A list generation formula for a two-dimensional list allows you to generate a list where each element itself is also a list. This is very useful when dealing with matrix or tabular data.
Here is an example of how to use list generation to create a two-dimensional list:
Basic syntax
[[expression for variable in iterable] for variable in iterable]
Here, the outer layer generates rows and the inner layer generates columns.
Example
1. Create a 3x3 unit matrix
identity_matrix = [[1 if i == j else 0 for i in range(3)] for j in range(3)] print(identity_matrix) # Output:# [[1, 0, 0], # [0, 1, 0], # [0, 0, 1]]
2. Create a 4x4 multiplication table
multiplication_table = [[i * j for i in range(1, 5)] for j in range(1, 5)] print(multiplication_table) # Output:# [[1, 2, 3, 4], # [2, 4, 6, 8], # [3, 6, 9, 12], # [4, 8, 12, 16]]
3. Create a 5x5 diagonal matrix (the elements on the diagonal are 1 and the rest are 0)
diagonal_matrix = [[1 if i == j or i + j == 4 else 0 for i in range(5)] for j in range(5)] print(diagonal_matrix) # Output:# [[1, 0, 0, 0, 1], # [0, 1, 0, 1, 0], # [0, 0, 1, 0, 0], # [0, 1, 0, 1, 0], # [1, 0, 0, 0, 1]]
Practical
# Create a 2D listlst = [
['City', 'Monopoly', 'Year-to-year'],
['Beijing', 102, 103],
['Shanghai', 104, 504],
['Shenzhen', 100, 39]
]
print(lst)
for row in lst:
for item in row:
print(item, end='\t')
print()
# List generation formula generates a 2D list of 4 rows and 5 columnslst2 = [[j for j in range(5)] for i in range(4)]
print(lst2)
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