List comprehension on a nested list

Question

I have this nested list   l      40    20    10    30      20    20    20    20    20    30    20      30    20    30    50    10    30    20    20    20      100    100      100    100    100    100    100      100    100    100    100      Now  what I want to do is convert each element in a list to float  My solution is this   newList      for x in l    for y in x      newList append float y     But can this be done using nested list comprehension  right   what I ve done is    float y  for y in x for x in l    But then the result is bunch of 100 s with the sum of 2400   any solution  an explanation would be much appreciated  Thanks

User · Answer

Yes  you can do it with such a code   l     float y  for y in x  for x in l

User · Answer

gt  gt  gt  l      40    20    10    30      20    20    20    20    20    30    20      30    20    30    50    10    30    20    20    20      100    100      100    100    100    100    100      100    100    100    100     gt  gt  gt  new list    float x  for xs in l for x in xs   gt  gt  gt  new list  40 0  20 0  10 0  30 0  20 0  20 0  20 0  20 0  20 0  30 0  20 0  30 0  20 0  30 0  50 0  10 0  30 0  20 0  20 0  20 0  100 0  100 0  100 0  100 0  100 0  100 0  100 0  100 0  100 0  100 0  100 0

User · Answer

I had a similar problem to solve so I came across this question  I did a performance comparison of Andrew Clark s and narayan s answer which I would like to share    The primary difference between two answers is how they iterate over inner lists  One of them uses builtin map  while other is using list comprehension  Map function has slight performance advantage to its equivalent list comprehension if it doesn t require the use lambdas  So in context of this question map should perform slightly better than list comprehension    Lets do a performance benchmark to see if it is actually true  I used python version 3 5 0 to perform all these tests  In first set of tests I would like to keep elements per list to be 10 and vary number of lists from 10-100 000   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 10    10    gt  gt  gt  100000 loops  best of 3  15 2 usec per loop     gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 10    10    gt  gt  gt  10000 loops  best of 3  19 6 usec per loop    gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 10    100    gt  gt  gt  100000 loops  best of 3  15 2 usec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 10    100    gt  gt  gt  10000 loops  best of 3  19 6 usec per loop    gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 10    1000    gt  gt  gt  1000 loops  best of 3  1 43 msec per loop     gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 10    1000    gt  gt  gt  100 loops  best of 3  1 91 msec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 10    10000    gt  gt  gt  100 loops  best of 3  13 6 msec per loop     gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 10    10000    gt  gt  gt  10 loops  best of 3  19 1 msec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 10    100000    gt  gt  gt  10 loops  best of 3  164 msec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 10    100000    gt  gt  gt  10 loops  best of 3  216 msec per loop     In the next set of tests I would like to raise number of elements per lists to 100    gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 100    10    gt  gt  gt  10000 loops  best of 3  110 usec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 100    10    gt  gt  gt  10000 loops  best of 3  151 usec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 100    100    gt  gt  gt  1000 loops  best of 3  1 11 msec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 100    100    gt  gt  gt  1000 loops  best of 3  1 5 msec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 100    1000    gt  gt  gt  100 loops  best of 3  11 2 msec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 100    1000    gt  gt  gt  100 loops  best of 3  16 7 msec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 100    10000    gt  gt  gt  10 loops  best of 3  134 msec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 100    10000    gt  gt  gt  10 loops  best of 3  171 msec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 100    100000    gt  gt  gt  10 loops  best of 3  1 32 sec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 100    100000    gt  gt  gt  10 loops  best of 3  1 7 sec per loop     Lets take a brave step and modify the number of elements in lists to be 1000   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 1000    10    gt  gt  gt  1000 loops  best of 3  800 usec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 1000    10    gt  gt  gt  1000 loops  best of 3  1 16 msec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 1000    100    gt  gt  gt  100 loops  best of 3  8 26 msec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 1000    100    gt  gt  gt  100 loops  best of 3  11 7 msec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 1000    1000    gt  gt  gt  10 loops  best of 3  83 8 msec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 1000    1000    gt  gt  gt  10 loops  best of 3  118 msec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 1000    10000    gt  gt  gt  10 loops  best of 3  868 msec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 1000    10000    gt  gt  gt  10 loops  best of 3  1 23 sec per loop   gt  gt  gt  python -m timeit   list map float k   for k in  list range 0 1000    100000    gt  gt  gt  10 loops  best of 3  9 2 sec per loop  gt  gt  gt  python -m timeit    float y  for y in x  for x in  list range 0 1000    100000    gt  gt  gt  10 loops  best of 3  12 7 sec per loop     From these test we can conclude that map has a performance benefit over list comprehension in this case  This is also applicable if you are trying to cast to either int or str   For small number of lists with less elements per list  the difference is negligible  For larger lists with more elements per list one might like to use map instead of list comprehension  but it totally depends on application needs   However I personally find list comprehension to be more readable and idiomatic than map  It is a de-facto standard in python  Usually people are more proficient and comfortable specially beginner  in using list comprehension than map

User · Answer

If you don t like nested list comprehensions  you can make use of the map function as well     gt  gt  gt  from pprint import pprint   gt  gt  gt  l   l      40    20    10    30      20    20    20    20    20    30    20      30    20    30    50    10    30    20    20    20      100    100      100    100    100    100    100      100    100    100    100       gt  gt  gt  pprint l     40    20    10    30      20    20    20    20    20    30    20      30    20    30    50    10    30    20    20    20      100    100      100    100    100    100    100      100    100    100    100      gt  gt  gt  float l    map float  nested list  for nested list in l    gt  gt  gt  pprint float l    40 0  20 0  10 0  30 0    20 0  20 0  20 0  20 0  20 0  30 0  20 0    30 0  20 0  30 0  50 0  10 0  30 0  20 0  20 0  20 0    100 0  100 0    100 0  100 0  100 0  100 0  100 0    100 0  100 0  100 0  100 0

User · Answer

The best way to do this in my opinion is to use python s itertools package    gt  gt  gt import itertools  gt  gt  gt l1    1 2 3   gt  gt  gt l2    10 20 30   gt  gt  gt  l 2 for l in itertools chain   l1 l2     2  4  6  20  40  60

User · Answer

Yes you can do the following     float y  for y in x  for x in l

User · Answer

Not sure what your desired output is  but if you re using list comprehension  the order follows the order of nested loops  which you have backwards  So I got the what I think you want with    float y  for x in l for y in x    The principle is  use the same order you d use in writing it out as nested for loops

User · Answer

Since i am little late here but i wanted to share how actually list comprehension works especially nested list comprehension    New list    float y  for x in l    is actually same as    New list    for x in l      New list append x    And now nested list comprehension      float y  for y in x  for x in l    is same as    new list    for x in l      sub list        for y in x          sub list append float y        new list append sub list   print new list    output     40 0  20 0  10 0  30 0    20 0  20 0  20 0  20 0  20 0  30 0  20 0    30 0  20 0  30 0  50 0  10 0  30 0  20 0  20 0  20 0    100 0  100 0    100 0  100 0  100 0  100 0  100 0    100 0  100 0  100 0  100 0

User · Answer

Here is how to convert nested for loop to nested list comprehension     Here is how nested list comprehension works               l a b c d e f                           In  1   l               1             In  2   for a in l              for b in a                  for c in b                      for d in c                          for e in d                              for f in e                                  print float f                                    1 0  In  3    float f           for a in l             for b in a                 for c in b                     for d in c                         for e in d                             for f in e  Out 3    1 0    For your case  it will be something like this       In  4   new list    float y  for x in l for y in x

User · Answer

deck           for rank in ranks          for suit in suits              deck append    s s    rank  suit     This can be achieved using list comprehension    deck append  rank suit   for suit in suits for rank in ranks

User · Answer

This Problem can be solved without using for loop Single line code will be sufficient for this  Using Nested Map with lambda function will also works here   l      40    20    10    30      20    20    20    20    20    30    20      30    20    30    50    10    30    20    20    20      100    100      100    100    100    100    100      100    100    100    100     map lambda x map lambda y float y  x  l    And Output List would be as follows     40 0  20 0  10 0  30 0    20 0  20 0  20 0  20 0  20 0  30 0  20 0    30 0  20 0  30 0  50 0  10 0  30 0  20 0  20 0  20 0    100 0  100 0    100 0  100 0  100 0  100 0  100 0    100 0  100 0  100 0  100 0

User · Answer

Here is how you would do this with a nested list comprehension     float y  for y in x  for x in l    This would give you a list of lists  similar to what you started with except with floats instead of strings   If you want one flat list then you would use  float y  for x in l for y in x

[python] List comprehension on a nested list?

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