CODEWITHSUNDEEP
pythonlistmodulo
John
John

Reputation: 4981

Python reverse mod

I'm using Python3. I want to get 4 items from mylist to populate list2 and print my list2 every 4 loops. And I need to print the rest even if there are just 1, 2 or 3 items at the end.

I want to do this with mod % :

mylist = [1, 2, 3, 4, 5]
list2 = []
for count, el in enumerate(mylist):
    list2.append(el)
    if count % 4 == 0:
        print(list2)
        list2 = []

Output : 

[1]
[2, 3, 4, 5]

But I need the inverse.

I tried to start at 1 enumerate(mylist, 1): but the output is [1, 2, 3, 4] the last is ignored.

The output that I need is :

[1, 2, 3, 4]
[5]

The length of mylist is totally random. How can I do to get the output that I want ?

Upvotes: 1

Views: 338

Answers (4)

Christian Sloper
Christian Sloper

Reputation: 7510

Sounds to me you just want to chunk the data into groups of 4. This is done best here:

How do you split a list into evenly sized chunks?

def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in range(0, len(l), n):
        yield l[i:i + n]

for i in chunks(mylist,4):
    print(i)

Upvotes: 1

Slam
Slam

Reputation: 8572

@remcogerlich wrote a good answer for what you're asking for. Although, I'd recommend not using % 4 for what you're achieving (until its some kind of homework from school) and use list slicing instead:

n = 4
for i in range(0, len(mylist), n):
    print(mylist[i:i + n])

Python can handle index "overflow" so you can write more high-level code instead of caring about tail size. Take a look at similar old question

Upvotes: 1

RemcoGerlich
RemcoGerlich

Reputation: 31260

You have two problems. One is the start at 0, as you noticed. The second is that you only print if the list has exactly 4 elements, so if there is something left at the end, then there is no print call anymore after the loop that prints those remaining values. Add it at the end:

mylist = [1, 2, 3, 4, 5]
list2 = []
for count, el in enumerate(mylist, 1):
    list2.append(el)
    if count % 4 == 0:
        print(list2)
        list2 = []

if list2: print(list2)

Upvotes: 2

Matthieu Brucher
Matthieu Brucher

Reputation: 22023

count starts at 0, and 0%4==0.

So just use the enumerate as you did and then at the end

if len(list2) != 0
    print(list2)

Upvotes: 2

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