5.12 List Comprehensions

  • Concise way to create new lists.
  • Replaces using for to iterate over a sequence and create a list.
In [1]:
list1 = []
In [2]:
for item in range(1, 6):
    list1.append(item)
In [3]:
list1
Out[3]:
[1, 2, 3, 4, 5]

Using a List Comprehension to Create a List of Integers

In [4]:
list2 = [item for item in range(1, 6)]
In [5]:
list2
Out[5]:
[1, 2, 3, 4, 5]
  • for clause iterates over the sequence produced by range(1, 6).
  • For each item, the list comprehension evaluates the expression to the left of the for clause and places the expression’s value in the new list.

Mapping: Performing Operations in a List Comprehension’s Expression

  • Mapping is a common functional-style programming operation that produces a result with the same number of elements as the original data being mapped.
In [6]:
list3 = [item ** 3 for item in range(1, 6)]
In [7]:
list3
Out[7]:
[1, 8, 27, 64, 125]

Filtering: List Comprehensions with if Clauses

  • Another common functional-style programming operation is filtering elements to select only those that match a condition.
  • Typically produces a list with fewer elements than the data being filtered.
In [8]:
list4 = [item for item in range(1, 11) if item % 2 == 0]
In [9]:
list4
Out[9]:
[2, 4, 6, 8, 10]

List Comprehension That Processes Another List’s Elements

  • The for clause can process any iterable.
In [10]:
colors = ['red', 'orange', 'yellow', 'green', 'blue']
In [11]:
colors2 = [item.upper() for item in colors]
In [12]:
colors2
Out[12]:
['RED', 'ORANGE', 'YELLOW', 'GREEN', 'BLUE']
In [13]:
colors
Out[13]:
['red', 'orange', 'yellow', 'green', 'blue']

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