A list is a row of data: one item after another, each with its own index. That is fine for the names of five players. It is not enough for their scores in four spelling tests, because that data has two directions. Down the page there is one line per student, and across the page there is one column per test.

Bob      5    9   10    7
Mary     4   10    3    6
James    7    6    4    8

Data shaped like a table is stored in a 2D array, where 2D stands for two dimensional: it has rows and it has columns.

Creating a 2D array

Each row is written as a list, in square brackets, and the rows are written inside one more pair of square brackets, with a comma between them:

animals = [["Salmon", "Pollock", "Cod"],
           ["Parrot", "Duck", "Wren"],
           ["Camel", "Lion", "Tiger"]]

That is one identifier, animals, holding three rows of three items: fish on the first row, birds on the second and land animals on the third.

Python does not mind the line breaks inside the brackets, so a 2D array is usually written one row per line. It is still one assignment, and laid out this way the code looks like the table it stores.

Every item has two indices

To pick out one item you need two numbers: which row it is on, and where it is on that row. Both count from zero, exactly like the index of an ordinary list.

The row index comes first and the column index comes second, each in its own pair of square brackets. The first number says how far down to go, the second how far across.

print(animals[1][2])

That prints Wren. Python reads it in two steps, left to right: animals[1] is the second row, ["Parrot", "Duck", "Wren"], and [2] then picks the third item from that row.

One index on its own is allowed too. It gives you a whole row, as a list:

print(animals[0])

['Salmon', 'Pollock', 'Cod']

Changing an item

An item is changed by assigning to it, just like an item in a list. The two indices say which one:

animals[0][2] = "Plaice"

That replaces Cod, the item on row 0 at column 2, with Plaice. Every other item stays where it is.

Adding an item to a row

Each row is a list, so it has append. Pick the row with one index, then append to it:

animals = [["Salmon", "Pollock", "Cod"],
           ["Parrot", "Duck", "Wren"],
           ["Camel", "Lion", "Tiger"]]

animals[0].append("Trout")

print(animals)

[['Salmon', 'Pollock', 'Cod', 'Trout'], ['Parrot', 'Duck', 'Wren'], ['Camel', 'Lion', 'Tiger']]

Trout goes on the end of row 0, so that row now has four items and the other two still have three. Nothing says every row has to be the same length.

Printing the whole array works the same as printing a whole list, and shows the list of lists for what it is: square brackets inside square brackets.

Adding items in a loop

A row can start out empty and be filled while the program runs. This program keeps asking for words and adds each one to the end of row 0, until the user says they have had enough:

words = [[], []]
words_left = True
while words_left:
    print("Enter a word")
    word = input()
    words[0].append(word)
    print("Would you like to enter another word? Y/N")
    answer = input().upper()
    if answer == "N":
        words_left = False

print(words)

A few things in it are worth a closer look.

• words = [[], []] is a 2D array with two rows and nothing on either of them yet. [] is an empty list, as in the Random numbers chapter.
• words_left holds a Boolean value, True or False, rather than a number or a piece of text: the two values every condition produces. A variable like this is called a Boolean variable, and one that a loop keeps checking is often called a flag. A while loop can check it directly: while words_left: keeps going for as long as it is True, just like while words_left == True:. The loop ends once the user answers N and the variable is set to False. True and False start with a capital letter and have no quotes around them.
• input().upper() uses .upper() from the String manipulation chapter, so n works as well as N.

Run it, enter three words, and look at the last line of output. The three words are all on the first row, and the second row is still empty.

Finding where an item is

index does the opposite of an index. Give it an item, and it tells you where that item is. Like append, it belongs to one row:

print("Which word would you like to find?")
word = input()
location = words[0].index(word)

If the user types a word that is the third one on row 0, location holds 2.

animals = [["Salmon", "Pollock", "Cod"],
           ["Parrot", "Duck", "Wren"],
           ["Camel", "Lion", "Tiger"]]

print(animals[1].index("Duck"))

That prints 1: Duck is at column 1 of row 1. Notice that you have to know which row to search. animals[1].index("Lion") does not look on the other rows.