python map()

A map is used to :

Apply function to every item of iterable and return a list of the results. [...] The iterable arguments may be a sequence or any iterable object; the result is always a list.

Let’s create a function that simply adds 1 to any number:

def addone(x): 
    return x+1

Let’s define a list of numbers. If we call the function addone() on every item of “numbers”, we get the following:

numbers = [0,1,2,3,4]
print map(addone, numbers)
>>> [1, 2, 3, 4, 5]

If we wanted, we could actually do this with a list comprehension – it’s just a little longer – something like this:

print [addone(x) for x in numbers]
>>> [1, 2, 3, 4, 5]

So map() is actually just an efficient shortcut for a loop that calls the function on every item:

added_numbers = []
for number in numbers:
print added_numbers
>>> [1, 2, 3, 4, 5]

But now our code is getting out of hand…Let’s try something else. We can pass multiple arguments to a map(), which have to all be of the same size. Otherwise they will be defaulted to None.

Let’s say we have a multiplier function:

def multiply(x, y): 
    return x*y

Now let’s create two lists of numbers to be multiplied together, and use map() to do so:

numbers = [0,1,2,3,4]
multipliers = [10, 20, 30, 40, 50]
print map(multiply, numbers, multipliers)
>>> [0, 20, 60, 120, 200]

Same result, with a list comprehension (slightly longer) :

print [multiply(numbers[x], multipliers[x]) for x in xrange(len(numbers))]
>>> [0, 20, 60, 120, 200]

And just to be fully ridiculous, with the complete loop :

multiplied_numbers = []
for number_index in xrange(len(numbers)):
    multiplied_numbers.append(multiply(numbers[number_index], multipliers[number_index]))
print multiplied_numbers
>>> [0, 20, 60, 120, 200]

But really, don’t do this. Just use map()!