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Introducing and wWorking with lists

Hello and welcome back the third workshop of Pyladies - Ho Chi Minh chapter,

This time we will continue to learn about an introduction of list and work on some simple examples of this data structure.

For some of you who have not followed us since the first workshop begun, please checkout the last session for more references.

  • Workshop1: Get started with Jupyterlab and git
  • Workshop2: Variable with str and numbers. How to write a good script with PEP8

Again, the main content of this workshop is mostly adapted from "Python Crash Course: A Hands-on, Project-based Introduction to Programming" by Eric Matthews and "Developer Class" by Google in python 3.

Alright, let us get started!

Outline

  1. What is a list? How to access elements in a list
  2. Modifying, adding, inserting/ removing elements
  3. Loop through an entire list with some notices
  4. Make numerical lists with some statistics
  5. Simple statistics with a list of numbers
  6. Define a tuple and looping through all values in a tuples

0. What is a list?

In simple words, list is a collection of many items in a order. Every elements can be str, int, float, and event list itself. Open an your Jupyterlab and let us know how your list variable looks like.

"""
Example of a list
"""
vietnam_cities = ["Hanoi", "Hai Phong", "Hue", "Danang", "Saigon"]
print(vietnam_cities)

To get an access to an element or a sub-list from an original list, we use index, or a position of a list. Note Different from other programming language, an index of a list starts with 0.

Try the following print function and let us know how it works.

"""
Example of accessing a list
"""
print(vietnam_cities[0])    # The fist element
print(vietnam_cities[-1])   # The last element
print(vietnam_cities[::-1])   # Reverse an order
print(vietnam_cities[:-1:])   # Start from the second element from the last

1. Modifying, adding, inserting/ removing elements

In this section, we will try to show you how to manipulate with elements in python with simple and easy examples. To begin, let us start with a story.

[a picture here]

Summer is coming. Imagine that we are planning for a vacation. Let us say my_cities is a list of cities in a list of vietnam_cities you would like to travel in an upcoming trip.

First of all, let us create a copy of a list

my_cities = vietnam_cities[:]    #Get a copy of vietnam_cities
print(my_cities)

Remember: Use [:] at the end of a variable you want to copy unless any change you update to either vietnam_cities or my_cities will affect on another.

However, there are some missing, and some are not belong to your favorite one. Suppose "Vinh" is a city you would like to visit instead of "Hue". How would you do to update my_cities?

There are many ways to do. Let us try with some most basic manipulation.

Modifying

If we want to visit "Vinh" after "Hanoi" ...

vietnam_cities[1] = "Vinh"
print(vietnam_cities)
Adding

If we want to visit "Vinh" after arriving "Saigon" (in case still have money and time)

vietnam_cities.append("Vinh")
print(vietnam_cities)
Inserting/ Removing

If we want to visit "Vinh" first ...

vietnam_cities.insert(0,"Vinh")
print(vietnam_cities)

What if "Vinh" is the last ...

vietnam_cities.insert(-1,"Vinh")
print(vietnam_cities)

Ok, we have done a lot planning, there are something wrong, what should we do?

del vietnam_cities[0]
del vietnam_cities[-1]
print(vietnam_cities)

Well, if we would not like to visit "Vinh" anymore ...

vietnam_cities.remove("Vinh")
print(vietnam_cities)

5. Simple statistics with a list of numbers

visit_days = [1,2,3,4,5]
min(visit_days)
max(visit_days)
sum(visit_days)
triples = [value ** 3 for values in visit_days]
print(triples)

6. Define a tuple and looping through all values in a tuple

Tuple is an immutable list, which means that you can not change an oder or make any modification. To understand, let us come up with some examples

How to initialize a tuple
carried_items = ("t-shirt", "shoes", "pairs of pants", "sunglasses")
print(carried_items[0])
print(carried_items[-1])

Try to update a tuple?

carried_items[1] = "sandals"

No, it won't work. The only way is that we set up a new value of that tuple.

carried_items = ("t-shirt", "shoes", "pairs of pants", "sunglasses")
for item in carried_items:
  print(item)

carried_items = ("t-shirt", "sandals", "pairs of pants", "sunglasses")
for item in carried_items:
  print(item)

Review

To end this lesson, here are some of methods used for list (Adapted from Google Developer Class in Python)

  • list.append(elem) -- adds a single element to the end of the list. Common error: does not return the new list, just modifies the original.
  • list.insert(index, elem) -- inserts the element at the given index, shifting elements to the right.
  • list.extend(list2) adds the elements in list2 to the end of the list. Using + or += on a list is similar to using extend().
  • list.index(elem) -- searches for the given element from the start of the list and returns its index. Throws a ValueError if the element does not appear (use "in" to check without a ValueError).
  • list.remove(elem) -- searches for the first instance of the given element and removes it (throws ValueError if not present)
  • list.sort() -- sorts the list in place (does not return it). (The sorted() function shown below is preferred.)
  • list.reverse() -- reverses the list in place (does not return it)
  • list.pop(index) -- removes and returns the element at the given index. Returns the rightmost element if index is omitted (roughly the opposite of append()).