[Python Basic] Working With Lists — Part 2

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We already know what the Python list is and how to manipulate one with some functions and even list comprehensions. Do we?

For many of you, it is true. But there is still a growing number of new generations who want to participate in this fascinating new world of machine learning, artificial intelligence, data science or whatever. We should provide them some materials to read, think and learn.

This post is an attempt to prepare teaching materials to my students whose major is not computer science, but who are willing to learn this new fascinating world.

Making Numerical Lists

There are many reasons to store a set of numbers in computer programmings. Lists are ideal for storing sets of numbers and Python gives us tools to work with lists of numbers.

Using the range() Function

Python’s range() function makes it easy for us to create a series of numbers. With the help of for statement, we can see values in the range() function.

for value in range(1,5):

One of the must-remember things in Python is the indexing scheme of its own. It might help us to remember that a <= x < b in range(a, b) . That is, the left-hand side is included, while the right-hand side is excluded. So the output will be 1, 2, 3 and 4, excluding 5.

Using the range() to Make a List of Numbers

If we want to make a list of numbers, we can do it using the list() function as we did in the previous posting.

my_numbers = list(range(1,5))

The protocol of range() function is range(a, b, c) , where a is the starting number, b is the ending number +1 (remember the exclusion) and c is the increment. Some examples.

range(1,5)                 # 1, 2, 3, 4
range(2,7,1) # 2, 3, 4, 5, 6
range(3,10,2) # 3, 5, 7, 9

Here is a question where we can test our logical thinking. What is range(3) ? We are given the range() function with just one parameter? Is it a starting number, an ending number +1 or an increment? If it were a starting number, we will get an infinite sequence starting from 3, which is impossible. If it were an increment, then we still do not have the ending number, we can guess the starting number might be 0 though. So the final candidate is that 3 is the ending number +1 . Yes, range(3) is 0, 1 and 2.

Using enumerate() Function

It is useful to obtain an indexed list. It returns a pair of (index, value) in a loop. For example, modifying the previous post’s code with enumerate() function produces the index of the list as well as the value of the list.

my_string = "Hello, Python!"
my_list = list(my_string)
for i, item in enumerate(my_list):
print(i, item)

Remember that enumerate() always returns a pair so we need two variables (i and item in the code) to store the pair. Also, don’t forget the Python index starts at 0.

List Comprehensions

There are many reasons why people love Python, and list comprehensions must be one of them. It makes our code shorter while maintaining the readability once we master it.

Let us consider the following example of finding squared numbers from 1 to 10.

squares = []                   #1
for value in range(1,11): #2
squares.append(value**2) #3
print(squares) #4

We make an empty list named squares at #1. Then using the for loop and the range() function at #2, we make the list containing squared values from 1 to 10 at #3. Finally, print the result.

Just for a quick reminder, value**2 is a square operation and squares.append() is used for adding new values to the existing list.

With list comprehensions, we can make one line code for #1, #2 and #3.

squares = [value**2 for value in range(1,11)]

Isn’t it nice to have a shorter code while keeping the readability? We don’t have to initialize the list and don’t have to use the append method. Compare the list comprehension with the three lines at #1, #2 and #3.




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