Python Programming

We now want to apply some mathematical operation to every number in the list of numbers.

Using list comprehension, we might do it this way.

>>> numbers = [1, 3, 5, 7, 9]
>>> transformed = [3*x+5 for x in numbers]
>>> print(transformed)
[8, 14, 20, 26, 32]

The map solution

Using map(), it gets a bit messy because we might have to define the function first.

>>> def transform(x):
...    return 3*x + 5
...
>>> numbers = [1, 3, 5, 7, 9]
>>> transformed = list(map(transform, numbers))
>>> print(transformed)
[8, 14, 20, 26, 32]

>>> numbers = [1, 3, 5, 7, 9]
>>> transformed = list(map(lambda x: 3*x+5, numbers))
>>> print(transformed)
[8, 14, 20, 26, 32]

Another example: perhaps we want to Capitalize each word in the list, and also repeat them?

>>> greetings = ["hello", "bonjour", "hola", "ciao"]
>>> transformed = list(map(lambda x: x.capitalize()*2, greetings))
>>> print(transformed)
['HelloHello', 'BonjourBonjour', 'HolaHola', 'CiaoCiao']

This is the same as the following using list comprehension.

transformed = [greeting.capitalize()*2 for greeting in greetings]

Lambda functions are useful in these examples as they are simple expressions. Without lambda functions, you will have to explicitly define a function (e.g. capitalize_and_repeat). So in some cases, lambda functions will make the code more readable as the expression is in a single place.

Exercise

Use map() and lambda functions to convert a list of temperatures (in Celsius) to Fahrenheit.

The formula is $F=C\times\frac{9}{5}+32$, where $F$ is the temperature in Fahrenheit, and $C$ the temperature in Celsius.

As usual, feel free to also implement a list comprehension version as a comparison.

c_temperatures = [7, 50, 12, 22, 30]

f_temperatures = ????

assert f_temperatures == [44.6, 122.0, 53.6, 71.6, 86.0]

👀 Peek at solution

c_temperatures = [7, 50, 12, 22, 30]

f_temperatures = list(map(lambda c: c*9/5+32, c_temperatures))

assert f_temperatures == [44.6, 122.0, 53.6, 71.6, 86.0]

c_temperatures = [7, 50, 12, 22, 30]

f_temperatures = [temp*9/5 + 32 for temp in c_temperatures]

assert f_temperatures == [44.6, 122.0, 53.6, 71.6, 86.0]