Quiz: Nested Lists - 2D

Programming Python
#1

Hi Everyone!

I am new to programming language.

Can anyone explain what is the meaning of the countries2 variable.

countries = [['Egypt', 'USA', 'India'],
       ['Dubai', 'America', 'Spain'], 
       ['London', 'England', 'France']]
countries2  = [country for sublist in countries for country in 
                      sublist if len(country) < 6]
print(countries2).
#2

Because python uses indentation, you’ll need to paste your code into a code block:

This is a code block:
    It has indentation.
    It renders quote marks like " and ' without corrupting them into other characters.
    You create it by using the </> button that's in the top of the editing section of the page.

Your code is so hopelessly garbled that I can’t tell what it is, much less what it should be. So paste your code in a code block and let’s see what you have here.

#3

Thank you rob for the response.

I have updated the code.

#4

The easiest way to see this is to break this down into steps:

$ ipython
Python 3.9.19 (main, Mar 20 2024, 06:24:51)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.30.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: countries = [['Egypt', 'USA', 'India'],
   ...:        ['Dubai', 'America', 'Spain'],
   ...:        ['London', 'England', 'France']]

In [2]:  [country for sublist in countries for country in sublist]
Out[2]:
['Egypt',
 'USA',
 'India',
 'Dubai',
 'America',
 'Spain',
 'London',
 'England',
 'France']

In [3]:  [country for sublist in countries for country in sublist if len(country) < 6 ]
Out[3]: ['Egypt', 'USA', 'India', 'Dubai', 'Spain']

You start with a list of lists; we turn this into a (fairly complex) list comprehension, and then, we filter the result of that; that’s how you get to the answer.

#5

counties is a list of lists. Each sublist containing 3 countries

The comprehension that assigns the value to countries2 is a common trick in Python. It is called “list flattening”, to turn the list of lists into one list containing all the countries. It is shorthand for (and much faster than) this code

countries = [['Egypt', 'USA', 'India'],
       ['Dubai', 'America', 'Spain'], 
       ['London', 'England', 'France']]
countries2 = []

for sublist in countries:
    for country in sublist:
       countries2.append(country)

print(countries2)
#6

In plain english:

Countries2 is a dynamic list that asks for all the countries in each of the sublists within countries that have a length less than six.

The technical part:

Remember that countries is a list of lists

So it (countries 2 asks):

(Again this is NOT python, its plain english pseudo code)

for each sublist in countries:
    for each county within that sublist:
          only add the country to countries2 if the length is less than 6 characters

The ACTUAL python code for countries2 if taken from a one-liner to indented code (which, may or may not run, but in my opinion is slightly easier to understand whats going on) looks like this:

countries2 = [
    country for sublist in countries:
        for country in sublist:
               if len(country) < 6
]

Note: this code broken down like this might NOT actually run, due to indentationErrors and various other factors, it’s only shown like this for illustration / code readability purposes only, please dont send hatemail

Hope that breaks it down for you in a way that helps you read other code as well.