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Python – Check alternate peak elements in List
  • Last Updated : 08 Dec, 2020

Given a list, the task is to write a Python program to test if it’s alternate, i.e next and previous elements are either both smaller or larger across the whole list.

Input : test_list = [2, 4, 1, 6, 4, 8, 0] 
Output : True 
Explanation : 4, 6, 8 are alternate and peaks (2 1).

Input : test_list = [2, 4, 1, 6, 4, 1, 0] 
Output : False 
Explanation : 1 is not peak ( 4 < 1 < 0). 
 

Method #1: Using loop.

In this, we check for each element for next and previous elements using conditional if statement to be either smaller or larger, if any variation is found, the result is flagged false and loop is exited.



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# Python3 code to demonstrate working of
# Test for alternate peak List
# Using loop
  
# initializing list
test_list = [2, 4, 1, 6, 4, 8, 0]
               
# printing original list
print("The original list is : " + str(test_list))
  
res = True 
for idx in range(1, len(test_list) - 1):
      
    # breaking if not alternate peaks
    if not ((test_list[idx - 1] < test_list[idx] 
             and test_list[idx + 1] < test_list[idx])
            or (test_list[idx - 1] > test_list[idx] 
                and test_list[idx + 1] > test_list[idx])):
        
        res = False
          
# printing result
print("Is list forming alternate peaks ? : " + str(res))

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Output:

The original list is : [2, 4, 1, 6, 4, 8, 0]
Is list forming alternate peaks ? : True

Method #2 : Using all() + generator expression.

In this, we perform the task of checking for all the elements for getting alternate peaks using all(), and generator expression is used to iterate through the entire list. 

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# Python3 code to demonstrate working of
# Test for alternate peak List
# Using all() + generator expression
  
# initializing list
test_list = [2, 4, 1, 6, 4, 8, 0]
               
# printing original list
print("The original list is : " + str(test_list))
  
# checking for all the elements for alternate peaks 
# one liner solution to problem
res = all(((test_list[idx - 1] < test_list[idx]
            and test_list[idx + 1] < test_list[idx]) 
           or (test_list[idx - 1] > test_list[idx] 
               and test_list[idx + 1] > test_list[idx]))
          for idx in range(1, len(test_list) - 1))
          
# printing result
print("Is list forming alternate peaks ? : " + str(res))

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Output:

The original list is : [2, 4, 1, 6, 4, 8, 0]
Is list forming alternate peaks ? : True

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