Python List index()

Python List index()

Python List index()

Python List index() method returns the index of the element, that is passed as argument, in the list. If the element is not at all present in the list, then it throws ValueError. In this tutorial, we will learn how to use index() method, when there is one occurrence, multiple occurrences or no occurrence of the element in the list.

Syntax ? list.index()

The syntax of list.index() method is myList.index(element)

where myList is the list of elements. index is the name of the method. element is the one whose index in the list, the method returns.

Example 1 ? Python list.index()

In this example, we will take a list of strings, say fruit names, and find the index of mango in the list. The following screenshot depicts the list and how index() method finds a match for the element and returns its index.

myList

apple banana cherry orange mango kiwi

0

1

2

3

4

5

myList.index( mango )

return value



Python Program

myList = ['apple', 'banana', 'cherry', 'orange', 'mango', 'kiwi'] ind = myList.index('mango') print(ind)

Output

4

list.index() ? Multiple Occurrences of Element

When the element, whose index we are trying to find, occurs multiple times in the list, list.index() method returns the index of first match and ignores the other occurrences.

myList

apple banana mango cherry orange mango kiwi

0

1

2

3

4

5

6

myList.index( mango )

return value



Python Program

myList = ['apple', 'banana', 'mango', 'cherry', 'orange', 'mango', 'kiwi'] ind = myList.index('mango') print(ind)

Output 2

list.index() ? Get All Occurrences of Element

You can use list.index() along with slicing technique to get all the occurrences of the element in the list.

In the following program, we have list of strings, and we shall find all the occurrences of the string mango in the list.

The logic is that, each time we get the index of element in the list, we slice the list from that index and find the next occurrence of the element. We collect these indexes in a list.

Python Program

myList = ['apple', 'banana', 'mango', 'cherry', 'orange', 'mango', 'kiwi'] element = 'mango' indexes = []

while True: try: slicingPos = 0 if len(indexes) == 0 else indexes[len(indexes)-1]+1 index = myList[slicingPos:].index(element) indexes.append(index+slicingPos) except ValueError as e: break

print(indexes)

Output

[2, 5]

list.index() ? No Occurrences of Element

When there is no occurrence of element, whose index we are trying to find in the list, list.index() throws ValueError.

Python Program

myList = ['apple', 'banana', 'cherry', 'orange', 'mango', 'kiwi'] ind = myList.index('watermelon') print(ind)

Output

Traceback (most recent call last): File "d:/workspace/python/example.py", line 2, in ind = myList.index('watermelon')

ValueError: 'watermelon' is not in list

This could terminate our program execution. So, how do we handle this?

Prior to making a call to index() method, check if element is present in the list.

Python Program

myList = ['apple', 'banana', 'cherry', 'orange', 'mango', 'kiwi'] element = 'watermelon'

if element in myList: ind = myList.index('watermelon') print(ind)

else: print(element, 'not in the list.')

Output

watermelon not in the list.

Or you can also use try except to handle ValueError thrown by list.index() method.

Python Program

myList = ['apple', 'banana', 'cherry', 'orange', 'mango', 'kiwi'] element = 'watermelon' try:

ind = myList.index('watermelon') print(ind) except ValueError as e: print(element, 'not in the list.')

Output

watermelon not in the list.

Conclusion

In this Python Tutorial, we learned the syntax of list.index() method and how to use it in different scenarios in your Python programs.

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