String Slicing using a Negative Index || Start : Stop : Step Index || What? How? || Python
String Slicing using a Negative Index || Start : Stop : Step Index || What? How? || Python
This video will discuss WHAT and How of String Slicing using a Negative Index with a lot of EXAMPLES and ANIMATIONS. Please subscribe to our channel for more such videos.
— Types of Indexes — What is a Negative Index? — How are strings indexed using both Positive and Negative Indexes? — How are Strings sliced using a Negative Index? — Scenario 1: No Step Size while slicing using a Negative Index — Scenario 2: String Slicing using a combination of Positive and Negative Index — Scenario 3: Slicing using the Positive Indexes and a Negative Step Size — Scenario 4: How to Reverse a String?
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Content
0.02 -> In today’s video, we will be discussing:
2.23 -> 1) Types of Indexes
2) What is a Negative Index?
5.17 -> 3) How are strings indexed using both positive
and negative indexes?
8.98 -> 4) How are strings sliced using a negative
index?
12.16 -> 5) Scenario 1: No Step Size while slicing
using a Negative Index
16.04 -> 6) Scenario 2: String Slicing using a combination
of Positive and Negative Index.
20.66 -> 7) Scenario 3: Slicing using the Positive
Indexes and a Negative Step Size
24.8 -> 8) Scenario 4: How to Reverse a String
Hey champions, if you are interested in these
29.39 -> topics, then keep on watching. You can subscribe
to our channel for more such videos.
33.23 -> First, let’s understand - What exactly is
a Negative index?
36.66 -> To understand it, let’s first see what is
an index in Python?
40.09 -> Python Strings are an array of bytes. And
to access an individual element from this
44.75 -> array, we need to know its position in the
array. This position is called an index.
48.98 -> We have covered string indexing in detail
in one of our videos. If you are interested
53.79 -> in the topic then the link is in the description
box.
56.76 -> An Index in Python can be of two types:
58.7 -> 1) The first type is the Positive index – Here
the pointer moves from left to right in the
63.6 -> array. Since it is the default behaviour in
Python, hence we can refer to it by index.
69.09 -> The positive index starts with 0.
2) The next type is the negative index – As
73.259 -> the name suggests, the functionality of this
index is opposite to that of the positive
77.899 -> index. Here the pointer moves from right to
left in the array. It is also known as the
83.029 -> Reverse index. To depict the negative index,
a negative sign is added just before the index
88.009 -> and starts with -1.
90.009 -> Now that we know about both the indexes in
python, Let’s see how are strings indexed
94.99 -> using both of these indexes.
We have a string ‘Champ’ and here is how
98.899 -> it is indexed using a positive and a negative
index. As you can see a positive index starts
103.929 -> from the left of the string and has 0 as the
starting index, while a negative index starts
109.179 -> from the right of the string and has -1 as
the starting index.
112.619 -> Interesting, isn’t it?
114.459 -> We have made a separate video on String Slicing
with a positive index on this channel. If
118.61 -> you are interested in the topic then the link
is in the description box.
121.779 -> Let’s now move on to our next topic which
is how to slice a string using a negative
125.92 -> index.
Slicing using a negative index is quite similar
129.149 -> to that done using a positive index, but one
of the distinguishing features between both
134.02 -> is that when the slicing is done using a negative
index, the step size is negative while when
139.32 -> the slicing is done using a positive index,
the step size is positive. This is important
143.92 -> because the step size will direct the direction
of the pointer. If the step size is positive,
148.47 -> then the pointer will move from left to right
in the array but if this index is negative
152.65 -> then the pointer will move from right to left
in the array.
155.55 -> Let’s look at an example to better understand
this concept.
158.34 -> For our example, we have taken a string ‘Learning
Champion’ and to slice it using a negative
163.35 -> index we have given mystring[-1:-5:-1] as
the slicing command to Python. Once we execute
171.62 -> this command, it will return ‘noip’ as
the output. Here, -1 is the step size which
177.12 -> will move the pointer from right to left,
-1 is the start index which is the index of
181.63 -> the last character in the original string
and -5 is the stop index which is the index
186.27 -> of letter m in the string but as we all know
that this index will not be included in the
191.24 -> output. Also, note that in this example start
index is greater than the stop index which
196.32 -> is not the case when we do slicing using a
positive index.
199.57 -> Hence, whenever we want to do slicing using
a negative index, we have to keep 2 things
204.02 -> in mind :
1) The first is that step size should be negative
207.55 -> 2) And second is that the start index should
always be greater than the stop index because
212.74 -> now we are moving from right to left.
I hope the example made everything easier
216.91 -> to understand. Let’s now look at a few other
scenarios
219.99 -> Scenario 1: No Step Size while slicing using
a Negative Index
223.9 -> For this scenario, we have taken -1 as the
starting index and -5 as the stop index but
229.89 -> have not defined the step size. Since we have
not mentioned any step size, Python will assume
234.79 -> it as 1 by default.
After executing the command, we will notice
238.64 -> that the Python has returned Nothing!
But why? Isn’t it odd?
242.84 -> This is because the default step size is 1
which will move the pointer from left to right.
247.29 -> But when we gave the starting index as -1
and the ending index as -5, we are telling
253.27 -> the Python pointer to move from right to left
as the starting index is greater than the
257.509 -> stop index. Python did not return anything
because of the contradicting inputs.
262.61 -> Scenario 2: String Slicing using a combination
of Positive and Negative Index.
267.039 -> for our second scenario, We have given mystring[-1:13:-1].
When we run this command, Python will return
275.319 -> ‘noi’ as the result. But shouldn’t this
be an error? Here the starting index is not
279.93 -> greater than the stop index but still, Python
has given us a valid output. This is because
284.909 -> each element is represented by a negative
as well as a positive index. It doesn’t
289.18 -> matter which index you mention as long as
the start index is before the stop index when
293.75 -> we specify -1 as the step size.
Let’s move on to the next scenario, which
298.419 -> is
Scenario 3: Slicing using the Positive Indexes
301.27 -> and a Negative Step Size
In this scenario, we have written mystring[7:0:-1]
308.199 -> and when we execute this command the output
will be a substring that will start from the
312.919 -> 7th index till the 0th index but will not
include the element at the 0th index. Hence,
318.37 -> we can also slice the string from right to
left using both the positive start and stop
323.219 -> index as long as the step size is negative.
325.55 -> Scenario 4: How to Reverse a String?
327.55 -> Let’s move on to the last scenario, where
we try to reverse the string we have.
332.24 -> For it, we have written mystring[::-1] and
the output is the reverse of the initial string
338.28 -> we gave. Here notice a few things. First,
as we did not mention the starting index,
343.3 -> Python has assumed it as -1 by default based
on the step size we have given and, second,
348.719 -> we also did not mention the stop index, Python
has assumed it as the starting of the string
353.819 -> by default based on the step size we have
given. Here the step size is the governing
357.969 -> factor that will decide the default values
of start and stop indexes along with moving
362.639 -> the pointer from right to left.
That’s it for today’s video. I hope all
366.319 -> the examples made the concept easier for you.
You can share your thoughts or questions in
370.479 -> the comment box. Subscribe for more such videos.
Bye and meet you in the next one.
