Question

I have a list of many Python objects like this:

class RangeClass(object):

    def __init__(self,address,size):
        self.address=address
        self.size=size
        #other attributes and methods...

Then, I have a list (rangelist) of RangeClass objects.

I need to find within which range a given value is.

I can use some code like this:

for r in ragelist:
    if(value>=r.address and value<(r.address+r.size)):
        return r
return None

But I think there is a faster way. Ranges have arbitrary size, but we can assume that they don’t overlap.

Thank you.

Answer 1

If you have many values to test, then you could use the bisect module to find which range the values are in more quickly.

If

• m = the number of values to test, and
• n = len(rangelist)

then looping through the values and rangelist as you suggest would take O(m*n) time.

If you use bisection, then you must first sort the starting addresses O(nlogn) and find each value’s place in rangelist O(m*logn).
So if

O(nlogn + m*logn) < O(m*n)

then bisection wins. For large n, O(m*logn) is miniscule compared to O(m*n).
So the inequality above would be true if

O(nlogn) < O(m*n)

or equivalently, when

C log(n) < m

for some constant C.

---

Thus, when n is large and C log(n) < m, you might try something like

import bisect

class RangeClass(object):

    def __init__(self,address,size):
        self.address=address
        self.size=size
    def __str__(self):
        return '[{0},{1})'.format(self.address,self.address+self.size)
    def __lt__(self,other):
        return self.address<other.address

rangelist=sorted([RangeClass(i,1) for i in (1,3,4,5,7.5)])
starts=[r.address for r in rangelist]

def find_range(value):
    start_idx=bisect.bisect(starts,value)-1
    try:
        r=rangelist[start_idx]
    except IndexError:
        return None
    start=r.address
    end=r.address+r.size
    if start<=value<end:
        return rangelist[start_idx]
    return None    

print(','.join(str(r) for r in rangelist))

for value in (0,1,1.5,2,3,4,5,6,7,8,9,10):
    r=find_range(value)
    if r:
        print('{v} in {r}'.format(v=value,r=str(r)))
    else:
        print('{v} not in any range'.format(v=value))