5.16 Two-Dimensional Lists¶

Lists can contain other lists as elements.
Typical use is to represent tables of values consisting of information arranged in rows and columns.
To identify a particular table element, we specify two indices—the first identifies the element’s row, the second the element’s column.

Creating a Two-Dimensional List¶

a = [[77, 68, 86, 73], [96, 87, 89, 81], [70, 90, 86, 81]]

Writing the list as follows makes its row and column tabular structure clearer:

a = [[77, 68, 86, 73],  # first student's grades
     [96, 87, 89, 81],  # second student's grades 
     [70, 90, 86, 81]]  # third student's grades

Illustrating a Two-Dimensional List¶

The two-dimensional list 'a' with its rows and columns of exam grade values

Identifying the Elements in a Two-Dimensional List¶

The two-dimensional list 'a' labeled with the names of its elements

Output the rows of the preceding two-dimensional list.

for row in a:
    for item in row:
        print(item, end=' ')
    print()

77 68 86 73 
96 87 89 81 
70 90 86 81

How the Nested Loops Execute¶

for i, row in enumerate(a):
    for j, item in enumerate(row):
        print(f'a[{i}][{j}]={item} ', end=' ')
    print()

a[0][0]=77  a[0][1]=68  a[0][2]=86  a[0][3]=73  
a[1][0]=96  a[1][1]=87  a[1][2]=89  a[1][3]=81  
a[2][0]=70  a[2][1]=90  a[2][2]=86  a[2][3]=81

Outer for statement iterates over the list’s ows one row at a time.
During each iteration of the outer for statement, the inner for statement iterates over each column in the current row.

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