Is there a need for range len a

Question

One frequently finds expressions of this type in python questions on SO  Either for just accessing all items of the iterable  for i in range len a        print a i     Which is just a clumbersome way of writing   for e in a      print e    Or for assigning to elements of the iterable   for i in range len a        a i    a i    2   Which should be the same as   for i  e in enumerate a        a i    e   2   Or if it isn t too expensive to create a new iterable a    e   2 for e in a    Or for filtering over the indices   for i in range len a        if i   2    1  continue     print a i     Which could be expressed like this   for e in a    2       print e    Or when you just need the length of the list  and not its content   for   in range len a        doSomethingUnrelatedToA     Which could be   for   in a      doSomethingUnrelatedToA     In python we have enumerate  slicing  filter  sorted  etc    As python for constructs are intended to iterate over iterables and not only ranges of integers  are there real-world use-cases where you need in range len a

User · Answer

Going by the comments as well as personal experience, I say no, there is no need for range(len(a)). Everything you can do with range(len(a)) can be done in another (usually far more efficient) way.

You gave many examples in your post, so I won't repeat them here. Instead, I will give an example for those who say "What if I want just the length of a, not the items?". This is one of the only times you might consider using range(len(a)). However, even this can be done like so:

>>> a = [1, 2, 3, 4]
>>> for _ in a:
...     print True
...
True
True
True
True
>>>

Clements answer (as shown by Allik) can also be reworked to remove range(len(a)):

>>> a = [6, 3, 1, 2, 5, 4]
>>> sorted(range(len(a)), key=a.__getitem__)
[2, 3, 1, 5, 4, 0]
>>> # Note however that, in this case, range(len(a)) is more efficient.
>>> [x for x, _ in sorted(enumerate(a), key=lambda i: i[1])]
[2, 3, 1, 5, 4, 0]
>>>

So, in conclusion, range(len(a)) is not needed. Its only upside is readability (its intention is clear). But that is just preference and code style.

User · Answer

My code is   s   9   int input    for I in range len s        while not set s I   lt  set  01   s i  input i  print bin sum  int x 2 for x in s    2      It is a binary adder but I don t think the range len or the inside can be replaced to make it smaller better

User · Answer

If you have to iterate over the first len a  items of an object b  that is larger than a   you should probably use range len a     for i in range len a        do something with b i

User · Answer

If you need to work with indices of a sequence  then yes - you use it    eg for the equivalent of numpy argsort       gt  gt  gt  a    6  3  1  2  5  4   gt  gt  gt  sorted range len a    key a   getitem     2  3  1  5  4  0

User · Answer

It s nice to have when you need to use the index for some kind of manipulation and having the current element doesn t suffice  Take for instance a binary tree that s stored in an array  If you have a method that asks you to return a list of tuples that contains each nodes direct children then you need the index    0 - gt  1 2   1 - gt  3 4   2 - gt  5 6   3 - gt  7 8     nodes    0 1 2 3 4 5 6 7 8 9 10  children      for i in range len nodes      leftNode   None   rightNode   None   if i 2   1  lt  len nodes       leftNode   nodes i 2   1    if i 2   2  lt  len nodes       rightNode   nodes i 2   2    children append  leftNode rightNode   return children   Of course if the element you re working on is an object  you can just call a get children method  But yea  you only really need the index if you re doing some sort of manipulation

User · Answer

Short answer  mathematically speaking  no  in practical terms  yes  for example for Intentional Programming   Technically  the answer would be  no  it s not needed  because it s expressible using other constructs  But in practice  I use for i in range len a   or for   in range len a   if I don t need the index  to make it explicit that I want to iterate as many times as there are items in a sequence without needing to use the items in the sequence for anything   So   Is there a need        yes  I need it to express the meaning intent of the code for readability purposes   See also  https   en wikipedia org wiki Intentional programming  And obviously  if there is no collection that is associated with the iteration at all  for     in range len N   is the only option  so as to not resort to i   0  while i  lt  N  i    1

User · Answer

Sometimes matplotlib requires range len y    e g   while y array  1 2 5 6    plot y  works fine  scatter y  does not   One has to write scatter range len y   y     Personally  I think this is a bug in scatter  plot and its friends scatter and stem should use the same calling sequences as much as possible

User · Answer

I have an use case I don t believe any of your examples cover   boxes    b1  b2  b3  items    i1  i2  i3  i4  i5  for j in range len boxes        boxes j  putitemin items j     I m relatively new to  python though so  happy to  learn a more elegant approach

User · Answer

What if you need to access two elements of the list simultaneously   for i in range len a 0 -1         something new i    a i    a i 1    You can use this  but it s probably less clear   for i    in enumerate a 0 -1         something new i    a i    a i 1    Personally I m not 100  happy with either

User · Answer

Sometimes  you really don t care about the collection itself  For instance  creating a simple model fit line to compare an  approximation  with the raw data   fib raw    1  1  2  3  5  8  13  21    Fibonacci numbers  phi    1   sqrt 5     2 phi2    1 - sqrt 5     2  def fib approx n   return  phi  n - phi2  n    sqrt 5   x   range len data   y    fib approx n  for n in x     Now plot to compare fib raw and y   Compare error  etc   In this case  the values of the Fibonacci sequence itself were irrelevant  All we needed here was the size of the input sequence we were comparing with

User · Answer

Very simple example   def loadById self  id       if id in range len self itemList            self load self itemList id     I can t think of a solution that does not use the range-len composition quickly   But probably instead this should be done with try    except to stay pythonic i guess

[python] Is there a need for range(len(a))?

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