Meet map()

Same rule, one item at a time.

Want to double every number in a list? map() applies a function to each item. Think of a function as a reusable rule.

1× 22

2× 24

3× 26

4× 28

4 items in. 4 results out. Same order.

Each item gets its own result. In this example, the original list stays [1, 2, 3, 4].

A function to use, and items to use it on.

list(map(double, numbers))

doubleThe function: The rule for one item. Here, multiply a number by 2.
numbersThe items: The list to work through, one number at a time.
list(...)Collect the results: map() produces results when asked. list() asks for them all and collects them into a list you can see and use.

Use double, without (). You’re handing map the rule; it calls double(1), double(2), and so on for you.

Define the rule, choose your list, then collect the results.

Choose a rule

PythonMultiply each number by 2

def double(x):
    return x * 2

numbers = [1, 2, 3, 4]
result = list(map(double, numbers))
print(result)

Output[2, 4, 6, 8]

Change the rule or numbers to see the code and output update.

New to writing functions?

def double(x): names the rule double. x stands for one number. return x * 2 sends back that number multiplied by 2.

The pattern to remember

list(map(function, items))

Apply the function to each item. Collect the results.

Explore when results are produced and how inputs pair up.