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))?

Answer 1

If you need to work with indices of a sequence, then yes - you use it... eg for the equivalent of numpy.argsort...:

>>> a = [6, 3, 1, 2, 5, 4] >>> sorted(range(len(a)), key=a.__getitem__) [2, 3, 1, 5, 4, 0] 

Answer 2

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!