Question

I need to iterate on two lists in the following way:

Pseudo code:

j=1
for i=1 to n:
   print a[i], b[j]
   while b[j+1] <= a[i]:
      j++
      print a[i], b[j]

For example:

a = [1 3 5 7]
b = [2 4 9] 

Desired output:

1 2
3 2
5 2
5 4
7 4

How do you do it cleanly in python?

Answer 1

Your pseudo code will almost work in Python. Some working code that does what you want is:

a = [1, 3, 5, 7]
b = [2, 4, 9] 
j = 0
for i in range(len(a)):
    print a[i], b[j]
    while j<len(b)-1 and b[j+1] <= a[i]:
        j += 1
        print a[i], b[j]

Note the few changes to make it work in Python:

1. When declaring the list, commas are required between items.
2. List indices start at 0, so both i and j should start there.
3. len(a) returns the length of a (4 in this case), and iterating i through range(len(a)) executes the loop for each integer from 0 to len(a)-1, which is all of the indices in a.
4. The ++ operation is not supported in Python, so we use j +=1 instead.
5. We have to avoid using out of bounds indices of b, so we test to make sure j will be in bounds before incrementing it.

This code can be made more pythonic by iterating through the list as follows:

a = [1, 3, 5, 7]
b = [2, 4, 9] 
j = 0
for element in a:
   print element, b[j]
   while j<len(b)-1 and b[j+1] <= element:
      j += 1
      print element, b[j]

In general, you probably don’t want to just print list elements, so for a more general use case you can create a generator, like:

def sync_lists(a, b)
    if b:
        j = 0
        for element in a:
            yield (element, b[j])
            while j<len(b)-1 and b[j+1] <= element:
                j += 1
                yield (element, b[j])

And then you can print them as before with

a = [1, 3, 5, 7]
b = [2, 4, 9]
for (e1, e2) in sync_lists(a, b):
    print e1, e2