5.14 Filter, Map and Reduce

• Built-in filter and map functions also perform filtering and mapping.

Filtering a Sequence’s Values with the Built-In filter Function

numbers = [10, 3, 7, 1, 9, 4, 2, 8, 5, 6]

def is_odd(x):
    """Returns True only if x is odd."""
    return x % 2 != 0

list(filter(is_odd, numbers))

[3, 7, 1, 9, 5]

• Functions are objects that you can assign to variables, pass to other functions and return from functions.
• Functions that receive other functions as arguments are a functional-style capability called higher-order functions.
• filter’s first argument must be a function that receives one argument and returns True if the value should be included in the result.
• Higher-order functions may also return a function as a result.
• filter returns an iterator, so filter’s results are not produced until you iterate through them—lazy evaluation.

[item for item in numbers if is_odd(item)]

[3, 7, 1, 9, 5]

Using a lambda Rather than a Function

• For simple functions like is_odd that return only a single expression’s value, you can use a lambda expression (or simply a lambda) to define the function inline.

list(filter(lambda x: x % 2 != 0, numbers))

[3, 7, 1, 9, 5]

• A lambda expression is an _anonymous function
• Begins with the lambda keyword followed by a comma-separated parameter list, a colon (:) and an expression.
• A lambda implicitly returns its expression’s value.

Mapping a Sequence’s Values to New Values

numbers

[10, 3, 7, 1, 9, 4, 2, 8, 5, 6]

list(map(lambda x: x ** 2, numbers))

[100, 9, 49, 1, 81, 16, 4, 64, 25, 36]

• Function map’s first argument is a function that receives one value and returns a new value.
• equivalent list comprehension:

[item ** 2 for item in numbers]

[100, 9, 49, 1, 81, 16, 4, 64, 25, 36]

Combining filter and map

list(map(lambda x: x ** 2, 
         filter(lambda x: x % 2 != 0, numbers)))

[9, 49, 1, 81, 25]

• Equivalent list comprehension:

[x ** 2 for x in numbers if x % 2 != 0]

[9, 49, 1, 81, 25]

Reduction: Totaling the Elements of a Sequence with sum

• Reductions process a sequence’s elements into a single value.
  • E.g., len, sum, min and max.
• Can create custom reductions using the functools module’s reduce function.

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