sec02 - list Overview

list is a structure that can store multiple values in a single variable.

Initialization of list

First, let's initialize the minimum list. Write the code as follows.

num_list = []

Assign empty square brackets ([]) to a variable. This is a list with zero elements.

A list does not necessarily need to have elements. After initializing it with zero elements, you can add elements using the append() method or the + operator. (The method for adding elements will be explained in detail in a dedicated lecture.)

A list can contain one or more elements. You can initialize a list as follows.

numbers = [1, 2, 3, 4, 5]
primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]
alphabets = ['a', 'b', 'c', 'd', 'e']
cities = ['Tokyo', 'New York', 'Paris', 'Rio de Janeiro', 'Cairo']

The above code initializes (1) a list of numbers, (2) a list of prime numbers, (3) a list of alphabets, and (4) a list of city names. Each element is separated by a comma (,).

Generally, lists are easier to understand and manage when they contain only one data type, such as a list of numbers or a list of strings. However, as shown in the following code, you can also create a list that contains different data types.

mixed = [1, 'two', 3.0, [4]]

The list in the above code contains the data types int, string, float, and list. As you can see, Python makes it easy to manage multiple types of data.

Extracting elements from a list

List elements are written in the same way as strings, using [n]. Let's try extracting the elements of the “mixed” variable from earlier.

mixed = [1, 'two', 3.0, [4]]
print(mixed[0])  # 1
print(mixed[1])  # 'two'
print(mixed[2])  # 3.0
print(mixed[3])  # [4]

As shown in the code above, you can retrieve elements one by one using indexes.

(Slicing lists will be explained in a separate lecture.)

Number of elements in list - len() function

To check the number of elements in a list, use the len() function.

mixed = [1, 'two', 3.0, [4]]
print(len(mixed))  # 4

2D list

Python lists can contain other lists as elements. Embedding one data structure within another is called nesting.

Data with nested lists can be treated like data in rows and columns, so it is called a 2D list. Nesting can be done to any depth.

Initialization of a 2D list is as follows.

multiplication_table = [[0, 0, 0, 0, 0], [0, 1, 2, 3, 4, 5], [0, 2, 4, 6, 8, 10], [0, 3, 6, 9, 12, 15], [0, 4, 8, 12, 16, 20], [0, 5, 10, 15, 20, 25]]

If there are many elements and the code becomes difficult to read, you can write the elements on separate lines as shown in the following code.

multiplication_table = [
    [0, 0, 0, 0, 0],
    [0, 1, 2, 3, 4, 5],
    [0, 2, 4, 6, 8, 10],
    [0, 3, 6, 9, 12, 15],
    [0, 4, 8, 12, 16, 20],
    [0, 5, 10, 15, 20, 25]
]

As shown above, when describing elements with line breaks, combining them with indentation (inserting spaces at the beginning of program lines) makes the code easier to read.

The 2D list that was initialized earlier is now a ‘5x5 multiplication table’. Let's try outputting the results of ‘2x3’ and ‘4x5’.

multiplication_table = [
    [0, 0, 0, 0, 0],
    [0, 1, 2, 3, 4, 5],
    [0, 2, 4, 6, 8, 10],
    [0, 3, 6, 9, 12, 15],
    [0, 4, 8, 12, 16, 20],
    [0, 5, 10, 15, 20, 25]
]
answer_2x3 = multiplication_table[2][3]
answer_4x5 = multiplication_table[4][5]
print(f'2x3={answer_2x3}, 4x5={answer_4x5}')

When you run the above code, you will get the result “2x3=6, 4x5=20”. To retrieve elements from a 2D list, use the notation [n][m].

Using “2x3” as an example, first, you can retrieve the list [0, 2, 4, 6, 8, 10] corresponding to element number [2] from multiplication_table[2]. By specifying the element number [3] within the list [0, 2, 4, 6, 8, 10] retrieved earlier, you can extract the desired “6.” Therefore, by concatenating [2] and [3](multiplication_table[2][3]), you can obtain the result ‘6’ for “2x3.”

Examples of using list

As an example of using a list, we will explain the glob function. glob is a function that returns a list of folders and files existing in the specified directory. In this example, we will execute a simple code to obtain a list of folders/files existing directly under the C drive in Windows.

import glob
c_drive_list = glob.glob('c:/*')
print(c_drive_list)

When you run the above code, you should get a result similar to the one below. (The result may vary depending on your C drive environment. Since a large amount of data is returned, some data has been replaced with “...”.)

['c:/Documents and Settings',... 'c:/Program Files', 'c:/Program Files (x86)',... 'c:/ProgramData',... 'c:/Users', 'c:/Windows']

As in the example above, lists are used when returning large amounts of data. (Detailed explanations of the code will be provided in a separate lecture, so please focus on the results returned here.)

As a supplementary explanation, when displaying a list containing a large number of elements as shown above, printing the output results in long lines that can be difficult to read. In such cases, using the pprint function will output each element on a separate line, making the output easier to read.

import glob
from pprint import pprint

c_drive_list = glob.glob('c:/*')
pprint(c_drive_list)

The result output by pprint is as follows. Each element is output with a line break, making it easier to see what directories (folders/files) exist in the C drive.

['c:/Documents and Settings',
.....
 'c:/Program Files',
 'c:/Program Files (x86)',
 'c:/ProgramData',
.....
 'c:/Users',
 'c:/Windows']

from pprint import pprint
pprint(variable)