9.5 List comprehensions

Creating a new list with a list comprehension

A list comprehension shown below in the second code has the same effect as the regular for loop shown in the first code. The resultant list is [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] in both cases.

Creating a list of squares using a for loop.

    # Create an empty List.
    squares_list = []

    # Add items to a list, as squares of numbers starting at 0 and ending at 9.
    for i in range(10):
      squares_list.append(i*i)
    

Creating a list of squares using the list comprehension.

    square_list = [i*i for i in range(10)]
    

The expression i*i is applied for each value of the loop_variable i.

A Dr. Seuss poem

A list comprehension can be used to create a list based on another list. In line 6, the for loop is written on the list poem_lines.

1
# Create a list of words
2
    words_list = ["one", "two", "red", "blue"]
3
    
4
    # Use a list comprehension to create a new list called poem_lines
5
    # Inserting the word "fish" attached to each word in words_list
6
    poem_lines = [w + " fish" for w in words_list]
7
    for line in poem_lines:
8
      print(line)
    
The above code's output is:
    
    one fish
    two fish
    red fish
    blue fish