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Deleting the items from the end of the list
needs a bit more thought.

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If there are billions of items, we don't to
work through the entire list to find the first

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value to be deleted.

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Instead, we can process the list from the end.

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So let's have a go at doing that.

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On line 18 I'm going to type a comment; we're going to process the high values in the list.

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We're going to type start is equal to zero.

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On line 19, we're initializing start to zero.

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Next, we need a for loop.

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And we want to work backwards,

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which means that we'll start at the index position 1 less

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than the length of the list.

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So I'm going to type for index in range
parenthesis len parenthesis data right parenthesis

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minus 1 comma.

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Now we can't use 0 as the stop value.

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We discussed that in the videos on indexing.

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Remember that the last value in a range or slice isn't included.

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If we use 0, the first item in the list won't
be included.

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And that's why we're using -1 as the stop value.

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I'm going to go ahead and put -1 there.

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And finally, to work backwards, we use a step of -1, so I'm going to do that.

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right parenthesis and a colon.

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When you're creating ranges like this, I suggest
you print out the values that your range produces.

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When you're starting out with all this, it
can be very confusing.

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Heck, when you've been doing it for years,
it can still be confusing.

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So let's make sure we're getting the index
numbers we want.

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So there are 16 items left in our list, so
we want to get the values from 15 to 0.

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So what I'll do is I'll print out index to confirm
that our range is correct.

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I said 16 there because the earlier code
has deleted the first two items.

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Our list started off with 18 items,  but by the

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time we get to this code that we're writing, it'll contain only 16 items.

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Let me go ahead and print(index)

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Then we'll run this

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You can see that's working, we've got 0 to 15 being output.

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Alright, so we're going to be iterating backwards through the list, from 15 to 0.

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If we find a value that's within the valid
range - so a value that's less than or equal

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to 200 - we can break out of the loop.

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So let's look at doing that.

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I'll delete the printout, and change the code to be

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if data left square brackets, index within those brackets,

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that's less than or equal to max underscore valid

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then a colon, then on the next line, start is equal to index, and then we're going to break.

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This gives us a start value that's equal to

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the position of the last item that we want
to keep.

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So with our data,

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that should be 13.

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The last value we want is 191, and that's at index position

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13.

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So we'll print out start to confirm
it's what we expect.

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So print start, in parentheses, and a comment there for debugging.

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We'll run this, to confirm that we get 13.

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We got 13, so that's working fine.

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Next, we want to delete the slice that includes the values at the end of the list.

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Let's go ahead and write the code for that.

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del data left square bracket, start colon within the square brackets.

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Let's print data after that, as well.

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So our slice on line 26 includes everything
from the start value to the end of the list.

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So we're going to delete everything from position 13 to the end.

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Now that, by the way, will include the value 191, which isn't what we want.

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For our data, we want to start deleting at
position 14, not from 13.

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So what we we need to do is add one to the start value.

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And we can do that on line 26.

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Come up here and type start plus one.

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That'll work - our slice will now start at
position 14.

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Now we can do the same thing by changing line
22 instead.

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So I'm going to undo the change I've just made.

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So let's put the change, or modify, line 22, to be start = index + 1

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Now that does the same thing, but the code's a bit cleaner.

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As far as the computer's concerned, there's no difference.

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We'll still be removing all the items from
position 14. However, the code's safer for humans

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to work with.

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The purpose of our for loop is to identify
the first item to delete.

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It sets start to the position to start deleting from, rather than from the position of the last

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item we want to keep, so that just makes more sense.

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This is the sort of thing that comes with
experience.

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Both versions would work fine, but it's cleaner
to have the correct start position when the

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loop terminates.

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Having to alter start, after the loop, is
a bit clumsy and it is something that's easy

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to forget.

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But don't worry too much about that.

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When you're starting, you'll be pleased to just get code that works.

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You'll understand

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why this version's slightly easier to work
with, when we've looked at writing our own functions.

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What I probably should do,

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is add a comment to explain why we're adding 1 to the value.

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Let's go ahead and do that, we'll do it right under the if statement, on line 21.

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So basically we have the index of the last item to keep

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So we want to set start to the position of
the first item to delete, which is one after

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index.

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Let's just type that.

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and that's one after index.

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I'm putting the variable names in single quotes there, in the comments.

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Alright, so now that I've done that, let's see if it works. I'm going to run the program

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And you can see that the list no longer contains the first two

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values, and the last two values have also been stripped out.

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So that seems to be working fine
- at least it's working fine with this particular

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set of data.

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In the next video, we'll look at testing it
more thoroughly.

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See you in the next video.