How to iterate through two lists in parallel

Question

I have two iterables in Python  and I want to go over them in pairs  foo    1  2  3  bar    4  5  6   for  f  b  in some iterator foo  bar       print  quot f   quot   f   quot   b   quot   b   It should result in  f  1  b  4 f  2  b  5 f  3  b  6  One way to do it is to iterate over the indices  for i in range len foo        print  quot f   quot   foo i    quot   b   quot   bar i    But that seems somewhat unpythonic to me  Is there a better way to do it

User · Answer

You should use  zip  function  Here is an example how your own zip function can look like  def custom zip seq1  seq2       it1   iter seq1      it2   iter seq2      while True          yield next it1   next it2

User · Answer

Python 3  for f  b in zip foo  bar       print f  b    zip stops when the shorter of foo or bar stops   In Python 3  zip returns an iterator of tuples  like itertools izip in Python2   To get a list of tuples  use list zip foo  bar    And to zip until both iterators are exhausted  you would use itertools zip longest   Python 2  In Python 2  zip returns a list of tuples  This is fine when foo and bar are not massive  If they are both massive then forming zip foo bar  is an unnecessarily massive temporary variable  and should be replaced by itertools izip or itertools izip longest  which returns an iterator instead of a list   import itertools for f b in itertools izip foo bar       print f b  for f b in itertools izip longest foo bar       print f b    izip stops when either foo or bar is exhausted  izip longest stops when both foo and bar are exhausted  When the shorter iterator s  are exhausted  izip longest yields a tuple with None in the position corresponding to that iterator  You can also set a different fillvalue besides None if you wish  See here for the full story     Note also that zip and its zip-like brethen can accept an arbitrary number of iterables as arguments  For example   for num  cheese  color in zip  1 2 3     manchego    stilton    brie                                     red    blue    green         print            format num  color  cheese     prints  1 red manchego 2 blue stilton 3 green brie

User · Answer

Here s how to do it with list comprehension   a    1  2  3  b    4  5  6   print  f    i     b   j  for i  j in zip a  b     prints   f  1   b 4 f  2   b 5 f  3   b 6

User · Answer

You want the zip function   for  f b  in zip foo  bar       print  f     f     b     b

User · Answer

Python 3  import itertools as it  for foo  bar in list it izip longest list1  list2        print foo  bar

User · Answer

You can bundle the nth elements into a tuple or list using comprehension  then pass them out with a generator function   def iterate multi  lists       for i in range min map len lists             yield tuple l i  for l in lists   for l1  l2  l3 in iterate multi  1 2 3   4 5 6   7 8 9        print str l1      str l2      str l3

User · Answer

Building on the answer by  unutbu  I have compared the iteration performance of two identical lists when using Python 3 6 s zip   functions  Python s enumerate   function  using a manual counter  see count   function   using an index-list  and during a special scenario where the elements of one of the two lists  either foo or bar  may be used to index the other list  Their performances for printing and creating a new list  respectively  were investigated using the timeit   function where the number of repetitions used was 1000 times  One of the Python scripts that I had created to perform these investigations is given below  The sizes of the foo and bar lists had ranged from 10 to 1 000 000 elements  Results   For printing purposes  The performances of all the considered approaches were observed to be approximately similar to the zip   function  after factoring an accuracy tolerance of   -5   An exception occurred when the list size was smaller than 100 elements  In such a scenario  the index-list method was slightly slower than the zip   function while the enumerate   function was  9  faster  The other methods yielded similar performance to the zip   function    For creating lists  Two types of list creation approaches were explored  using the  a  list append   method and  b  list comprehension  After factoring an accuracy tolerance of   -5   for both of these approaches  the zip   function was found to perform faster than the enumerate   function  than using a list-index  than using a manual counter  The performance gain by the zip   function in these comparisons can be 5  to 60  faster  Interestingly  using the element of foo to index bar can yield equivalent or faster performances  5  to 20   than the zip   function     Making sense of these results  A programmer has to determine the amount of compute-time per operation that is meaningful or that is of significance  For example  for printing purposes  if this time criterion is 1 second  i e  10  0 sec  then looking at the y-axis of the graph that is on the left at 1 sec and projecting it horizontally until it reaches the monomials curves  we see that lists sizes that are more than 144 elements will incur significant compute cost and significance to the programmer  That is  any performance gained by the approaches mentioned in this investigation for smaller list sizes will be insignificant to the programmer  The programmer will conclude that the performance of the zip   function to iterate print statements is similar to the other approaches  Conclusion Notable performance can be gained from using the zip   function to iterate through two lists in parallel during list creation  When iterating through two lists in parallel to print out the elements of the two lists  the zip   function will yield similar performance as the enumerate   function  as to using a manual counter variable  as to using an index-list  and as to during the special scenario where the elements of one of the two lists  either foo or bar  may be used to index the other list  The Python3 6 Script that was used to investigate list creation  import timeit import matplotlib pyplot as plt import numpy as np   def test zip  foo  bar        store          for f  b in zip foo  bar            print f  b          store append   f  b      def test enumerate  foo  bar        store          for n  f in enumerate  foo             print f  bar n           store append   f  bar n       def test count  foo  bar        store          count   0     for f in foo           print f  bar count           store append   f  bar count             count    1  def test indices  foo  bar  indices        store          for i in indices           print foo i   bar i           store append   foo i   bar i      def test existing list indices  foo  bar        store          for f in foo           print f  bar f           store append   f  bar f       list sizes     10  100  1000  10000  100000  1000000   tz      te      tc      ti      tii      tcz      tce      tci      tcii      for a in list sizes      foo     i for i in range a        bar     i for i in range a        indices     i for i in range a        reps   1000      tz append  timeit timeit   test zip  foo  bar                                    from   main   import test zip  foo  bar                                 number reps                                                      te append  timeit timeit   test enumerate  foo  bar                                    from   main   import test enumerate  foo  bar                                 number reps                                                      tc append  timeit timeit   test count  foo  bar                                    from   main   import test count  foo  bar                                 number reps                                                      ti append  timeit timeit   test indices  foo  bar  indices                                    from   main   import test indices  foo  bar  indices                                 number reps                                                      tii append  timeit timeit   test existing list indices  foo  bar                                     from   main   import test existing list indices  foo  bar                                  number reps                                                         tcz append  timeit timeit     f  b  for f  b in zip foo  bar                                     from   main   import foo  bar                                  number reps                                                        tce append  timeit timeit     f  bar n   for n  f in enumerate  foo                                      from   main   import foo  bar                                  number reps                                                        tci append  timeit timeit     foo i   bar i   for i in indices                                     from   main   import foo  bar  indices                                  number reps                                                        tcii append  timeit timeit     f  bar f   for f in foo                                      from   main   import foo  bar                                   number reps                                                       print  f te     te     print  f ti     ti     print  f tii    tii     print  f tc     tc     print  f tz     tz      print  f tce     te     print  f tci     ti     print  f tcii    tii     print  f tcz     tz      fig  ax   plt subplots  2  2   ax 0 0  plot  list sizes  te  label  enumerate     marker       ax 0 0  plot  list sizes  ti  label  index-list   marker       ax 0 0  plot  list sizes  tii  label  element of foo   marker       ax 0 0  plot  list sizes  tc  label  count     marker       ax 0 0  plot  list sizes  tz  label  zip     marker      ax 0 0  set xscale  log   ax 0 0  set yscale  log   ax 0 0  set xlabel  List Size   ax 0 0  set ylabel  Time  s    ax 0 0  legend   ax 0 0  grid  b True  which  major   axis  both   ax 0 0  grid  b True  which  minor   axis  both    ax 0 1  plot  list sizes  np array te  np array tz   label  enumerate     marker       ax 0 1  plot  list sizes  np array ti  np array tz   label  index-list   marker       ax 0 1  plot  list sizes  np array tii  np array tz   label  element of foo   marker       ax 0 1  plot  list sizes  np array tc  np array tz   label  count     marker       ax 0 1  set xscale  log   ax 0 1  set xlabel  List Size   ax 0 1  set ylabel  Performances   vs zip   function     ax 0 1  legend   ax 0 1  grid  b True  which  major   axis  both   ax 0 1  grid  b True  which  minor   axis  both    ax 1 0  plot  list sizes  tce  label  list comprehension using enumerate      marker      ax 1 0  plot  list sizes  tci  label  list comprehension using index-list      marker      ax 1 0  plot  list sizes  tcii  label  list comprehension using element of foo    marker      ax 1 0  plot  list sizes  tcz  label  list comprehension using zip      marker      ax 1 0  set xscale  log   ax 1 0  set yscale  log   ax 1 0  set xlabel  List Size   ax 1 0  set ylabel  Time  s    ax 1 0  legend   ax 1 0  grid  b True  which  major   axis  both   ax 1 0  grid  b True  which  minor   axis  both    ax 1 1  plot  list sizes  np array tce  np array tcz   label  enumerate     marker       ax 1 1  plot  list sizes  np array tci  np array tcz   label  index-list   marker       ax 1 1  plot  list sizes  np array tcii  np array tcz   label  element of foo   marker       ax 1 1  set xscale  log   ax 1 1  set xlabel  List Size   ax 1 1  set ylabel  Performances   vs zip   function     ax 1 1  legend   ax 1 1  grid  b True  which  major   axis  both   ax 1 1  grid  b True  which  minor   axis  both    plt show

[python] How to iterate through two lists in parallel?

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