Merge Sort Problems - Python

Question

What's wrong with my code? It prints only a part of the vect values. Seems that the while loop breaks at some point. I don't understand why.

def print_list(vect):
    for i in range(0, len(vect)):
        print(vect[i])

def merge_sort(vect):
    left = []
    right = []    
    result = []

    for i in range(0, int(len(vect)/2)):
        left.append(vect[i])
    for i in range(int(len(vect)/2), len(vect)):
        right.append(vect[i])

    left.sort()
    right.sort()
    i = 0
    j = 0

    while i < len(left) and j < len(right):
        if left[i] <= right[j]:
            result.append(left[i])
            i += 1
        else:
            result.append(right[j])
            j += 1

    print(len(result))
    return result

vect = [3, 1, 5, 7, 10, 2, 0]

vect = merge_sort(vect)

Answer 1

Well, your mistake is that after your while loop

while i < len(left) and j < len(right):
  ...

it may be (and most probably would) that either i < len(left) or j < len(right) , so you need to append appropriate part's suffix to the answer. It's easy to do with

result += left[i:]
result += right[j:]

Explanation:

Imagine the merge procedure: you have i and j at 0 at start, and at every step you move one of them forward. When you stop ? When one of them reaches the end. Let's say i has reached the end. Hereby you added the whole left part to the result, but there are still some elements in right between j and len(right), so you have to add them to the answer too.

Offtop :

You are implementing merge sort, so please have

left = merge_sort( left )
right = merge_sort( right )

instead of

left.sort()
right.sort()

Attention: you have to add the following check to your code at the beginning of merge function to avoid infinite recursion:

if len( vect ) == 1:
   return vect

Also in your print_list function you can just use

print vect

or at least

for x in vect
print x

Answer 2

The while loop exits as soon as either left or right is exhausted. This leaves all the elements left in the unexhausted list unmerged.

Change your code to this:

def print_list(vect):
    for i in range(0, len(vect)):
        print(vect[i])

def merge_sort(vect):
    left = []
    right = []

    result = []
    for i in range(0, int(len(vect)/2)):
        left.append(vect[i])
    for i in range(int(len(vect)/2), len(vect)):
        right.append(vect[i])

    left.sort();
    right.sort();
    i = 0
    j = 0

    while i < len(left) and j < len(right):
        if left[i] <= right[j]:
            result.append(left[i])
            i += 1
        else:
            result.append(right[j])
            j += 1

    if i < len(left):
        result.extend(left[i:])
    else:
        result.extend(right[j:])

    print(len(result))
    return result

vect = [3, 1, 5, 7, 10, 2, 0]

vect = merge_sort(vect)
print vect

If you're using the sort method on left and right, I'm a little confused as to why you're not just using it on the list as a whole. But I suppose you have your reasons. :)

Edit: I put the entire block of code there so you can see it. When I run this, it prints [0, 1, 2, 3, 5, 7, 10], which is correct.