Two-Dimensional List

 Two-Dimensional List

Pay close attention, folks, because 2D lists are basically tables.
Tables are two-dimensional data structures where we can store data both vertically and horizontally.
Usually this means that vertical data is used for fields (one category - name, ID, favorite biscuit, etc.) and horizontal data is used for records (all the data for each category).
Behind the scenes, we see a list inside a list. Forget what you know about reading a table with math or geography: 'across the corridor (x-axis) first and then down the stairs (y-axis)'.

Here, we will do row index first and then the column index.

Remember...

Here's a 1D list. We have the list name as a variable, single equals to set the value, and sqaure brackets to show this is a list. 2D lists are very similar.

my1DList = ["Johnny", 21, "Mac"]

Adding The Second Dimension

To add the second dimension, we put lists inside the first list.

Each new list has its own set of square brackets and is separated by a comma. This layout of code is nice to help us visualise the 2D list as a table, but...

my2DList = [ ["Johnny", 21, "Mac"],

                       ["Sian", 19, "PC"],

                       ["Gethin", 17, "PC"] ]

...you can also lay it out like this below:

my2DList = [ ["Johnny", 21, "Mac"], ["Sian", 19, "PC"], ["Gethin", 17, "PC"] ]

Now, let's print this list.

Printing From a 2D List

Remember, any comments about the code are written in green like this:

the entire list

We can print an entire 2D list just like we do with a 1D list. However, this will output (print) all of the square brackets, commas, etc.

print(my2DList)

a single row
To print a single row, use a single square bracket [] in the print command. However, you will still get all of those square brackets and commas.

In this example, I'm outputting the first row (index 0) - all of the data about Johnny.

print(my2DList[0])

a single piece of data

The first square bracket references the list, while the second references the item inside that list.

Here are a couple of examples:

my2DList = [ ["Johnny", 21, "Mac"],

             ["Sian", 19, "PC"],

             ["Gethin", 17, "PC"] ]

print(my2DList[0][0])

# This code outputs 'Johnny'. It's Johnny's name from list 0 (first square bracket), item 0 (second square bracket)

print(my2DList[1][2])

# This code outputs 'PC'. It's Sian's computing preferene from list 1 (first square bracket), item 2 (second square bracket)

Editing a 2D List

We can edit values in a 2D list in the same way as variables and 1D lists. You just need to change the value to the new row and column index numbers.
In this example, Sian has joined the dark side, so we're updating her computing preference to Linux.

my2DList = [ ["Johnny", 21, "Mac"],

             ["Sian", 19, "PC"],

             ["Gethin", 17, "PC"] ]

my2DList[1][2] = "Linux"

# The line above changes list 1, item 2 from PC to Linux

print(my2DList[1])

# I'm using this line to output list 1 to check that the change has happened correctly.

Coding Challenge - 

Randomly generate a series of number between 0 and 90.
Allocate each number to a place in a 2D list.
The numbers should be in numerical order, left to right.
Numbers should not be repeated.
The center square should not contain a number. It should contain the word 'BINGO!'.

import random

bingo = []

def ran():

  number = random.randint(1,90)

  return number

def prettyPrint():

  for row in bingo:

    print(row)

numbers = []

for i in range(8):

  numbers.append(ran())

numbers.sort()

bingo = [ [ numbers[0], numbers[1], numbers[2]],

          [ numbers[3], "BINGO", numbers[4] ],

          [ numbers [5], numbers[6], numbers[7]]

        ]

prettyPrint()