local maximum in lists within list

Question

I have an optimization problem of trying to use less variety of products.

For example:

screws = [6,8,16,18,39]

I would like to exchange those 5 screws with 3. So I need to use the strongest so I could select, [8,18,39]. Using any other option will cause a waste—for example, screw 16 is too strong for location of 6, so [16,18,39] is not as good. I would like to write an algorithm that would be useful for a larger number of parts also. So far I tried this:

def split_list(data, n):
    from itertools import combinations, chain
    for splits in combinations(range(1, len(data)), n-1):
        result = []
        prev = None
        for split in chain(splits, [None]):
            result.append(data[prev:split])
            prev = split
        yield result
new_list = list(split_list(screws, 3))       
#print list(split_list(screws, 3))

As a result of running this code I got a list of lists:

[[[6], [8], [16, 18, 39]], 
 [[6], [8, 16], [18, 39]], 
 [[6], [8, 16, 18], [39]], 
 [[6, 8], [16], [18, 39]], 
 [[6, 8], [16, 18], [39]], 
 [[6, 8, 16], [18], [39]]]

I would like to find out local maximums from all the list. For example in the first list [[6], [8],[16, 18, 39]], maximum = 6, maximum = 8, maximum = 39, and so on. But I don't know how to. Is there a way to find local maximum of all nested lists? I'm stuck in this moment, could you help me? I would appreciate also help with further progress.

Later on, I would like to check the sum of differences between maximum and other elements in the same list. So, 6-6 = 0, 8-8 = 0, and last 39-16+30-18-39-39 = 35. This will allow me to find out the smallest value from all lists of lists. This will be the most optimal solution. So the final result should be [[6, 8], [16, 18], [39]] and from it I would select [8,18,39].

This is basically my first program after tutorials and online classes, so all help is very welcome.

Answer 1

You have a list of lists of lists so just iterate over the list then get the max of each of the sublists inside the sublists.

l = [[[6], [8], [16, 18, 39]],
 [[6], [8, 16], [18, 39]],
 [[6], [8, 16, 18], [39]],
 [[6, 8], [16], [18, 39]],
 [[6, 8], [16, 18], [39]],
 [[6, 8, 16], [18], [39]]]
for sub in l: # each sublist in l -> [[6], [8], [16, 18, 39]]etc..
    print([max(ele) for ele in sub]) # each sublist inside each sublist -> [6], [8], [16, 18, 39]

[6, 8, 39]
[6, 16, 39]
[6, 18, 39]
[8, 16, 39]
[8, 18, 39]
[16, 18, 39]

So in your code just do the following:

for sub in split_list(screws, 3):
    print([max(ele) for ele in sub])

To get the max minus each element you will have a lot of nested loops:

l = [[[6], [8], [16, 18, 39]],
 [[6], [8, 16], [18, 39]],
 [[6], [8, 16, 18], [39]],
 [[6, 8], [16], [18, 39]],
 [[6, 8], [16, 18], [39]],
 [[6, 8, 16], [18], [39]]]

result = []
for sub in l:
    for sub_ele in sub:
        mx = max(sub_ele)
        result.append([mx]+map(lambda x: mx-x,sub_ele))


[[6, 0], [8, 0], [39, 23, 21, 0], [6, 0], [16, 8, 0], [39, 21, 0], [6, 0], [18, 10, 2, 0], [39, 0], [8, 2, 0], [16, 0], [39, 21, 0], [8, 2, 0], [18, 2, 0], [39, 0], [16, 10, 8, 0], [18, 0], [39, 0]]