How to duplicate set of items in list into list? (Python3)

Question

Say my list is this:
l=[1,2,3,4,5,6]

Say I want to duplicate every set of three numbers so my result is this:
l=[1,2,3,1,2,3,4,5,6,4,5,6]

It has to duplicate right after the set of numbers. I can't think of a good way to do this through a for loop, although I might just be tired.

I need this for expanding an images y values, if you were curious why I need this.

Answer 1

A very crude, but effective version. I'll be reducing it to an error-resistant one-liner tomorrow. But for the time being - there you go :)
https://code.sololearn.com/cZlLfe9tWoP2/?ref=app

Answer 2

Thank you Kuba! You always Luba when I need it the most!

Answer 3

@Kirk Schafer
The best challenges are real problems! I actually needed this and you guys saved me alot of trouble. So thank you all!!!

Answer 4

Adding this because "me" trying to do this with Pythonic features was almost disastrously convoluted...so at this point it's just funny to me (3 ways):

https://code.sololearn.com/cL1DOm8zF6rS/?ref=app

Answer 5

@Kirk I spent some time still yesterday but all I got was a lousy map object or a 2D list :D

Answer 6

yeah, the iterators gave me fits... (@Kuba) I started with starmap and even an iterator of iterators, (imap? izip?) which was undefined on SoloLearn & I can't find now.

Noticing things like accumulate() today and @richard's chain() it's clear I need to learn more.

Thanks @Ahri for the challenge.

Answer 7

Ah...I was looking for something like count() yesterday. Here's an Ahri-while-compatible, less terrible version (only because I need to clarify iterators for myself + maybe it helps to note):

l=[1,2,3,4,5,6,7,8,9]
from itertools import count

out=[]
ctr=count(0,3)
while True:
c=next(ctr) # ctr.next() should work; doesn't.
out += l[c:c+3] * 2
if c>=len(l): break

print(out)

Answer 8

That is Sololearn's idea how I get it - learn playing, play learning ;)

P.S. Now I understand why Guido wanted to dispose of all those lambdas, map and reduce :D

Answer 9

# there is also the itertools option with a comprehension
from itertools import chain

def test(Alist,size):
return list(chain(*(Alist[x:x+size]*2 for x in range(0,len(Alist),size))))

print(test([1,2,3,4,5,6,7,8,9],3))
print(test([1,2,3,4,5,6,7,8,9,10],2))