Custom slice from list of lists

Question

I have this list

MAIN = [
        ['ABC', '562', '112', '80', '231', '217', '433', '115', '10'],
        ['ABC', '562', '112', '80', '231', '322', '202', '432', '12'],
        ['ABC', '562', '112', '80', '231', '677', '133', '255', '64'], 
        ['DEF', '711', '87', '319', '433', '981', '400', '100', '09'],
        ['DEF', '711', '87', '319', '433', '113', '210', '321', '51'],
        ['DEF', '711', '87', '319', '433', '921', '711', '991', '44']
       ]

and I want to generate 2 lists from MAIN list.

1- First get list A that would have elements from index 0 to index 4 for each sublist in MAIN, resulting in this

A = [ 
     ['ABC', '562', '112', '80', '231'],
     ['ABC', '562', '112', '80', '231'],
     ['ABC', '562', '112', '80', '231'],
     ['DEF', '711', '87', '319', '433'],
     ['DEF', '711', '87', '319', '433'],
     ['DEF', '711', '87', '319', '433']
    ]

and remove duplicates to finally get this A list:

A = [ 
     ['ABC', '562', '112', '80', '231'],
     ['DEF', '711', '87', '319', '433'],
    ]

2 - Get list B that would have elements with index 0 and from index 5 to index 8 for each sublist in MAIN, resulting in this

B = [
     ['ABC', '217', '433', '115', '10'],   
     ['ABC', '322', '202', '432', '12'],
     ['ABC', '677', '133', '255', '64'], 
     ['DEF', '981', '400', '100', '09'], 
     ['DEF', '113', '210', '321', '51'], 
     ['DEF', '921', '711', '991', '44']
    ]

Below my attemps so far:

To get list A

A = []
for z in MAIN:
    y = z[:5]
    if not (y in A):
        A.append(y)

To get list B

B = []
for z in MAIN:
    B.append(list(set(z) - set(z[1:5])))

In results below it seems list A is ok, but list B has the sublists in different order and last sublist has a missing element.

A = [
     ['ABC', '562', '112', '80', '231'], 
     ['DEF', '711', '87', '319', '433']
    ]

B = [
     ['217', '433', 'ABC', '10', '115'], 
     ['322', '202', '432', 'ABC', '12'], 
     ['255', '64', '677', 'ABC', '133'], 
     ['09', '100', '400', '981', 'DEF'], 
     ['113', '51', '210', '321', 'DEF'], 
     ['DEF', '44', '991', '921']
    ]

How would be a best method to assure the correct output for A and B? Thanks for any help.

user_2988730 · Answer

You can apply a slice to each element using a comprehension:

[x[:5] for x in MAIN]

The best way I know to remove duplicates is to use a set. However, you can't add lists to it, so you'll have to wrap the slices in a tuple:

A = list(set(tuple(x[:5]) for x in MAIN))

If you want the elements to be lists instead of tuples, you'll have to convert them explicitly:

A = list(map(list, set(tuple(x[:5]) for x in MAIN)))

You can't rely on set to create slices for you because it does not guarantee order or preserve duplicates. Instead, just append slices together:

B = [x[:1] + x[5:9] for x in MAIN]

Notice that the slice x[:1] (a.k.a. x[0:1]) creates a one-element list, while x[0] would return the same element as a scalar. Any index n can be rewritten as slice n:n+1 this way.

Custom slice from list of lists

1 Answers