Python map() function
map() function in Python applies a given function to each element of an iterable (list, tuple, set, etc.) and returns a map object (iterator). It is a higher-order function used for uniform element-wise transformations, enabling concise and efficient code.
Let's start with a simple example of using map() to convert a list of strings into a list of integers.
s = ['1', '2', '3', '4']
res = map(int, s)
print(list(res))
Output
[1, 2, 3, 4]
Explanation: map() applies int() to each element in s which changes their datatype from string to int.
Syntax
map(function, iterable,..)
Parameters:
- function: The function to apply to every element of the iterable.
- iterable: One or more iterable objects (list, tuple, etc.) whose elements will be processed.
Note: You can pass multiple iterables if the function accepts multiple arguments.
Converting map object to a list
By default, map() function returns a map object, which is an iterator. In many cases, we will need to convert this iterator to a list to work with the results directly.
Example: Let's see how to double each elements of the given list.
# Custom function to be applied
# in map
def double(val):
return val * 2
# Let us apply double on every member
a = [1, 2, 3, 4]
res = list(map(double, a))
print(res)
Output
[2, 4, 6, 8]
Explanation:
- map(double, a) applies double() to each element in a.
- list() converts the map object to a list.
map() with lambda
We can use a lambda function instead of a custom function with map() to make code shorter and easier. Let's see how to improve above code for better readability.
a = [1, 2, 3, 4]
res = list(map(lambda x: x ** 2, a))
print(res)
Output
[1, 4, 9, 16]
Explanation: lambda x: x ** 2 squares each number and the results are converted into a list.
map() with multiple iterables
We can use map() with multiple iterables if the function we are applying takes more than one argument.
Example: In this example, map() takes two iterables (a and b) and applies lambda function to add corresponding elements from both lists.
a = [1, 2, 3]
b = [4, 5, 6]
res = map(lambda x, y: x + y, a, b)
print(list(res))
Output
[5, 7, 9]
Explanation: map() takes x from a and y from b and adds them.
Converting strings to Uppercase
This example shows how we can use map() to convert a list of strings to uppercase.
fruits = ['apple', 'banana', 'cherry']
res = map(str.upper, fruits)
print(list(res))
Output
['APPLE', 'BANANA', 'CHERRY']
Explanation: str.upper method is applied to each element in the list fruits using map(). The result is a list of uppercase versions of each fruit name.
Extracting first character from strings
In this example, we use map() to extract the first character from each string in a list.
words = ['apple', 'banana', 'cherry']
res = map(lambda s: s[0], words)
print(list(res))
Output
['a', 'b', 'c']
Explanation: lambda s: s[0] extracts first character from each string in the list words. map() applies this lambda function to every element, resulting in a list of the first characters of each word.
Removing whitespaces from strings
In this example, We can use map() to remove leading and trailing whitespaces from each string in a list.
s = [' hello ', ' world ', ' python ']
res = map(str.strip, s)
print(list(res))
Output
['hello', 'world', 'python']
Explanation: str.strip method removes leading and trailing whitespaces from each string in list strings. map() function applies str.strip to each element and returning a list of trimmed strings.
Calculate fahrenheit from celsius
In this example, we use map() to convert a list of temperatures from Celsius to Fahrenheit.
celsius = [0, 20, 37, 100]
fahrenheit = map(lambda c: (c * 9/5) + 32, celsius)
print(list(fahrenheit))
Output
[32.0, 68.0, 98.6, 212.0]
Explanation: lambda c: (c * 9/5) + 32 converts each Celsius temperature to Fahrenheit using standard formula. map() function applies this transformation to all items in the list celsius.
