Python - Extract Preceding Record from list values

Python – Extract Preceding Record from list values

Last Updated : 10 Jul, 2020

Sometimes, while working with Tuple records, we can have a problem in which we need to extract the record, which is preceeding to particular key provided. This kind of problem can have application in domains such as web development and day-day programming. Let’s discuss certain ways in which this task can be performed.

Input : test_list = [(‘Gfg’, 3), (‘is’, 4), (‘best’, 1), (‘for’, 10), (‘geeks’, 11)], key = ‘geeks’
Output : (‘for’, 10)
Input : test_list = [(‘Gfg’, 3), (‘is’, 4), (‘best’, 1), (‘for’, 10), (‘geeks’, 11)], key = ‘is’
Output : (‘Gfg’, 3)

Method #1 : Using zip() + enumerate() + loop
The combination of above functions can be used to perform this particular task. In this, we create 2 lists, one with starting with next index. The zip(), helps to connect them and return the desired result using enumerate() for extracting index.

# Python3 code to demonstrate working of 
# Extract Preceeding Record
# Using zip() + enumerate() + loop
  
# initializing list
test_list = [('Gfg', 3), ('is', 4), ('best', 1), ('for', 10), ('geeks', 11)]
  
# printing original list
print("The original list is : " + str(test_list))
  
# initializing Key
key = 'for'
  
# Extract Preceding Record
# Using zip() + enumerate() + loop
for idx, (a, b) in enumerate(zip(test_list, test_list[1:])):
    if b[0] == key:
        res = (a[0], a[1])
  
# printing result 
print("The Preceeding record : " + str(res)) 

Output :

The original list is : [('Gfg', 3), ('is', 4), ('best', 1), ('for', 10), ('geeks', 11)]
The Preceeding record : ('best', 1)

Method #2 : Using list comprehension + enumerate()
The combination of above functions can be used to solve this problem. In this, we perform the task of extracting the previous element to manually test previous key, rather than creating a separate list as in previous method.

# Python3 code to demonstrate working of 
# Extract Preceeding Record
# Using list comprehension + enumerate()
  
# initializing list
test_list = [('Gfg', 3), ('is', 4), ('best', 1), ('for', 10), ('geeks', 11)]
  
# printing original list
print("The original list is : " + str(test_list))
  
# initializing Key
key = 'for'
  
# Extract Preceding Record
# Using list comprehension + enumerate()
res = [(test_list[idx - 1][0], test_list[idx - 1][1]) 
      for idx, (x, y) in enumerate(test_list) if x == key and idx > 0]
  
# printing result 
print("The Preceeding record : " + str(res)) 

Output :

The original list is : [('Gfg', 3), ('is', 4), ('best', 1), ('for', 10), ('geeks', 11)]
The Preceeding record : [('best', 1)]

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