Python find element position in list

How to Find the Position of an Element in a List Using Python — A Comprehensive Guide

Learn how to find the position of an element in a list using Python. This guide explores different methods, including index(), for loop, bisect module, dictionary, and sorted() function.

• Using index() method to find element position in list
• Finding all occurrences of an element in a list in Python
• How to Find Position in List (Index)
• Using bisect module to find the position of an element in a list in Python
• Using dictionary to find element position in list in Python
• Using sorted() function to find element position in list in Python
• Other helpful code examples for «How to Find the Position of an Element in a List Using Python»: «Additional Python code examples for finding element position in a list
• Conclusion
• How do you find the position of an element in a list in Python?
• How do you find the position of an element in a list in Python using for loop?
• How do I print a specific position in a list Python?

Python is a powerful and versatile programming language that has become increasingly popular over the years. Python provides a wide range of built-in functions, including index(), which can be used to find the position of an element in a list. In this blog post, we will explore different ways to find the position of an element in a list using Python.

Using index() method to find element position in list

The index() method takes an element as an argument and returns the index of the first occurrence of that element in the list. The syntax of the method is:

list.index(element, start, end) 

• list is the list in which the element is to be searched.
• element is the element whose position is to be found.
• start (optional) is the starting index of the search. If not specified, the search starts from the beginning of the list.
• end (optional) is the ending index of the search. If not specified, the search ends at the end of the list.

Here is an example code to find the position of an element in a list using index() method:

list1 = [10, 20, 30, 40, 50] element = 30 position = list1.index(element) print("The position of", element, "in the list is", position) 

The position of 30 in the list is 2 

However, it is important to note that the index() method raises a ValueError if the element is not found in the list. Therefore, it is advisable to use the in keyword to check if the element exists in the list before using the index() method.

Finding all occurrences of an element in a list in Python

Sometimes, you may need to find all occurrences of an element in a list. To achieve this, you can use a for loop and the index() method. Here is an example code to find all positions of an element in a list using a for loop and index() method:

list1 = [10, 20, 30, 20, 40, 50, 20] element = 20 positions = [] for i in range(len(list1)): if list1[i] == element: positions.append(i) print("The positions of", element, "in the list are", positions) 

The positions of 20 in the list are [1, 3, 6] 

It is worth noting that this method is not very efficient for large lists with multiple occurrences of the element. In such cases, it is advisable to use other methods such as the bisect module.

How to Find Position in List (Index)

How to find position in list (Index) | Python Tutorial | ProgrammerParkerHow to find an Duration: 2:29

Using bisect module to find the position of an element in a list in Python

The bisect module provides a function named bisect_left() that can be used to find the position where an element should be inserted in a sorted list to maintain the order of the list. The bisect_left() function has the following syntax:

bisect_left(a, x, lo=0, hi=len(a)) 

• a is the sorted list in which the element is to be searched.
• x is the element whose position is to be found.
• lo (optional) is the starting index of the search. If not specified, the search starts from the beginning of the list.
• hi (optional) is the ending index of the search. If not specified, the search ends at the end of the list.

Here is an example code to find the position of an element in a sorted list using bisect_left() function:

import bisectlist1 = [10, 20, 30, 40, 50] element = 30 position = bisect.bisect_left(list1, element) print("The position of", element, "in the list is", position) 

The position of 30 in the list is 2 

It is important to note that the bisect_left() function returns the position where the element should be inserted to maintain the order of the list. Therefore, if the element is not found in the list, the returned position may not be correct.

Using dictionary to find element position in list in Python

Another way to find the position of an element in a list is by creating a dictionary with list elements as keys and their indexes as values. Here is an example code to create a dictionary with list elements as keys and their indexes as values:

list1 = [10, 20, 30, 40, 50] dict1 = <> for i in range(len(list1)): dict1[list1[i]] = i print(dict1) 

Once the dictionary is created, you can use it to find the position of an element in the list. Here is an example code to find the position of an element in a list using the dictionary:

element = 30 position = dict1.get(element, -1) print("The position of", element, "in the list is", position) 

The position of 30 in the list is 2 

It is important to note that this method requires extra memory to create the dictionary. Therefore, it may not be suitable for large lists.

Using sorted() function to find element position in list in Python

If the list is not sorted, you can use the sorted() function to sort the list elements and then use the index() method to find the position of an element in the sorted list. Here is an example code to find the position of an element in a list using sorted() function:

list1 = [50, 10, 40, 20, 30] element = 30 sorted_list = sorted(list1) position = sorted_list.index(element) print("The position of", element, "in the list is", position) 

The position of 30 in the list is 2 

It is important to note that this method changes the original order of the list.

Other helpful code examples for «How to Find the Position of an Element in a List Using Python»: «Additional Python code examples for finding element position in a list

In python, position in list python code example

animals = ['cat', 'dog', 'rabbit', 'horse'] index = animals.index('dog') print(index) #Output => 1 

In python, how to get something from a certain position in a list python code example

lst = [1,2,3] print(lst[1]) # will print 2

In python, how to find the position in a list python code example

lst = [4, 6, 5] lst.index(6) # will return 1 

In python, find an index of an item in a list python code example

#Example List list = ['apples', 'bannas', 'grapes'] #Use Known Entites In The List To Find The Index Of An Unknown Object Index_Number_For_Bannas = list.index('apples') #Print The Object print(list[Index_Number_For_Bannas]) 

In python, python list find index of item code example

In python, get index of item in list code example

list.index(element, start, end)

Conclusion

In this blog post, we have discussed different ways to find the position of an element in a list using Python. We explored the index() method, for loop and index() method, bisect module, dictionary, and sorted() function. By using these methods, you can easily find the position of an element in a list and perform further operations on it. It is important to choose the appropriate method based on the size and characteristics of the list. We hope that this comprehensive guide has been helpful to you.

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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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