Python list of lists

Question

Running the code  listoflists      list      for i in range 0 10       list append i      if len list  gt 3          list remove list 0           listoflists append  list  list 0    print listoflists   returns     7  8  9   0     7  8  9   0     7  8  9   0     7  8  9   1     7  8  9   2     7  8  9   3     7  8  9   4     7  8  9   5     7  8  9   6     7  8  9   7     so somehow the first argument of each tuple  list  is being updated each time in the list of lists  but the second argument list 0  is not  Can someone explain what s going on here and suggest a way to fix this  I d like to output     0  0     0 1  0

User · Answer

I came here because I m new with python and lazy so I was searching an example to create a list of 2 lists  after a while a realized the topic here could be wrong    this is a code to create a list of lists   listoflists      for i in range 0 2       sublist          for j in range 0 10          sublist append  i j       listoflists append sublist  print listoflists   this the output                0  0    0  1    0  2    0  3    0  4    0  5    0  6    0  7    0  8    0  9             1  0    1  1    1  2    1  3    1  4    1  5    1  6    1  7    1  8    1  9          The problem with your code seems to be you are creating a tuple with your list and you get the reference to the list instead of a copy  That I guess should fall under a tuple topic

User · Answer

Time traveller here  List of list     z for z in range x-2 x 1  if z  gt   0  y  for y in range 10  for x in range 10     This should do the trick  And the output is this      0   0     0  1   0     0  1  2   0     1  2  3   0     2  3  4   0      3  4  5   0     4  5  6   0     5  6  7   0     6  7  8   0     7  8  9   0     0   1     0  1   1     0  1  2   1     1  2  3   1     2  3  4   1     3  4  5   1     4  5  6   1     5  6  7   1     6  7  8   1     7  8  9   1     0   2     0  1   2     0  1  2   2     1  2  3   2     2  3  4   2     3  4  5   2     4  5  6   2     5  6  7   2     6  7  8   2     7  8  9   2     0   3     0  1   3     0  1  2   3     1  2  3   3     2  3  4   3     3  4  5   3     4  5  6   3     5  6  7   3     6  7  8   3     7  8  9   3     0   4     0  1   4     0  1  2   4     1  2  3   4     2  3  4   4     3  4  5   4     4  5  6   4     5  6  7   4     6  7  8   4     7  8  9   4     0   5     0  1   5     0  1  2   5     1  2  3   5     2  3  4   5     3  4  5   5     4  5  6   5     5  6  7   5     6  7  8   5     7  8  9   5     0   6     0  1   6     0  1  2   6     1  2  3   6     2  3  4   6     3  4  5   6     4  5  6   6     5  6  7   6     6  7  8   6     7  8  9   6     0   7     0  1   7     0  1  2   7     1  2  3   7     2  3  4   7     3  4  5   7     4  5  6   7     5  6  7   7     6  7  8   7     7  8  9   7     0   8     0  1   8     0  1  2   8     1  2  3   8     2  3  4   8     3  4  5   8     4  5  6   8     5  6  7   8     6  7  8   8     7  8  9   8     0   9     0  1   9     0  1  2   9     1  2  3   9     2  3  4   9     3  4  5   9     4  5  6   9     5  6  7   9     6  7  8   9     7  8  9   9         This is done by list comprehension which makes looping elements in a list via one line code possible   The logic behind this one-line code is the following    1  for x in range 10  and for y in range 10  are employed for two independent loops inside a list   2   a list  y  is the general term of the loop  which is why it is placed before two for s in  1    3  the length of the list in  2  cannot exceed 3  and the list depends on x  so     z for z in range x-2 x 1      is used   4  because z starts from zero but range x-2 x 1  starts from -2 which isn t what we want  so a conditional statement if z    0 is placed at the end of the list in  2    z for z in range x-2 x 1  if z  gt   0

User · Answer

When you run the code  listoflists append  list  list 0      You are not  as I think you expect  adding a copy of list to the end of listoflists  What you are doing is adding a reference to list to the end of listoflists  Thus  every time you update list  it updates every reference to list  which in this case  is every item in listoflists  What you could do instead is something like this   listoflists      for i in range 1  10       listoflists append  range i   0

User · Answer

You re also not going to get the output you re hoping for as long as you append to listoflists only inside the if-clause   Try something like this instead   import copy  listoflists      list      for i in range 0 10       list append i      if len list  gt 3          list remove list 0       listoflists append  copy copy list   copy copy list 0     print listoflists

User · Answer

First  I strongly recommend that you rename your variable list to something else  list is the name of the built-in list constructor  and you re hiding its normal function  I will rename list to a in the following   Python names are references that are bound to objects  That means that unless you create more than one list  whenever you use a it s referring to the same actual list object as last time  So when you call  listoflists append  a  a 0      you can later change a and it changes what the first element of that tuple points to  This does not happen with a 0  because the object  which is an integer  pointed to by a 0  doesn t change  although a 0  points to different objects over the run of your code    You can create a copy of the whole list a using the list constructor   listoflists append  list a   a 0      Or  you can use the slice notation to make a copy   listoflists append  a     a 0

User · Answer

The list variable  which I would recommend to rename to something more sensible  is a reference to a list object  which can be changed   On the line  listoflists append  list  list 0      You actually are only adding a reference to the object reference by the list variable  You ve got multiple possibilities to create a copy of the list  so listoflists contains the values as you seem to expect   Use the copy library  import copy listoflists append  copy copy list   list 0      use the slice notation  listoflists append  list     list 0

User · Answer

Lists are a mutable type - in order to create a copy  rather than just passing the same list around   you need to do so explicitly  listoflists append  list     list 0     However  list is already the name of a Python built-in - it d be better not to use that name for your variable  Here s a version that doesn t use list as a variable name  and makes a copy  listoflists      a list      for i in range 0 10       a list append i      if len a list  gt 3          a list remove a list 0           listoflists append  list a list   a list 0    print listoflists   Note that I demonstrated two different ways to make a copy of a list above      and list    The first       is creating a slice  normally often used for getting just part of a list   which happens to contain the entire list  and thus is effectively a copy of the list  The second  list    is using the actual list type constructor to create a new list which has contents equal to the first list   I didn t use it in the first example because you were overwriting that name in your code - which is a good example of why you don t want to do that

[python] Python: list of lists

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