Pairs of numbers from two lists

Question

I have two lists:

a = [1,3,6,10,20] 
b = [2,4,9,12,15,22,24,25] 

Now, I would like to make a new list, containing pairs from the two previous lists. The pairs are defined as such:

• The left value [l,..] : a[i]
• The right [..,r]: The highest number in b between a[i] and a[i+1] if a[i+1] exists, else just greater than a[i] , if a[i] exists, else just greater than a[-1] with 0 < i < max(len(a),len(b))

The result would look like that:

pair = [[1,2],[3,4],[6,9],[10,15],[20,25]]

Anyone knows how to do that ?

This is what I have done so far:

a = [1,3,6,10,20] 
b = [2,4,9,12,15,22,24,25]  

pairs = []
counter = 0
for i in range(max(len(a),len(b))):

try: 
    # get a[i]
    ai = a[i]
except: 
    ai = a[-1]

try: 
    # get a[i+1]
    ai1 = a[i+1]
except:
    ai1 = b[-1]+1

temp = []
for bi in b:

    if ai < bi and bi < ai1:
        temp.append(bi)

# Avoid adding the last element of b again and again until i = len(b)
if max(temp) == b[-1]:
    counter = counter +1

if counter <= 1:
    print(max(temp))
    pairs.append([ai,max(temp)])

It's okay, since it gets the job done, but I was wondering, if there is a better, more efficient way to do it ?

Answer 1

You can do a binary search, since the arrays are sorted, you don't have to search for a[i] < b[j] , only for a[i+1] > b[j] (This code will return invalid results if there are no elements in b such that a[i] < b < a[b+1] ):

import bisect

def pairs(a, b):
    for (a1, a2) in zip(a, a[1:]):
        yield a1, b[bisect.bisect_left(b, a2) - 1]

    yield a[-1], b[-1]

print(list(pairs([1,3,6,10,20], [2,4,9,12,15,22,24,25])))

---

[(1, 2), (3, 4), (6, 9), (10, 15), (20, 25)]

Answer 2

Here is another way of doing it:

a = [1,3,6,10,20] 
b = [2,4,9,12,15,22,24,25]

merged = sorted(a + b, reverse=True)
mask = [(False, True)[x in a] for x in merged]


flag = True
good = []
for m, n in zip(mask, merged):
    if m is flag:
        continue
    else:
        good.append(n)
        flag = not flag

pairs = list(zip(good[1::2], good[::2]))[::-1]
pairs
>>> [(1, 2), (3, 4), (6, 9), (10, 15), (20, 25)]