find the sublist whose first element is the maximum that is no greater than a given number

Question

I have a list of sublists, and the first element of each sublist is a number. I would like to find the sublist whose first element is the maximum that is no greater than a given number. I wonder how to implement that?

For example, I would like to find the sublist in list a , so that its first element is the maximum one which is no greater than 3 . The sublist is [2,'b'] .

>>> a=[[5,'d'] ,[1,'a'],[4,'c'],[2,'b'] ]
>>> a = sorted(a)
>>> a
[[1, 'a'], [2, 'b'], [4, 'c'], [5, 'd']]
>>> [3>=x for [x,_] in a]
[True, True, False, False]
>>> a[1]
[2, 'b']

Thanks and regards!

Answer 1

>>> a=[[5,'d'] ,[1,'a'],[4,'c'],[2,'b'] ]
>>> max(filter(lambda sl: sl[0]<3, a), key=lambda sl: sl[0])
[2, 'b']

Break that down:

1) Produce the sub list of the list of lists that meets the criteria of sl[0]<3 using filter :

>>> filter(lambda sl: sl[0]<3, a)
[[1, 'a'], [2, 'b']]

1.a) You could use a list comprehension also:

>>> [sl for sl in a if sl[0]<3]
[[1, 'a'], [2, 'b']]

2) Then find the max of that subset list using a key function:

>>> max([[1, 'a'], [2, 'b']], key=lambda sl: sl[0])
[2, 'b']

3) Combine -- one line -- no sort -- be happy...

Answer 2

def grab_max_pair(lst_of_pairs, num):
    result = None
    for pair in lst_of_pairs:
        if result and pair[0] <= num:
            if pair[0] > result[0]:
                result = pair
        elif pair[0] <= 3:
            result = pair
    return result

a=[[5,'d'] ,[1,'a'],[4,'c'],[2,'b'] ]    
print grab_max_pair(a, 3)  # prints [2,b]

Answer 3

You could use a list comprehension with something like:

a = # define your list here
new_list = [list for list in a if list[0] < 4]  # only grab sub-lists that meet your criterion
new_list = sorted(new_list)  # sort them now (shorter list)
result = new_list[0]  # grab the first result

You could throw this all into a function if it is something that you do a lot:

def get_first(my_list, criterion=4):
    new_list = [list for list in my_list if list[0] < criterion]
    new_list = sorted(new_list)
    return new_list[0] if new_list is not None else None  # avoid a crash if new_list[0] does not have meaning

You could then call this (with or without a value for criterion, default is 4) from Python after importing whatever module you have placed it inside, or after defining it in your environment:

>> my_list = # define your list here
>> smallest_match = get_first(my_list)