Finding even numbers in lists of lists python

Question

I want to how to find even numbers from lists of lists.
The example was:

maximum_even_each_list_in_lol([[1,2,3],[6,5,4],[5,7,9]])

should return (maximum even numbers):

[2,6,0]

I found how to find even numbers from list, from stackoverflow:

def maximum_even_each_list_in_lol (lol):
    evens=[]
    for number in lol:
        if is_even(number):
            evens.append(number)
    return evens

but I want to know how I can make it so that it can find even numbers from each list, in list.

Answer 1

Though you don't say it explicitly, it seems you are looking for the maximum even number in each list within a list of lists. And from your example it seems you want the programme to add 0 if there is no even number. In that case this will work:

print [max(value for value in [0] + L if not value % 2) for L in list_of_lists]

Answer 2

You can build this algorithm up from the inside-out using some nice Pythonic language constructs:

# Function to determine if one number is even
is_even = lambda x : x%2==0

# Function to return just the even numbers of a list
just_evens = lambda x : filter(is_even, x)

# Function to return the maximum number in a list, or '0' if there is none
get_max = lambda x : max(x) if len(x) else 0

# Function to return the maximum even number of a list
max_even = lambda x : get_max(just_evens(x))

# Print out the maximum even number from each list:
map(max_even, [[1,2,3],[6,5,4],[5,7,9]])

This will print out:

[2, 6, None]

Answer 3

python 3.4 takes a default value for max so if after filtering there are no even numbers we can return 0 as a default value:

l = [[1,2,3],[6,5,4],[5,7,9]]
print([max(filter(lambda x: not x % 2,x),default=0) for x in l])
[2, 6, 0]

Answer 4

You're almost there! :-)

Check what the builtin max function does.

def maximum_even_each_list_in_lol (lol):
    evens=[0]  # Dirty trick to ensure that regardless of the `even` numbers
               # found, you'll always have a zero among the results.
    for number in lol:
        if is_even(number):
            evens.append(number)
    print "I found these even numbers (%s) among %s" % (evens, lol)
    return max(evens)  # Return the maximum value among the `evens` list.
results = []
for lst in [[1,2,3],[6,5,4],[5,7,9]]:
    print "Studying %s" % lst
    results.append(maximum_even_each_list_in_lol(lst))
print "This is what I found: %s" % results

I added some print statements to help follow what the code is doing. The code above will output:

Studying [1, 2, 3]
I found these even numbers ([0, 2]) among [1, 2, 3]
Studying [6, 5, 4]
I found these even numbers ([0, 6, 4]) among [6, 5, 4]
Studying [5, 7, 9]
I found these even numbers ([0]) among [5, 7, 9]
This is what I found: [2, 6, 0]

Answer 5

As I understand you want to iterate over a list of lists , grabbing biggest even number from each list or 0 if not even numbers were found. Here is your function then:

def maximum_even_each_list_in_lol(lol):
    evens = []
    for sublist in lol:
        subevens = []
        for number in sublist:
            if (not number % 2):
                subevens.append(number)
        # As I understand you want to insert 0
        # if no even numbers were found, right?
        if not subevens:
            subevens = [0]
        evens.append(max(subevens))
    return evens

# Testing
assert maximum_even_each_list_in_lol([[1,2,3],[6,5,4],[5,7,9]]) == [2,6,0]

Make sure to read about builtin max function :)

Answer 6

here you go:

>>> [ max(x) if len(x)>0 else 0 for x in [ list(filter(lambda x:x%2==0,x)) for x in a ]]
[2, 6, 0]

i used lambda to check for even number, then filter to save value for true, then max if lenght of list greater then 0 else 0