Picking up an item from a list of lists

Question

I'm pretty new in python and have am difficulty doing the next assignment. I am receiving a list of lists with numbers and the word none, for example:

 [[1,None],[2,4],[1.5,2]]

My problem is when I go through None (I need to sum up the lists) I need to replace it by the max number in the same place in the other lists. So my result should be None = max(4,2) and receive :

 [[1,4],[2,4],[1.5,2]]

If I go through a for loop I don't understand how can I go to the other sub lists and check them out (especially when I don't know how many subs lists I have)

Answer 1

Use a nested list comprehension with conditional

>>> l =  [[1,None],[2,4],[1.5,2]]
>>> def findMax(j):
...     return max(i[j] for i in l)
... 
>>> [[j if j is not None else findMax(k) for k,j in enumerate(i)] for i in l]
[[1, 4], [2, 4], [1.5, 2]]

Here the list comprehension checks if each element is None or not. If not it will print the number, else it will fnd the maximum and print that element.

Another way using map is

>>> l =  [[1,None],[2,4],[1.5,2]]
>>> maxVal = max(map(max, l))
>>> [[j if j is not None else maxVal for k,j in enumerate(i)] for i in l]
[[1, 4], [2, 4], [1.5, 2]]

Answer 2

Here is a hint: In Python, a for in loop iterates through all the elements in some iterable. If you have a list of lists, that means each element in the list can also have a for loop applied to it, as in a for loop inside a for loop. You could use this if and only if the maximum depth of a list is 2:

def get_deep_max(deeplst):
   new = []
   for elem in deeplst:
      for num in elem:
         new.append(num)
   return max(new)

Try writing the code for replacing the none value yourself for practice.