Python return element index

Python List index()

The index() method returns the index of the specified element in the list.

Example

animals = ['cat', 'dog', 'rabbit', 'horse'] 
# get the index of 'dog' index = animals.index('dog')
 print(index) # Output: 1

Syntax of List index()

The syntax of the list index() method is:

list.index(element, start, end)

list index() parameters

The list index() method can take a maximum of three arguments:

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

Return Value from List index()

• The index() method returns the index of the given element in the list.
• If the element is not found, a ValueError exception is raised.

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

Example 1: Find the index of the element

# vowels list vowels = ['a', 'e', 'i', 'o', 'i', 'u'] # index of 'e' in vowels 
index = vowels.index('e')
 print('The index of e:', index) # element 'i' is searched # index of the first 'i' is returned 
index = vowels.index('i')
 print('The index of i:', index)

The index of e: 1 The index of i: 2

Example 2: Index of the Element not Present in the List

# vowels list vowels = ['a', 'e', 'i', 'o', 'u'] # index of 'p' is vowels 
index = vowels.index('p')
 print('The index of p:', index)

ValueError: 'p' is not in list

Example 3: Working of index() With Start and End Parameters

# alphabets list alphabets = ['a', 'e', 'i', 'o', 'g', 'l', 'i', 'u'] # index of 'i' in alphabets 
index = alphabets.index('e') # 1
 print('The index of e:', index) # 'i' after the 4th index is searched 
index = alphabets.index('i', 4) # 6
 print('The index of i:', index) # 'i' between 3rd and 5th index is searched 
index = alphabets.index('i', 3, 5) # Error!
 print('The index of i:', index)

The index of e: 1 The index of i: 6 Traceback (most recent call last): File "*lt;string>", line 13, in ValueError: 'i' is not in list

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Функция index() в Python

Метод index() возвращает индекс указанного элемента в списке.

Синтаксис метода в Python:

list.index(element, start, end)

Список параметров

• element – элемент для поиска;
• start (необязательно) – начать поиск с этого индекса;
• end (необязательно) – искать элемент до этого индекса.

Возвращаемое значение из списка

• Метод возвращает индекс данного элемента в списке.
• Если элемент не найден, возникает исключение ValueError.

Примечание: Команда возвращает только первое вхождение соответствующего элемента.

Пример 1: Найти индекс элемента

# vowels list vowels = ['a', 'e', 'i', 'o', 'i', 'u'] # index of 'e' in vowels index = vowels.index('e') print('The index of e:', index) # element 'i' is searched # index of the first 'i' is returned index = vowels.index('i') print('The index of i:', index)

The index of e: 1 The index of i: 2

Пример 2: Указатель элемента, отсутствующего в списке

# vowels list vowels = ['a', 'e', 'i', 'o', 'u'] # index of'p' is vowels index = vowels.index('p') print('The index of p:', index)

ValueError: 'p' is not in list

Пример 3: Работа с параметрами начала и конца

# alphabets list alphabets = ['a', 'e', 'i', 'o', 'g', 'l', 'i', 'u'] # index of 'i' in alphabets index = alphabets.index('e') # 2 print('The index of e:', index) # 'i' after the 4th index is searched index = alphabets.index('i', 4) # 6 print('The index of i:', index) # 'i' between 3rd and 5th index is searched index = alphabets.index('i', 3, 5) # Error! print('The index of i:', index)

The index of e: 1 The index of i: 6 Traceback (most recent call last): File "*lt;string>", line 13, in ValueError: 'i' is not in list

Источник

How to find index of element in List in Python?

To find the index of a specific element in a given list in Python, you can call the list.index() method on the list object and pass the specific element as argument.

index = mylist.index(element)

The list.index() method returns an integer that represents the index of first occurrence of specified element in the List.

You can also provide start and end positions in the List, to specify the bounds where the search has to happen in the list.

Syntax of list.index()

The syntax of index() function to find the index of element x in the list mylist is

index = mylist.index(x, [start[,end]])

start parameter is optional. If you provide a value for start, then end is optional.

Examples

In the following examples, we take a list, and an element. We then get the index of the element in the list with following use cases.

1. Get index of element in the list.
2. Get index of element in the list, where searching for the element must happen from a specific starting position in the list.
3. Get index of element in the list, where searching for the element must happen in the [starting position, ending position] bounds.
4. The item whose index we are trying to find out, is not in the list.

1. Find index of a specific item in the list

In the following example, we have taken a list with numbers. Using index() method we will find the index of item 8 in the list.

Python Program

#list mylist = [21, 5, 8, 52, 21, 87] #search item item = 8 #find index of item in list index = mylist.index(item) #print output print(f"index of in the list : ")

index of 8 in the list : 2

The element is present at 3rd position, so mylist.index() function returned 2.

2. Find index of a specific item in the list in [start, end] index bounds

In the following example, we have taken a list with numbers.

Using index() method we will find the index of item 8 in the list.

Also, we shall pass start and end indices/positions. index() function considers only those elements in the list starting from start index, till end index in mylist.

Python Program

mylist = [21, 8, 67, 52, 8, 21, 87] item = 8 start=2 end=7 #search for the item index = mylist.index(item, start, end) #print output print(f"index of in the list : ")

index of 8 in the list : 4

Explanation

mylist = [21, 8, 67, 52, 8, 21, 87] ----------------- only this part of the list is considered for searching ^ index() finds the element here 0 1 2 3 4 => 4 is returned by index() 

3. Find index of item in list where item has multiple occurrences

A Python List can contain multiple occurrences of an element. If we try to find the index of an element which has multiple occurrences in the list, then index() function returns the index of first occurrence of specified element only.

Python Program

mylist = [21, 5, 8, 52, 21, 87, 52] item = 52 #search for the item index = mylist.index(item) #print output print(f"index of in the list : ")

index of 52 in the list : 3

The element 52 is present two times, but only the index of first occurrence is returned by index() method.

Let us understand how index() method works. The function scans the list from starting. When the item matches the argument, the function returns that index. The later occurrences are ignored.

4. Find index of item in list where item is not in the list

If the element that we are searching in the List is not present, you will get a ValueError with the message item is not in list.

In the following program, we have taken a list and shall try to find the index of an element that is not present in the list.

Python Program

mylist = [21, 5, 8, 52, 21, 87, 52] item = 67 #search for the item/element index = mylist.index(item) print('The index of', item, 'in the list is:', index)

Traceback (most recent call last): File "example.py", line 5, in index = mylist.index(item) ValueError: 67 is not in list

As index() can throw ValueError, use Python Try-Except while using index(). In the following example, we shall learn how to use try-except statement to handle this ValueError.

Python Program

mylist = [21, 5, 8, 52, 21, 87, 52] item = 67 try: #search for the item index = mylist.index(item) print(f"index of in the list is : ") except ValueError: print("item not present in list")

The item, whose index we are trying to find, is not present in the list. Therefore, mylist.index(item) throws ValueError. except ValueError: block catches this Error and the corresponding block is executed. Since we have surrounded the index() function call with try-except, the program is not terminated abruptly, even when there was ValueError thrown.

Instead of using a try-except, we can first check if the element is present in the list or not. We find the index of the element in the list, if only the element is present. We shall demonstrate this using the following program.

Python Program

mylist = [21, 5, 8, 52, 21, 87, 52] item = 67 if item in mylist: index = mylist.index(item) print(f"index of in the list is : ") else: print("item not present in list")

Summary

In this Python Tutorial, we learned how to find the index of an element/item in a list using list.index() method, with the help of well detailed examples.

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