Python: Find the indices for which the numbers in the list drops

Last update on March 22 2025 13:48:16 (UTC/GMT +8 hours)

Indices of Drops in List

Write a Python program to find the indices at which the numbers in the list drop.

NOTE: You can detect multiple drops just by checking if nums[i] < nums[i-1].

Input:
[0, -1, 3, 8, 5, 9, 8, 14, 2, 4, 3, -10, 10, 17, 41, 22, -4, -4, -15, 0]
Output:
[1, 4, 6, 8, 10, 11, 15, 16, 18]

Input:
[6, 5, 4, 3, 2, 1]
Output:
[1, 2, 3, 4, 5]

Input:
[1, 19, 5, 15, 5, 25, 5]
Output:
[2, 4, 6]

Sample Solution-1:

Python Code:

# License: https://bit.ly/3oLErEI

# Define a function named 'test' that takes a list of numbers 'nums' as input
def test(nums):
    # Initialize an empty list 'drop_indices' to store indices where numbers drop
    drop_indices = []
    
    # Iterate through the list starting from the second element
    for i in range(1, len(nums)):
        # Check if the current number is less than the previous number
        if nums[i] < nums[i - 1]:
            # If true, append the index to 'drop_indices'
            drop_indices.append(i)
    
    # Return the list of indices where the numbers drop
    return drop_indices

# Assign a specific list of numbers 'nums' to the variable
nums = [0, -1, 3, 8, 5, 9, 8, 14, 2, 4, 3, -10, 10, 17, 41, 22, -4, -4, -15, 0]

# Print the original list of numbers 'nums'
print("Original list:")
print(nums)

# Print a message indicating the operation to be performed
print("Indices for which the numbers of the said list drops:")

# Print the result of the test function applied to the 'nums' list
print(test(nums))

# Assign a different list of numbers 'nums' to the variable
nums = [6, 5, 4, 3, 2, 1]

# Print the original list of numbers 'nums'
print("\nOriginal list:")
print(nums)

# Print a message indicating the operation to be performed
print("Indices for which the numbers of the said list drops:")

# Print the result of the test function applied to the updated 'nums' list
print(test(nums))

# Assign another list of numbers 'nums' to the variable
nums = [1, 19, 5, 15, 5, 25, 5]

# Print the original list of numbers 'nums'
print("\nOriginal list:")
print(nums)

# Print a message indicating the operation to be performed
print("Indices for which the numbers of the said list drops:")

# Print the result of the test function applied to the updated 'nums' list
print(test(nums))

Sample Output:

Original list:
[0, -1, 3, 8, 5, 9, 8, 14, 2, 4, 3, -10, 10, 17, 41, 22, -4, -4, -15, 0]
Indices for which the numbers of the said list drops.:
[1, 4, 6, 8, 10, 11, 15, 16, 18]

Original list:
[6, 5, 4, 3, 2, 1]
Indices for which the numbers of the said list drops.:
[1, 2, 3, 4, 5]

Original list:
[1, 19, 5, 15, 5, 25, 5]
Indices for which the numbers of the said list drops.:
[2, 4, 6]

Flowchart:

Sample Solution-2:

Python Code:

# License: https://bit.ly/3oLErEI

# Define a function named 'test' that takes a list of numbers 'nums' as input
def test(nums):
    # List comprehension to create a list of indices where the numbers drop
    # Iterate through the list starting from the second element
    # Check if the current number is less than the previous number
    # If true, include the index in the resulting list
    return [i for i in range(1, len(nums)) if nums[i] < nums[i - 1]]

# Assign a specific list of numbers 'nums' to the variable
nums = [0, -1, 3, 8, 5, 9, 8, 14, 2, 4, 3, -10, 10, 17, 41, 22, -4, -4, -15, 0]

# Print the original list of numbers 'nums'
print("Original list:")
print(nums)

# Print a message indicating the operation to be performed
print("Indices for which the numbers of the said list drops:")

# Print the result of the test function applied to the 'nums' list
print(test(nums))

# Assign a different list of numbers 'nums' to the variable
nums = [6, 5, 4, 3, 2, 1]

# Print the original list of numbers 'nums'
print("\nOriginal list:")
print(nums)

# Print a message indicating the operation to be performed
print("Indices for which the numbers of the said list drops:")

# Print the result of the test function applied to the updated 'nums' list
print(test(nums))

# Assign another list of numbers 'nums' to the variable
nums = [1, 19, 5, 15, 5, 25, 5]

# Print the original list of numbers 'nums'
print("\nOriginal list:")
print(nums)

# Print a message indicating the operation to be performed
print("Indices for which the numbers of the said list drops:")

# Print the result of the test function applied to the updated 'nums' list
print(test(nums))

Sample Output:

Original list:
[0, -1, 3, 8, 5, 9, 8, 14, 2, 4, 3, -10, 10, 17, 41, 22, -4, -4, -15, 0]
Indices for which the numbers of the said list drops.:
[1, 4, 6, 8, 10, 11, 15, 16, 18]

Original list:
[6, 5, 4, 3, 2, 1]
Indices for which the numbers of the said list drops.:
[1, 2, 3, 4, 5]

Original list:
[1, 19, 5, 15, 5, 25, 5]
Indices for which the numbers of the said list drops.:
[2, 4, 6]

Flowchart:

For more Practice: Solve these Related Problems:

Write a Python program to identify indices where the current element is less than the previous element in a list.
Write a Python program to iterate over a list and collect positions where a drop in value occurs using enumerate().
Write a Python program to compare each element with its predecessor and return all indices of decreases.
Write a Python program to use a list comprehension to gather indices where a drop in sequence value is detected.

Python Code Editor :

Have another way to solve this solution? Contribute your code (and comments) through Disqus.

Previous: Compute the sum of the ASCII values of the upper-case characters in a given string.
Next: Create a list whose ith element is the maximum of the first i elements of the input list.