- Sequence Operations 0:57
- Slices 4:29
- Creating Slices 1 objective
- Len, Min, and Max 3:04
- Finding len, min, max 3 objectives
- Membership Testing 2:16
- Count and Index 4:38
- Code Samples: Membership Testing, Count, and Index
- Concatenation and Multiplication 3:04
- Sequence Operations Cheat Sheet
- Recap of Sequence Operations 4 questions
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Slices
4:29Discover slicing in Python.
A slice is a portion of a sequence.
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Much like a range, a slice is created by
providing a start, stop, and step value.
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The start and stop values refer to
the index of elements in a sequence.
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You're already familiar with
the syntax for slicing.
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It's very similar to the syntax for
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accessing an individual
element of a sequence.
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You're just expanding on it by telling
the interpreter to access several
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elements of the sequence by telling
you where to start and stop.
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And if you want, adding a step value.
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A start value of 1, for
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instance, will create a slice that begins
with the second element in the sequence.
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And a stop value of 4,
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means the sliced sequence will end at
the fifth element in the sequence.
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But just like with ranges,
it won't include it.
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This slice will include all
of the elements in between.
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All right, let's take a look.
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I'm gonna work down in my command
line tool, and I'm gonna copy and
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paste in a Python list with six elements.
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Now, let's say I wanna access
the second element in this list.
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I would use the following code,
rainbow, then [1].
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Remember I use the index
1 there instead of 2,
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because python sequence
indexing begins at 0.
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So the first element in
the list will be index 0 and
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the second will be index 1 and so on.
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So this code gives us access to the second
element in the list, the string orange.
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Now if I want the second, third,
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and fourth elements in a list,
I would use a slice.
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To do that,
I will use pretty similar syntax,
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but I will add a colon after the index
1 and then I'll add the number 4.
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The 1 is my starting index of the slice
and the 4 is my ending index.
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The slice will be created with all
elements from the second element up to,
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but not including the fifth element.
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So that's basic slicing.
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But that's not all you can do with them.
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Stop values can be excluded if you want
your slice to go all the way to the very
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end of your original sequence,
including its last element.
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The syntax for that is the same, but
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just leave the stock value
blank after the colon.
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So that returned a slice from the fourth
element all the way to the end.
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Just like ranges,
slices can also accept a step value.
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If we wanna slice that only includes
every other element in the sequence,
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we could make a slice
with the step value of 2.
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In this example,
I'm going to leave the start value blank.
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By doing this, Python assumes I mean for
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the slide to start at
the beginning of the sequence.
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Then a lot of colon,
then I'll also skip the stop value,
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telling the interpreter that I want
the slice to go all the way to the end.
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And then I'll add another colon.
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Then finally, I'll add step value of 2.
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Let's see what it returns.
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Okay, good, it skipped every
other element of the list.
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Did you know you can add
a negative step value too?
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Wanna handy way to quickly
reverse a sequence?
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In our above example,
let's change the step value of 2 to -1.
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Pretty neat, right?
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It's important to note that slicing
does not change a sequence in place,
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it creates a new sequence.
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The rainbow sequence is
still in its initial order
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even after running this code.
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There are methods to reverse or
change mutable sequences in place, but
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slicing won't accomplish that.
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Slicing works with all Python sequences,
including strings.
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Take a look at this example.
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I can use a slice to grab just my
nickname from this string, Ash.
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So I've excluded a start value to indicate
that I want to start at the very beginning
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of the string.
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And I've included a stop value to indicate
that I want every element of the sequence
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up to but not including the fourth
element which has an index of 3.
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Okay, so that's suggestive slicing.
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Open the attached workspace,
inside you'll find a file with a couple of
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Python sequences and
splices of those sequences.
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For each one, I want you to manipulate
the start, stop, and step values.
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Use negative step values, leave values
blank, play around until the cause and
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effect of different permutations of start,
stop, and step values.
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When you feel you're ready,
move on to the next step.
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