Array manipulation in python

Question

Let's suppose I have this simple array:

simple_list = [    
    ('1', 'a', 'aa'),    
    ('2', 'b', 'bb'),    
    ('3', 'c', 'cc')
]

If we consider this list as a table, where columns are separated by comas and lines separated by tuples, I want to create a function that retrieves only the columns I want. for example, this function would look like something like this:

get_columns(array, tuple_columns_selector))

I want, for example, to collect only the first and third column out of it, in this case, it would return me another array with the new values:

if I do:

get_columns(simple_list, (0,2))     
get_columns(simple_list, (0,))

it will return something like:

[('1', 'aa'), ('2', 'bb'), ('1', 'cc')]    
[1, 2, 3]

And so on. Could you help me creating this get_columns function, please? Here's the code I've tried:

def get_columns(arr, columns): 
    result_list = [] 
    for ii in arr: 
        for i in columns: 
            result_list.append(ii[i]) 
    return result_list 


to_do_list = [
    ('Wake Up', True), 
    ('Brush Teeh', True), 
    ('Go to work', True), 
    ('Take a shower', True), 
    ('Go to bed', False) 
] 

print(get_columns(to_do_list, (0,)))

I show you my code if you show me yours :-) Even if its incorrect, just show what you've tried. We can help you fix it. — Chris
– Chris, Commented Sep 12, 2017 at 17:24
Welcome to StackOverflow. Please read How to Ask, and include details of what you have tried, specifically, show us some code that you may have tried to write. — Antimony
– Antimony, Commented Sep 12, 2017 at 17:30
These are not arrays. But actually, this sounds like a great use-case for actual arrays, specifically, structured numpy.arrays — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Sep 12, 2017 at 17:30
Shouldn't get_columns(simple_list, (0,)) return [('1'), ('2'), ('3')]? Since it should return a list of tuples... — francisco sollima
– francisco sollima, Commented Sep 12, 2017 at 17:30
OP, it might be worth using a pandas dataframe. This sort of operation has built-in support if you're using data frames. It's quite a rabbit hole but totally worth it, IMO. — RagingRoosevelt
– RagingRoosevelt, Commented Sep 12, 2017 at 17:34

Chris · Accepted Answer · 2017-09-12 17:44:45Z

4

Use the magic of operator.itemgetter and map:

from operator import itemgetter

simple_list = [
    ('1', 'a', 'aa'),
    ('2', 'b', 'bb'),
    ('3', 'c', 'cc')
]

cols = (1,) # can be (0, 2)
fn = itemgetter(*cols)
print map(fn, simple_list)

Returns:

[('1', 'aa'), ('2', 'bb'), ('3', 'cc')]

when cols is (0, 2).

And it returns:

[1,2,3]

when cols is (1,).

So your get_columns function can be

def get_columns(data, cols):
    return map(itemgetter(*cols), data)

edited Sep 12, 2017 at 17:44

Chris

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answered Sep 12, 2017 at 17:30

kopos

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2 Comments

Chris Over a year ago

Ah, very nice solution, much better than what I was originally thinking of. This handles the OP's logic of getting single elements or a tuple of elements very elegantly.

Felipe Marcelus Mantovani Over a year ago

Cheers, I'm new to programming, I am not very familiarized to modules but that's a very interesting way of solving it. Thanks once again.

francisco sollima · Accepted Answer · 2017-09-12 18:21:16Z

2

The answer of @kopos looks fine, I just wanted to share one without additional libraries.

simple_list = [
    ('1', 'a', 'aa'),
    ('2', 'b', 'bb'),
    ('3', 'c', 'cc')
]

def get_columns(array, tuple_columns_selector):
    return [tuple(elem[i] for i in tuple_columns_selector) for elem in array]

def get_columns_multiple_lines(array, tuple_columns_selector):
    # The only difference between the result of this version and the other is that this one returns a list of lists
    # while the other returns a list of tuples
    resulting_list = []  # Create the variable that will store the resulting list
    for elem in array:  # Loop for each element in array
        resulting_list.append([])  # We add a new "empty row" to store all the columns needed
        for i in tuple_columns_selector:  # Loop for each column needed
            resulting_list[-1].append(elem[i])  # We append the column value to the last item in resulting_list
    return resulting_list


print get_columns(simple_list, (0,2))  # outputs [('1', 'aa'), ('2', 'bb'), ('3', 'cc')]
print get_columns(simple_list, (0,))  # outputs [('1',), ('2',), ('3',)]
print get_columns_multiple_lines(simple_list, (0,2))  # outputs [['1', 'aa'], ['2', 'bb'], ['3', 'cc']]

The only difference is the return value when tuple_columns_selector is only one column. If it's an important difference, I can "correct" it, but you should think about how that value will be used and if it's convenient for it to have different possible structures.

edited Sep 12, 2017 at 18:21

answered Sep 12, 2017 at 17:34

francisco sollima

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2 Comments

Felipe Marcelus Mantovani Over a year ago

Thanks a mil, you've done exactly what I wanted. It's a bit confusing for me this one lined for though. I've not yet got used to it but I will, for sure. Nah, no need to change the (1,) to 1. It's fine.

francisco sollima Over a year ago

I added a more simple, not one-line approach.

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