Python map function with multiple arguments to a list of tuples

Question

The heading may sound bizarre but here is what I mean:

def f(x, y, z):
    return a_single_number_from_xyz
l = [(10, 'abc', 'def'), (20, 'efg', 'hij')]
print sum([ret_value_of_f_from_first_tuple, ret_value_of_f_from_second_tuple])

The three arguments of function f is the three elements of each of the tuple. Now, I want to apply function f to every tuple of list l and want to get the sum of those single numbers. How do I do that in a single statement? How do I map function and use list comprehension together here?

But in general, you are looking for `itertools.starmap`, so `list(itertools.starmap(f, iterable))` which is equivalent to `[f(*xs) for xs in iterable]` — juanpa.arrivillaga, Oct 31 '17 at 22:55
Is all you need the sum of those numbers? means is using map a requirement here? — ettanany, Oct 31 '17 at 22:56
i meant that there would be some processing with those three parameters. then a single value would be returned. — QuestionEverything, Oct 31 '17 at 22:57

juanpa.arrivillaga · Accepted Answer · 2017-10-31T23:04:18.740

11

In this particular case, I think you just want:

sum(n for n,_,_ in l)

But in general, you are looking for itertools.starmap, so

list(itertools.starmap(f, iterable))

is equivalent to

[f(*xs) for xs in iterable]

Hence the "star" map. Normally, I would just do:

sum(f(*xs) for xs in iterable)

For the general case, although:

sum(itertools.starmap(f, iterable))

Is similarly elegant to me.

edited Oct 31 '17 at 23:04

answered Oct 31 '17 at 22:57

juanpa.arrivillaga

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score 6 · Answer 2 · answered Oct 31 '17 at 23:01

6

First let's make that an actual working function using all values, for example:

>>> def f(x, y, z):
        return x + len(y) + len(z)

Now your example data:

>>> l = [(10, 'abc', 'def'), (20, 'efg', 'hij')]

And one way to do it, giving map three iterables (one for each "column"):

>>> sum(map(f, *zip(*l)))
42

Lol. Didn't see that coming.

answered Oct 31 '17 at 23:01

Stefan Pochmann

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yours and @juanpa.arrivillaga 's answer are both elegant and brilliant! however, i'm accepting his answer as it was posted moments earlier. – QuestionEverything Oct 31 '17 at 23:08

score 1 · Answer 3 · answered Oct 31 '17 at 22:57

1

You can try this:

def f(*args):
    return sum(map(lambda x:x[0], args))

l = [(10, 'abc', 'def'), (20, 'efg', 'hij'), (30, "klm", "nop")]
print(f(*l))

Output:

answered Oct 31 '17 at 22:57

Ajax1234

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i wanted to include other parameters into the calculation too... – QuestionEverything Oct 31 '17 at 23:15

score 1 · Answer 4 · answered Oct 31 '17 at 22:59

1

If what you need is just the sum, you can get it with this one-line code:

s = sum(t[0] for t in l)

Output:

>>> l = [(10, 'abc', 'def'), (20, 'efg', 'hij')]
>>> s = sum(t[0] for t in l)
>>> s
30

answered Oct 31 '17 at 22:59

ettanany

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Python map function with multiple arguments to a list of tuples

4 Answers4