Question

How do I compute the number of elements that would result from a call to range(start, stop, step) before I actually call it.

The context is that I’m implementing slice indexing into an object

def __init__(self, impl_object):
   self.impl=impl_object # the object that actually holds (or generates) an array of values
def __getitem__(self, key):
    if isinstance(key, slice):
         (start,stop,step)=key.indices( self.impl.numValues() )
         # It would be nice to know how many items I'm dealing with
         # here
    ...snip...

I’ve convinced myself that for step>0
len(range(start,stop,step))==(start-stop+step-1)/step

but I don’t see how to generalize this for negative steps.

Edit: I require (strongly prefer) that the solution take O(1) time.

Answer 1

The easiest way is

len(xrange(start, stop, step))

xrange.__len__ calculates the number of elements it would yield, without constructing the range in memory.