Double Iteration in List Comprehension

Question

In Python you can have multiple iterators in a list comprehension  like    x y  for x in a for y in b    for some suitable sequences a and b  I m aware of the nested loop semantics of Python s list comprehensions   My question is  Can one iterator in the comprehension refer to the other  In other words  Could I have something like this    x for x in a for a in b    where the current value of the outer loop is the iterator of the inner   As an example  if I have a nested list   a   1 2   3 4     what would the list comprehension expression be to achieve this result    1 2 3 4        Please only list comprehension answers  since this is what I want to find out

User · Answer

I feel this is easier to understand   row i  for row in a for i in range len a     result   1  2  3  4

User · Answer

This flatten nlevel function calls recursively the nested list1 to covert to one level  Try this out  def flatten nlevel list1  flat list       for sublist in list1          if isinstance sublist  type list                        flatten nlevel sublist  flat list          else              flat list append sublist   list1    1  1  2 3  4 6   4  5   items      flatten nlevel list1 items  print items    output    1  1  2  3  4  6  4  5

User · Answer

To answer your question with your own suggestion    gt  gt  gt   x for b in a for x in b    Works fine   While you asked for list comprehension answers  let me also point out the excellent itertools chain      gt  gt  gt  from itertools import chain  gt  gt  gt  list chain from iterable a    gt  gt  gt  list chain  a     If you re using python  lt  2 6

User · Answer

Order of iterators may seem counter-intuitive   Take for example   str x  for i in range 3  for x in foo i    Let s decompose it   def foo i       return i  i   0 5   str x      for i in range 3          for x in foo i       is same as for i in range 3       for x in foo i           yield str x

User · Answer

If you want to keep the multi dimensional array  one should nest the array brackets  see example below where one is added to every element    gt  gt  gt  a     1  2    3  4     gt  gt  gt    col  1 for col in row  for row in a    2  3    4  5     gt  gt  gt   col  1 for row in a for col in row   2  3  4  5

User · Answer

Additionally  you could use just the same variable for the member of the input list which is currently accessed and for the element inside this member  However  this might even make it more  list  incomprehensible   input     1  2    3  4    x for x in input for x in x    First for x in input is evaluated  leading to one member list of the input  then  Python walks through the second part for x in x during which the x-value is overwritten by the current element it is accessing  then the first x defines what we want to return

User · Answer

ThomasH has already added a good answer  but I want to show what happens    gt  gt  gt  a     1  2    3  4    gt  gt  gt   x for x in b for b in a  Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  NameError  name  b  is not defined   gt  gt  gt   x for b in a for x in b   1  2  3  4   gt  gt  gt   x for x in b for b in a   3  3  4  4    I guess Python parses the list comprehension from left to right  This means  the first for loop that occurs will be executed first   The second  problem  of this is that b gets  leaked  out of the list comprehension  After the first successful list comprehension b     3  4

User · Answer

Gee  I guess I found the anwser  I was not taking care enough about which loop is inner and which is outer  The list comprehension should be like    x for b in a for x in b    to get the desired result  and yes  one current value can be the iterator for the next loop

User · Answer

I hope this helps someone else since a b x y don t have much meaning to me  Suppose you have a text full of sentences and you want an array of words     Without list comprehension list of words      for sentence in text      for word in sentence         list of words append word  return list of words   I like to think of list comprehension as stretching code horizontally   Try breaking it up into     List Comprehension   word for sentence in text for word in sentence    Example    gt  gt  gt  text      Hi    Steve       What s    up      gt  gt  gt   word for sentence in text for word in sentence    Hi    Steve     What s    up      This also works for generators   gt  gt  gt  text      Hi    Steve       What s    up      gt  gt  gt  gen    word for sentence in text for word in sentence   gt  gt  gt  for word in gen  print word  Hi Steve  What s up

User · Answer

This memory technic helps me a lot      lt RETURNED VALUE gt    lt OUTER LOOP1 gt    lt INNER LOOP2 gt    lt INNER LOOP3 gt       lt OPTIONAL IF gt     And now you can think about Return   Outer-loop as the only Right Order  Knowing above  the order in list comprehensive even for 3 loops seem easy     c  111  222  333  b  11  22  33  a  1  2  3   print           i  j  k                                lt RETURNED VALUE gt       for i in a for j in b for k in c       in order  loop1  loop2  loop3     if i  lt  2 and j  lt  20 and k  lt  200         lt OPTIONAL IF gt          1  11  111       because the above is just a   for i in a                            outer loop1 GOES SECOND   for j in b                          inner loop2 GOES THIRD     for k in c                        inner loop3 GOES FOURTH       if i  lt  2 and j  lt  20 and k  lt  200          print  i  j  k                returned value GOES FIRST     for iterating one nested list structure  technic is the same  for a from the question   a     1 2   3 4    i2    for i1 in a      for i2 in i1  which return  1  2  3  4    for one another nested level  a      1  2    3  4      5  6    7  8  9      10     i3    for i1 in a      for i2 in i1     for i3 in i2  which return  1  2  3  4  5  6  7  8  9  10     and so on

[python] Double Iteration in List Comprehension

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