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)):
    prin         


        

Answer 1

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.

Answer 2

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])

Answer 3

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 -> 1,2 : 1 -> 3,4 : 2 -> 5,6 : 3 -> 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 < len(nodes):
    leftNode = nodes[i*2 + 1]
  if i*2 + 2 < 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.

Answer 4

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 < N; i += 1 ...

Answer 5

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!

Answer 6

My code is:

s=["9"]*int(input())
for I in range(len(s)):
    while not set(s[I])<=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.