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== 9.5#h(0.6em)List comprehensions


  
    
=== Learning objectives


    By the end of this section you should be able to


    - Identify the different components of a list comprehension statement.
- Implement filtering using list comprehension.


  
  
    
=== List comprehensions


    A #strong[list comprehension] is a Python statement to compactly create a new list using a pattern.


    The general form of a list comprehension statement is shown below.


    list\_name = \[expression for loop\_variable in iterable\]


    list\_name refers to the name of a new list, which can be anything, and the for is the for loop keyword. An expression defines what will become part of the new list. loop\_variable is an iterator, and iterable is an object that can be iterated, such as a list or string.


    #examplebox("Example 1")[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.
]


    #examplebox("Example 2")[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.


      
    \# Create a list of words
    words\_list = \["one", "two", "red", "blue"\]
    
    \# Use a list comprehension to create a new list called poem\_lines
    \# Inserting the word "fish" attached to each word in words\_list
    poem\_lines = \[w + " fish" for w in words\_list\]
    for line in poem\_lines:
      print(line)
      The above code's output is:


      
    one fish
    two fish
    red fish
    blue fish
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[List comprehensions]
]


  
  
    
=== Filtering using list comprehensions


    List comprehensions can be used to filter items from a given list. A #emph[condition] is added to the list comprehension.


    list\_name = \[expression for loop\_variable in container if condition\]


    In a filter list comprehension, an element is added into list\_name only if the condition is met.


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Filtering a list]


      #link("https://www.openstax.org/r/filtering-list")[Filtering a list; ch 9, video 5]
]


    #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Filtering using list comprehensions]


      For each code using list comprehension, select the correct resultant list in new\_list.
]


  
  #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Selecting five-letter words]


    Write a program that creates a list of only five-letter words from the given list and prints the new list.


    \# A list
word\_list = \["notebook", "capsule", "ruler", "divider", "compass", "chart"\]
]


  #notebox("Note", rgb("#8a94a6"), rgb("#556666"), rgb("#f7f8fa"))[
#emph[Books starting with "A"]


    Write a program that selects words that begin with an "A" in the given list. Make sure the new list is then sorted in dictionary order. Finally, print the new sorted list.


    \# A list of book titles
book\_list = \["Pachinko", "A Hitchhiker's Guide to the Galaxy", "Moby Dick", "Beloved", "A Farewell to Arms", "Anna Karenina", "The Golden Notebook"\]
]