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In this video let's write the first bit of logic in our merge sort algorithm - the dividing into sublists.

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The first bit of logic we are going to
write is the divide step of the algorithm.
0:00

This step is fairly straight forward and
only requires a few lines of code, but
0:04

is essential to get
the sorting process going.
0:09

All right, so as we saw earlier,
we are going to call the function for
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the divide step split.
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So we'll say def split, and split is going
to take as an argument a list to split up.
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Let's document how this function works.
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So I'll say, divide the unsorted
list at midpoint into sublists.
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And it's always good to say,
what we're returning as well.
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So I'll say returns to sublists,
left and right.
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All right, so the first step is to
determine the midpoint of this list,
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of this array.
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We're going to use the floor
division operator for this.
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Floor division carries out
a division operation and
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if we get a non-integer value like 2.5
back, it just gets rounded down to 2.
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We'll define the midpoint to be
the length of the list divided by 2 and
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then rounded down.
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So len(list) and
using the two forward slashes for
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the floor division operator,
we'll put number 2 after it.
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Okay, once we have the midpoint,
we can use the slicing notation in Python,
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to extract portions of
the list we want to return.
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For instance, we can define
left as the left sublist that
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goes all the way from
the start of the list,
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all the way up to the midpoint
without including the midpoint.
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Now, over here we are using
this slicing syntax,
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where it's like using the subscript
notation to access a value from a list.
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But instead, we gave two index
values as a start and stop.
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If we don't include a start
value as I've done here,
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Python interprets that as starting from
the zeroth index or the start of the list.
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As similarly, we can define right to
be values on the right of the midpoint.
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So starting at the midpoint and going
all the way up to the end of the list.
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So couple things to note,
as I said earlier, when you don't include
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the starting index, it interprets it as to
start at the very beginning of the list.
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The index you give as
the stopping condition,
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that value is not included in the slice.
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So over here we're starting at
the very beginning of list, and
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we go all the way up to midpoint,
but not including midpoint.
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And then write start at midpoint,
so it includes the value and
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then goes all the way
to the end of the list.
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Now, once we have these two sublists,
we can return them.
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So we return left and right.
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Notice that we're returning
two values here and
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then in the merge to spot function,
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when we call that split function we're
declaring two variables, left half and
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right half, to assign so that we can
assign these two sublists to them, okay?
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And that's all there is
to these split function.
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In the next video, let's implement the
crucial portion of the merge short logic.
3:09

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