Python program to find the Decreasing point in List

Given a list, get the index of element where the list shows the first negative trend, i.e first point where the next element < current element. If not found return -1.

Input : test_list = [3, 6, 8, 9, 12, 5, 18, 1] 
Output : 4 
Explanation : At 12 -> 5, first decreasing point occurs.

Input : test_list = [3, 9, 12, 5, 18, 1] 
Output : 2 
Explanation : At 12 -> 5, first decreasing point occurs. 

Method #1 : Using loop

In this, we check for the next element to be less than the current element, at a point where it is first found, we break the loop. 

Python3

# Python3 code to demonstrate working of
# Decreasing point in List
# Using loop

# initializing list
test_list = [3, 6, 8, 9, 12, 5, 18, 1]

# printing original list
print("The original list is : " + str(test_list))

res = -1
for idx in range(0, len(test_list) - 1):

    # checking for 1st decreasing element
    if test_list[idx + 1] < test_list[idx]:
        res = idx
        break

# printing result
print("Decreasing Point : " + str(res))

Output:

The original list is : [3, 6, 8, 9, 12, 5, 18, 1]
Decreasing Point : 4

Method #2 : Using enumerate() + loop

In this, we check for index and value simultaneously using enumerate, a similar method to above, the difference being separate index element access.

Python3

# Python3 code to demonstrate working of
# Decreasing point in List
# Using enumerate() + loop

# initializing list
test_list = [3, 6, 8, 9, 12, 5, 18, 1]

# printing original list
print("The original list is : " + str(test_list))

res = -1
for idx, ele in enumerate(test_list):

    # checking for 1st decreasing element
    if test_list[idx + 1] < ele:
        res = idx
        break

# printing result
print("Decreasing Point : " + str(res))

Output:

The original list is : [3, 6, 8, 9, 12, 5, 18, 1]
Decreasing Point : 4