Question

I have a variable number of lists and I wanted to find the maximum of each of these numbers from all the lists.

I’m trying to use the map function to map the maximum of all the lists and I need to use the generator expression to pass the parameters in the map function.

I’m using something like this but it does not work:

map(max,x[c] for c in x.keys())

As an example my lists are in a dictionary:

{1: [0, 1, 0, 0, 3, 0],
 2: [1, 0, 0, 0, 0, 4, 5],
 3: [0, 5, 6, 0, 1, 1]}

You probably noticed that the lists are not all of equal sizes and that’s how I want them to be because of the way I have the data that I need to parse.

The result of this should be: [1, 5, 6, 0, 3, 4, 5]

Please do suggest if there are other ways to doing this.

Above is an example of the list. I cannot use x.values() to retrieve the lists because of the way my actual dictionary is set up. Lists are embedded at the bottom of hierarchy and I need to iterate over the top of hierarchy to retrieve the lists to compare. I must use a generator expression to retrieve all the lists so please provide solutions without the use of x.values()

Answer 1

To get the maxs past the end of the shortest value, you need izip_longest:

from itertools import izip_longest

d={1: [0, 1, 0, 0, 3, 0],
 2: [1, 0, 0, 0, 0, 4, 5],
 3: [0, 5, 6, 0, 1, 1]}

# there are other variations like
# [max(column) for column in izip_longest(*d.itervalues())]
print map(max, izip_longest(*d.values()))
# [1, 5, 6, 0, 3, 4, 5]

It’s known as zip_longest on Python 3.

---

This is still the way to do it even if you don’t have a simple method to access all of the lists. You can get the lists however you want:

d = {1: {'a': [0, 1, 0, 0, 3, 0]},
     2: {'a': [1, 0, 0, 0, 0, 4, 5]},
     3: {'a': [0, 5, 6, 0, 1, 1]}}

def value_gen(obj):
    for key in obj:
        yield obj[key]['a']

Then use:

map(max, izip_longest(*value_gen(d)))