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In this video we will cover lists and tuples.

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These are called compound data types and are one of the key types of data

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structures in Python. Tuples.

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Tuples are an ordered sequence. Here is a tuple

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ratings. Tuples are expressed as comma separated elements within parentheses.

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These are values inside the parentheses. In Python there are different types:

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strings, integer, float. They can all be contained

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in a tuple but the type of the variable is tuple.

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Each element of a tuple can be accessed via an index.

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The following table represents the relationship between the index and the

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elements in the tuple. The first element can be accessed by the

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name of the tuple followed by a square bracket with the index number,

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in this case zero. We can access the second element as follows.

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We can also access the last element. In Python we can use negative index.

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The relationship is as follows. The corresponding values are shown here.

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We can concatenate or combine tuples by adding them.

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The result is the following with the following index.

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If we would like multiple elements from a tuple we could also slice

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tuples. For example, if we want the first three elements

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we use the following command. The last index is one larger than the

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index you want; similarly if we want the last two elements,

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we use the following command. Notice how the last index is one larger than the

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length of the tuple. We can use the len command to obtain the

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length of a tuple. As there are five elements the result is 5.

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Tuples are immutable which means we can't change them.

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To see why this is important let's see what happens when we set the variable

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ratings 1 to ratings. Let's use the image to

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provide a simplified explanation of what's going on.

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Each variable does not contain a tuple, but references the same immutable tuple

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object. See the objects and classes module for

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more about objects. Let's say we want to change the

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element at index 2. Because tuples are immutable we can't,

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therefore ratings 1 will not be affected by a change in rating

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because the tuple is immutable, i.e we can't change it.

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We can assign a different tuple to the ratings variable.

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The variable ratings now references another tuple.

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As a consequence of immutability, if we would like to manipulate a tuple

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we must create a new tuple instead. For example,

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if we would like to sort a tuple we use the function sorted.

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The input is the original tuple, the output is a new sorted

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list. For more on functions see our video on functions.

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A tuple can contain other tuples as well as other complex data types.

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This is called nesting. We can access these elements using the standard

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indexing methods. If we select an index with a tuple, the

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same index convention applies. As such,

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we can then access values in the tuple. For example, we could access the second

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element. We can apply this indexing directly to

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the tuple variable NT. It is helpful to visualize this as a

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tree. We can visualize this nesting as a tree.

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The tuple has the following indexes. If we consider indexes with other tuples,

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we see the tuple at index 2 contains a tuple with two

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elements. We can access those two indexes. The same convention applies to index

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3. We can access the elements in those

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tuples as well. We can continue the process. We can even

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access deeper levels of the tree by adding another square bracket.

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We can access different characters in the string or various elements in the

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second tuple contained in the first. Lists are also a popular

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data structure in Python. Lists are also an ordered sequence.

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Here is a list, "L." A list is represented with square brackets.

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In many respects lists are like tuples. One key difference is they are mutable.

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Lists can contain strings, floats, integers.

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We can nest other lists. We also nest tuples and other data structures.

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The same indexing conventions apply for nesting

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Like tuples, each element of a list can be accessed via an index.

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The following table represents the relationship between the index and the

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elements in the list. The first element can be accessed by the name of the list

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followed by a square bracket with the index number,

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in this case zero. We can access the second element as follows.

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We can also access the last element. In Python, we can use a negative index;

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the relationship is as follows. The corresponding indexes

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are as follows. We can also perform slicing in lists. For example, if we want

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the last two elements in this list we use the following command.

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Notice how the last index is one larger than the length of the list.

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The index conventions for lists and tuples are identical.

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Check the labs for more examples. We can concatenate or combine lists by adding

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them. The result is the following. The new list

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has the following indices. Lists are mutable, therefore we

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can change them. For example, we apply the method extends

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by adding a dot followed by the name of the method then

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parentheses. The argument inside the parentheses is a

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new list that we are going to concatenate

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to the original list. In this case, instead of creating a new list,

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"L1," the original list, "L," is modified by adding two new elements.

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To learn more about methods check out our video on objects and classes.

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Another similar method is append. If we apply append instead of extended,

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we add one element to the list. If we look at the index

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there is only one more element. Index 3 contains the list we appended.

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Every time we apply a method the list changes.

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If we apply extend, we add two new elements to the list.

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The list L is modified by adding two new elements.

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If we append the string A, we further change the list,

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adding the string A. As lists are mutable we can change them.

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For example, we can change the first element as follows.

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The list now becomes hard rock 10 1.2. We can delete an element of a list

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using the del command. We simply indicate the list item we

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would like to remove as an argument. For example, if we would like to remove

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the first element the result becomes 10 1.2.

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We can delete the second element. This operation removes the second element off

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the list. We can convert a string to a list using

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split. For example, the method split converts

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every group of characters separated by a space

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into an element of a list. We can use the split function to separate strings on a

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specific character known, as a delimiter. We simply pass the

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delimiter we would like to split on as an argument, in this case a comma.

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The result is a list. Each element corresponds to a set of characters that

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have been separated by a comma. When we set one variable B equal to A,

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both A and B are referencing the same list.

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Multiple names referring to the same object is known as aliasing.

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We know from the list slide that the first element in B is set as hard rock.

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If we change the first element in A to banana,

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we get a side effect, the value of B will change as a consequence.

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A and B are referencing the same list, therefore if we change A,

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list B also changes. If we check the first element of B

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after changing list A, we get banana instead of hard rock.

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You can clone list A by using the following syntax.

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Variable A references one list. Variable B

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references a new copy or clone of the original list.

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Now if you change A, B will not change. We can get more info on lists, tuples, and

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many other objects in Python using the help command. Simply pass in the list, tuple, or any other Python object.

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See the labs for more things you can do with lists.