easily 'plugged into' when these questions come up, including those situations when we need to know if the key is found.

def searchTupleList(list_of_tuples, coord_value, coord_index):
    for i in range(0, len(list_of_tuples)):
        if list_of_tuples[i][coord_index] == coord_value:
            return i               # matching index in list
    return -1                      # not found