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Python with two iterators increments one at a time

I am new to python.

I tried to implement something like merge sort in python. I used a single list from https://pythonhosted.org/llist/#sllist-objects . To combine two sorted lists, I need to traverse both lists using an iterator. The pseudocode looks something like this:

n3 = sllist()
for n1 in list1 and n2 in list2:
    if (n1 > n2):
        n3.append(n1)
        n1++               # c way of doing thing
    else:
        n3.append(n2)
        n2++               # c way of doing thing

But I don't know how to get this to work in python. Any pointer or hint would help.

Edit: After all the discussions and suggestions, I came up with a code like this. Can anyone tell me how to get rid of the last two loops. I was planning on using "extension" but was unable to use it.

final_list = sllist()
node1 = list1.first
node2 = list2.first

while node1 and node2:
    if node1.value < node2.value:
        final_list.append(node1)
        node1 = node1.next              
    else:
        final_list.append(node2)
        node2 = node2.next
while node1:
    final_list.append(node1)
    node1 = node1.next
while node2:
    final_list.append(node2)
    node2 = node2.next

return final_list
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2 answers


I usually do this with iteration and next

:

lst1 = iter(list1)
lst2 = iter(list2)
out = sllist()
sentinel = object()
n1 = next(lst1, sentinel)
n2 = next(lst2, sentinel)
while n1 is not sentinel and n2 is not sentinel:
    if n1 > n2:
        out.append(n2)
        n2 = next(lst2, sentinel)
    elif n2 >= n1:
        out.append(n1)
        n1 = next(lst1, sentinel)
out.extend(lst1)
out.extend(lst2)


As pointed out in the comments, you can also write it like:

lst1 = iter(list1)
lst2 = iter(list2)
out = sllist()
try:
    n1 = next(lst1)
    n2 = next(lst2)
    while True:
        if n1 > n2:
            out.append(n2)
            n2 = next(lst2)
        elif n2 >= n1:
            out.append(n1)
            n1 = next(lst1)

except StopIteration:  # raised when next(...) fails.
    out.extend(lst1)
    out.extend(lst2)

It is functionally equivalent. Pick your choice :-)

+3


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Ok how you do it you need indices

So:



while i < len(list1) and j < len(list2):
  if list1[i] > list2[j]:
    n3.append(list1[i])
    i+=1
  else:
    n3.append(list2[j])
    j+=1

n3.extend(list1[i:])
n3.extend(list2[j:])
+1


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