Python map, filter and lambda

Links: 108 Python Index

---

lambda

• They are generally used in map and filter functions.
• lambda functions are single line functions defined without a name.
• Syntax: lambda arg1, arg2, ... : arg1 + arg2 + ... - any action on arguments that can be done in one line and will be returned.

• Using lambda without filter or map doesn't make sense since we can use def for defining functions.

  • 

• Sorting using lambda functions

some_dict = {"key1": 10, "key2": 5, "key3": 22}

sorted_dict = sorted(some_dict.items(), key=lambda item: item[1])
# using item[1] sorts the dictionary based on values
# using item[0] would have sorted the dictionary based on keys
print(dict(sorted_dict))

filter

• It takes 2 arguments: a function and an iterable.
  • It then applies the function on each item of the iterable.
  • The function should return a boolean value.
  • Filter filters out the items for which the function returns True.
  • If it returns some kind of number or string, it will always be considered as a truthy value since numbers represent True (except 0) in python.

filter returns a filter object which is iterable.

We can either iterate over it using a loop or get its values using list(filter(...))

def filter_func(item):
    if item % 2 == 0:
        return True
    return False


numbers = list(range(1, 10))

x = filter(filter_func, numbers)
print(list(x))

# or

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

# Output:
# 2 4 6 8

• Note that we just pass the reference to the function.

map

• It takes a function and one or more iterables.
  • map will apply the function on each iterable passed to it.
  • For passing multiple iterables the function should accept multiple items.
• map also returns a map object which is iterable.

numbers = list(range(1, 10))

x = map(lambda item: item**2, numbers)
print(list(x))

# Output:
# [1, 4, 9, 16, 25, 36, 49, 64, 81]

• Multiple iterables

  numbers = list(range(1, 10))
  
  x = map(lambda item1, item2: item1**2 + item2 // 2, numbers, numbers)
  print(list(x))
  

---

Last updated: 2022-12-19