Python Program to Insertion at the beginning in OrderedDict

In this tutorial, we will learn how to write a program that will insert at the beginning in an OrderedDict using Python. For a given dictionary that is an OrderedDict, we have to insert items at the beginning of the dictionary.

In an OrderedDict the order of the insertion of keys is maintained, unlike in a normal dictionary where the order of the keys is not saved.

Let us look at the sample input and output of the program.

Input: original_dict = {1: 'a', 2: 'b', 3:'c'} item to be inserted (4, 'd')

Output: {4: 'd', 1: 'a', 2: 'b', 3: 'c'}

To execute this task we can use the following approaches:

1. Using concatenation operator (+)
2. Using move_to_end() method

Approach 1: Using concatenation operator (+)

The concatenation operator (+) works for joining strings and lists in Python. We can convert the values in the dictionary in the list format and then using this operator, we can join the item which has to be inserted, and the given dictionary in a new dictionary, so that the item will be added in the beginning.

Algorithm

Follow the algorithm to understand the approach better.

Step 1- Import OrderedDict from collections

Step 2- Initialise dictionary with items

Step 3- Initialise item which has to be inserted

Step 4- Declare a dictionary that will store the result

Step 5- Use (+) to add the item at the beginning of the dictionary

Step 6- Print the new dictionary as the result

Python Program 1

Look at the program to understand the implementation of the above-mentioned approach. To get the key-value pairs from the dictionary, we have used the items() method which will return all the items present in a dictionary. We have used the list() method to convert the items of the dictionary in list format.

from collections import OrderedDict
dic1 = OrderedDict([('A', '100'), ('B', '200'), ('C', '300')])
insrt = OrderedDict([("D", '400')])
  
final = OrderedDict(list(insrt.items()) + list(dic1.items()))
  
# print result
print ("Resultant Dictionary :")
print(final)

Resultant Dictionary :
OrderedDict([('D', '400'), ('A', '100'), ('B', '200'), ('C', '300')])

Approach 2: Using move_to_end() method

The move_to_end() is a method of the OrderedDict subclass. This method can be used to move an existing key to either end of the dictionary. To add the new item in the dictionary, we can use the update() method. This method will add the specified item at the end of the list.

Algorithm

Follow the algorithm to understand the approach better.

Step 1- Import OrderedDict in the program

Step 2- Initialise a dictionary with items

Step 3- Use update() to add the new item in the dictionary

Step 4- Use move_to_end() to move the inserted item at the beginning

Step 5- Print the new dictionary as the result

Python Program 2

Look at the program to understand the implementation of the above-mentioned approach. The item which has to be inserted at the beginning is first added in the dictionary using update() then it is shifted to the beginning using the move_to_end() where the position is given as last=False.

from collections import OrderedDict  
dic1 = OrderedDict([('A', '100'), ('B', '200'), ('C', '300')])

dic1.update({"D": '400'})
print(dic1)

dic1.move_to_end("D", last=False)
  

print ("Resultant Dictionary :")
print(dic1)

OrderedDict([('A', '100'), ('B', '200'), ('C', '300'), ('D', '400')])
Resultant Dictionary :
OrderedDict([('D', '400'), ('A', '100'), ('B', '200'), ('C', '300')])

Conclusion

In this tutorial, we have seen two different approaches for inserting an item at the beginning of an OrderedDict. We have also seen what an OrderedDict is and how we can use different built-in methods to execute the task.