NickName:oz123 Ask DateTime:2012-04-12T21:38:17

Convert a flat list to list of lists in python

One may want to do the contrary of flattening a list of lists, like here: I was wondering how you can convert a flat list into a list of lists.

In numpy you could do something like:

>>> a=numpy.arange(9)
>>> a.reshape(3,3)
>>> a
array([[0, 1, 2],
   [3, 4, 5],
   [6, 7, 8]])

I was wondering how you do the opposite, and my usual solution is something like:

>>> Mylist
['a', 'b', 'c', 'd', 'e', 'f']
>>> newList = []
for i in range(0,len(Mylist),2):
...     newList.append(Mylist[i], Mylist[i+1])
>>> newList 
[['a', 'b'], ['c', 'd'], ['e', 'f']]

is there a more "pythonic" way to do it?

Copyright Notice：Content Author:「oz123」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/10124751/convert-a-flat-list-to-list-of-lists-in-python

Answers

jamylak 2012-04-12T13:40:16

>>> l = ['a', 'b', 'c', 'd', 'e', 'f']\n>>> zip(*[iter(l)]*2)\n[('a', 'b'), ('c', 'd'), ('e', 'f')]\n\n\nAs it has been pointed out by @Lattyware, this only works if there are enough items in each argument to the zip function each time it returns a tuple. If one of the parameters has less items than the others, items are cut off eg.\n\n>>> l = ['a', 'b', 'c', 'd', 'e', 'f','g']\n>>> zip(*[iter(l)]*2)\n[('a', 'b'), ('c', 'd'), ('e', 'f')]\n\n\nIf this is the case then it is best to use the solution by @Sven Marnach\n\nHow does zip(*[iter(s)]*n) work",

Sven Marnach 2012-04-12T13:41:32

This is usually done using the grouper recipe from the itertools documentation:\n\ndef grouper(n, iterable, fillvalue=None):\n \"grouper(3, 'ABCDEFG', 'x') --> ABC DEF Gxx\"\n args = [iter(iterable)] * n\n return itertools.izip_longest(fillvalue=fillvalue, *args)\n\n\nExample:\n\n>>> my_list = ['a', 'b', 'c', 'd', 'e', 'f', 'g']\n>>> list(grouper(2, my_list))\n[('a', 'b'), ('c', 'd'), ('e', 'f'), ('g', None)]\n",

Ryan M 2015-04-29T03:48:01

Another way to create a list of lists can be simplified as shown below:\n\n>>>MyList = ['a','b','c','d','e','f']\n# Calculate desired row/col\n>>>row = 3\n>>>col = 2\n>>>NewList = [MyList[col*i : col*(i+1)] for i in range(row)]\n>>>NewList\n[['a', 'b', 'c'], ['d', 'e', 'f']]\n\n\nThis can method can be extended to produce any row and column size. If you select row and column values such that row*col >len(MyList), the sublist (row) containing the last value in MyList will end there, and NewList will simply be filled with the appropriate number of empty lists to satisfy the row/col specifications\n\n>>>MyList = ['a','b','c','d','e','f','g','h']\n>>>row = 3\n>>>col = 3\n>>>NewList = [MyList[col*i : col*(i+1)] for i in range(row)]\n>>>NewList\n[['a', 'b', 'c'], ['d', 'e', 'f'], ['g','h']]\n\n>>>row = 4\n>>>col = 4\n>>>NewList = [MyList[col*i : col*(i+1)] for i in range(row)]\n[['a', 'b', 'c', 'd'], ['e', 'f', 'g','h'], [], []]\n",