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- [Instructor] In this
video we'll continue

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the iPython session
from the preceding video

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and we'll keep working with
our list called numbers

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this time to modify the
contents of the list

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using slicing operations.

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So let me paste in our
initial slice operation

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that we're going to discuss here.

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We have the list numbers, and we're saying

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select the subset of
that list with the slice

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of that list from index zero,

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up to but not including index three.

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Now just at the top here,
here's the original list

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that we're talking about.

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So from index zero we're
going to go zero, one, two,

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up to but not including
index position three.

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So we're talking about
the slice containing two,

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three, and five, and we're
going to assign to that slice

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this list with the same
number of elements.

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In this case two, three,
and five as the words

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that we would like to
replace the numeric values.

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And by the way, when you're
assigning to a slice,

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you do not need to

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have the same number of elements

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in the new slice as in the original slice,

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so you can actually
change the size of a slice

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by assigning a sequence
of values to that slice,

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and we'll talk about that a
couple of times in this video.

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So let's execute that and
of course to check what's

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now in the list numbers
we can evaluate it,

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and you can see that
we've replaced the first

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three elements with the word
equivalents of those values.

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Now in addition to simply
replacing values within a slice

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we can delete elements using
slice notation as well.

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So, for example, if I say number
sub-zero colon three again

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we're still selecting the first
three elements of that list.

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And if we assign to that an empty list,

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that literally says delete that slice

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from the original list.

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So in this case to prove
that we can go ahead

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and say numbers, and now you see the first

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three elements have been
physically removed from the list.

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Now

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so far, we've demonstrated

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a couple of modification operations.

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Let's bring back the original
list for the next operation

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that we're going to present to you.

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And by the way, here's
a handy little trick

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that you can use to
recall a prior snippet.

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So I know at some point previously,

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I assigned something to
the variable numbers.

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If I type in the beginning
of that assignment

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and hit the up arrow key it
will recall the last statement

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that started with what I had typed so far.

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So I'm now going to reset
the variable numbers

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to a list containing the same values

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that we started with
in the preceding video.

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Now,

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we can

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assign

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slices to the list

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in non-consecutive elements,

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and so that's what I want
to demonstrate next here.

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So if I say numbers, colon, colon, two,

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that says for every other element

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starting from the first
element of the list numbers,

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we would like to select that slice.

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And let's say we wanted
to assign the value 100

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to every other element.

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So we have eight elements
in this case, we want to

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place 100 into element zero,

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two, four, and six.

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So as a result of this it's going to take

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the first value in the right-hand side and

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place it into the first
element of the resulting slice,

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the second value on the
right-hand side will be placed

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into the second element
of the resulting slice,

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and we'll do the same thing for the third

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and fourth elements of the
resulting slice as well.

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And of course if we now say numbers,

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you can see the four 100 values placed

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into the corresponding
locations within that list.

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Now for the next part of this example,

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the first thing I want to do
is just take the ID of numbers,

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because I want to show you
when new objects are being

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created versus modifying
the existing object.

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So we'll simply leave
this here for the next

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part of the discussion.

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So we showed you up above that you can

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assign an empty list to a slice

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to delete elements from that list.

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Well, it turns out that
you can use that technique

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to delete all of the
elements of the list as well.

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So for example, if I
say numbers, sub-colon,

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like that,

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it's going to implicitly
insert the zero for

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the starting index, and
the length of numbers for

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the ending index.

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And I can then assign
to that an empty list

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which literally deletes all the elements

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from the original list.

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So if I go ahead and
execute that operation,

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I can evaluate numbers to
see that indeed it's empty.

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And I just want to prove
to you that it's the same

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list object still.

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So let me recall my ID expression here,

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and when I execute that you can see

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that the ID's of these
two objects are identical,

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so it's still the same numbers list,

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we've simply deleted all
the elements by using

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a slice operation on the
left side of the assignment.

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Now that's very different
from doing this assignment

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which simply aims the variable numbers

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at a completely new list object
that contains no elements.

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So let's go ahead and execute that,

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and of course if we evaluate numbers

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we still see that it's empty,

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but if we recall that ID
expression once again,

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you can see this time that indeed

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we have a new object identity,

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so this was a physically new object,

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as opposed to modifying the existing one.