How to make a new list of first elements from existing list of lists
Solution 1
You have two ways
Using a for loop
countries = []
for e in x:
countries.append(e[0])
or with list comprehensions, which would be in most cases the better option
countries = [e[0] for e in x]
Furthermore, if your data source is a generator (which it isn't in this case), or if you're doing some expensive processing on each element (not this case either), you could use a generator expression
by changing the square brackets []
for parenthesis ()
countries = (e[0] for e in x)
This will compute on demand the elements, and if the data source is too long or a generator will also reduce the memory footprint compared to a list comprehension.
Solution 2
The most readable way is probably:
>>> data = [["UK", "LONDON", "EUROPE"],
["US", "WASHINGTON", "AMERICA"],
["EG", "CAIRO", "AFRICA"],
["JP","TOKYO","ASIA"]]
>>> countries = [country for country, city, continent in data]
>>> countries
['UK', 'US', 'EG', 'JP']
This list comprehension makes it clear what the three values in each item from data
are, and which will be in the output, whereas the index 0
doesn't tell the reader much at all.
Solution 3
As cities LONDON
, WASHINGTON
, CAIRO
, TOKYO
are present in 1st position(starting from 0) on list items. So get all 1st item from the list items by list compression.
e.g.
>>> x= [["UK", "LONDON", "EUROPE"],["US", "WASHINGTON", "AMERICA"],["EG", "CAIRO", "AFRICA"],["JP","TOKYO","ASIA"]]
>>> [i[1] for i in x]
['LONDON', 'WASHINGTON', 'CAIRO', 'TOKYO']
>>>
same for countries:
>>> [i[0] for i in x]
['UK', 'US', 'EG', 'JP']
Sergiu Costas
Updated on January 01, 2021Comments
-
Sergiu Costas over 3 years
I have a list below. I need to use it to create a new list with only country names. How do I loop
x
in order to have a list of country names?x = [["UK", "LONDON", "EUROPE"], ["US", "WASHINGTON", "AMERICA"], ["EG", "CAIRO", "AFRICA"], ["JP", "TOKYO", "ASIA"]]
The outcome should look like
UK US EG JP