在列表列表中查找偶数python

Question

我想如何从列表列表中查找偶数。
示例是：

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

应该返回（最大偶数）：

[2,6,0]

我发现如何从列表中从stackoverflow查找偶数：

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

但我想知道如何做到这一点，以便它可以在列表中的每个列表中找到偶数。

Answer 1

尽管您没有明确说出来，但似乎您正在列表列表中的每个列表中寻找最大偶数。 从您的示例看来，如果没有偶数，您似乎希望程序加0。 在这种情况下，它将起作用：

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

Answer 2

您可以使用一些不错的Python语言构造从内到外构建此算法：

# 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]])

这将打印出：

[2, 6, None]

Answer 3

python 3.4采用max的默认值，因此如果在过滤后没有偶数，我们可以返回0作为默认值：

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

你快到了！ :-)

检查内置的max函数的作用。

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

我添加了一些print语句，以帮助遵循代码的作用。 上面的代码将输出：

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

据我了解，您想遍历列表列表 ，从每个列表中获取最大的偶数，如果找不到偶数则为0。 然后是您的功能：

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]

确保阅读有关内置最大功能的信息:)

Answer 6

干得好：

>>> [ 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]

我使用lambda检查偶数，然后过滤以将值保存为true，如果列表长度大于0则为max，否则为max