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5 tips you need to know before use Python List Index Method

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In Python, you can use the index() method to get the index of an item in a list. This is a very useful tool for finding specific items in a list. In this blog post, we will show you how to use the index() method to find the index of an item in a list.

We will also provide some examples so that you can understand how it works. Let’s get started!

Syntax of Python List index method

The syntax of the list index() method is: list.index(element, start, end). The list index() method can take a maximum of three arguments.The index() method returns the index of the given element in the list. If the element is not found, a ValueError exception is raised.

  • element – the element to be searched
  • start (optional) – start searching from this index
  • end (optional) – search the element up to this index

 

Note: The index() method only returns the first occurrence of the matching element.

Example of Python List index method

The first thing you need to do is create a list. You can do this by using the list() method. For example, we will create a list of numbers called my_list.my_list = [0, 1, 20, 30, 40]

Now that you have created a list, you can use the index() method to find the index of an item in the list. The syntax for this method is as follows: list_name.index(item). For our example, we will use the following code to find the index of 20 in our list:my_list.index(20)

As you can see from the output, the index() method returns the index of the first occurrence of the item in the list. In our example, 20 is at index position two.

using start parameter in Python List index method

If you want to find the index of an item that appears more than once in a list, you can use the following syntax:list_name.index(item, start). The start parameter is optional and it specifies where in the list we should start looking for the item. For example, if we want to find the second occurrence of 20 in our list, we would use this code:

my_list = [0, 1, 20, 30, 40, 20]
my_list.index(20, 3 )

As you can see from the output, this time we get 20 at index position five. We can find all indexes of an item in list with this command. [ i for i in range(len(my_list)) if my_list[i] == 20 ]

using negative numbers in Python List index method

You can also use negative numbers for the start parameter. If you do this, the index() method will start looking for the item at the end of the list and work its way backwards. For example, if we want to find the last occurrence of 20 in our list, we would use this code: my_list.index(20, -1 ).As you can see from the output, this time we get 20 at index position five.

Error message in Python List index method

One important thing to note is that if the index you are trying to find is not in the list, Python will throw an error. For example, if we try to find the index of 50 in our list, we get an error: my_list.index(50). As you can see, Python raises a ValueError when the item is not in the list.

One final note on the index() method is that it is case-sensitive. This means that if you are looking for an item in a list that is all lowercase, you will not find it if the item in the list is uppercase. For example, if we try to find the index of “a” in our list, we get an error:

my_list = [‘A’, ‘B’, ‘C’]
my_list.index(‘a’)

As you can see from the output, Python raises a ValueError when the item is not in the list.

 Best practice to use Python list index method

  • When you know the index of the element you want to retrieve, use the list[index] notation.
  • If you want to find the index of the last occurrence of an item in a list, use the list.index(item, -1) notation.
  • If you want to find the index of the first occurrence of an item in a list, use the list.index(item, 0) notation.

 

That’s all for this tutorial! In summary, the index() method is a powerful tool that you can use to find the index of an item in a list. I hope you found this tutorial helpful. Thanks for reading!