Adding items to nested lists-CodePudding

I am new to python and struggling with adding values to nested lists. I have a list that contains 6 values. My lists of lists contains 3 lists and I would like to add the values the 6 values in such a way that the 1 value goes to the 1 list, 2 value to 2nd list, 3rd value to 3 list. Then I would like to continue looping so that the 4 value goes to the first list, 5 value to second list and 6 value to 3rd list. In the end I want to end up with a list of three lists which each contains 2 items. I am struggling to get anything wise to run. Can someone help me on this, thank you!

CodePudding user response：

You can use the position of the item in the list of values (modulo the total number of target lists) to get the right list to append to.

lists = [[], [], []]                                                    
values = [1, 2, 3, 4, 5, 6]                                             
for i, x in enumerate(values): 
     lists[i % len(lists)].append(x)

Or use itertools.cycle and zip:

from itertools import cycle                                            
lists = [[], [], []]                                                   
values = [1, 2, 3, 4, 5, 6]                                            
for x, l in zip(values, cycle(lists)): 
    l.append(x)

Both ways, lists ends up as [[1, 4], [2, 5], [3, 6]].

In case you want more "regular" loops, you can also use range with start, stop and step parameters to create the sublists and then add those to the list of lists:

n = 3
lists = []
values = [1, 2, 3, 4, 5, 6]                                             
for k in range(n):
    lst = []
    for i in range(k, len(values), n):
        lst.append(values[i])
    lists.append(lst)

The same also works in a quite readable one-list list copmrehension:

lists = [[values[i] for i in range(k, len(values), 3)] for k in range(3)]

Or using zip, map and iter, in case you want to show off:

lists = list(map(list, zip(*zip(*[iter(values)]*3))))

CodePudding user response：

# list with 6 items
# each item in the list has an index/position;

#          0         1       2         3        4      5
cars = ['Subaru', 'Audi', 'Honda', 'Toyota', 'BMW', 'Mazda']

# two empty lists

first_3 = [] # for first 3 items of the "cars" list
last_3 = [] # for last 3 items of the "cars" list


#          0   1  2
finallist=[[],[],[]] # a list with three empty lists inside a list


for car in cars:
    # if the index/position of the car in the "cars" list is below 3
    # add the car to the end of the "first_3" list
    
    if cars.index(car) < 3:
          first_3.append(car)
          
    # if the index/position of the car in the "cars" list is above 2
    # add the car to the end of the "last_3" list
    
    if cars.index(car) > 2:
          last_3.append(car)


# loop the cars in the "first_3" list

for car in first_3:
    
    # get the index of the car in that list
    car_pos = first_3.index(car)
    
    # reference the list with the index "car_pos"
    # i.e - if car_pos is 2, that would be the last list inside of "finallist"
    
    finallist[car_pos].append(car) # append the car
    
    
# loop the cars in the "last_3" list and do the same thing
for car in last_3:
    car_pos = last_3.index(car)
    finallist[car_pos].append(car)


# print the final list
print(finallist)