Python

Lists

Organizing information in lists

What is a list?

A list is used to group certain information together. They are ordered, and the information in them can be changed.

Creating a List

To create a list in python, simply add information in between ‘[]’ and put commas between each value. Here is an example…

rainbow_colors = ["violet", "indigo", "blue", "green", "yellow", "orange", "red"]

Indexing and Accessing Values

Like we said earlier, lists are ordered. So, whatever order you put something into the list remains throughout the program. Each value added to the list is given an index. The first value has index 0. Every subsequent value has an index of 1, 2, 3, etc. To access a particular value in a list, type the list’s name, followed by square brackets and the index of the value you want to access. For example…

rainbow_colors = ["violet", "indigo", "blue", "green", "yellow", "orange", "red"]

color = rainbow_colors[3]
print(color)    

Output:

green

It will take some getting used to, but typically, in programming languages, all indices start from 0, not 1. In addition to accessing via index, you can also slice a part of a list out using an initial and final index in the form ‘list_name[initial:final]’. For example…

rainbow_colors = ["violet", "indigo", "blue", "green", "yellow", "orange", "red"]

colors = rainbow_colors[0:3]
print(colors)    

Output:

["violet","indigo","blue"]

Here, a part of the list is sliced and assigned to a new variable "colors" and it is printed out to the console.

Using For Loops

We can also use for loops to access and work with each value in a list. To write a for loop, use the following format…

for item_name in list_name:
	#code to run with each item_name

Here, "list_name" is replaced by the name of your list, and "item_name" should be replaced by any descriptive name you can think of to call each item in the list. You can then use that descriptive name as a variable inside the for loop to work with that value. For example…

rainbow_colors = ["violet", "indigo", "blue", "green", "yellow", "orange", "red"]
for color in rainbow_colors:
    print(color)    

Output:

violet
blue
indigo
blue
green
yellow
orange
red    

Here, we gave a random name to each item in the list, "color" and then used it like a variable, inside the for loop. So, each item in the "rainbow_colors" list is printed on a new line.

String Indexing and Slicing

Just like you index lists, you can also index a string to access a certain character or group of characters. To do so, just take the variable the string is assigned to, and access an index of it, like a list. You can also set a range of characters using ‘[starting index:finishing index]’. For example…

test_string = "I am happy today"
feeling = test_string[5:10]
person = test_string[0]
print("Feeling: " + feeling)
print("Person: " + person)

Output:

Feeling: happy
Person: I

Here, the word "happy" is sliced from the string "test_string" by slicing it like a list. The letter "I" is taken from the string "test_string" by accessing it using a list index.

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