Python | Alternate Rear iteration

The iteration of numbers is done by looping techniques in python. There are many techniques in Python which facilitate looping. Sometimes we require to perform the looping backwards in alternate way and having shorthands to do so can be quite useful. Let’s discuss certain ways in which this can be done. 

Method #1 : Using reversed() The simplest way to perform this is to use the reversed function for the for loop and the iteration will start occurring from the rear side than the conventional counting. 

Python3

# Python3 code to demonstrate
# Alternate Rear iteration
# using reversed()

# Initializing number from which
# iteration begins
N = 6

# using reversed() to perform the Alternate Rear iteration
print ("The reversed numbers are : ", end = "")
for num in reversed(range(0, N + 1, 2)) :
    print (num, end = " ")

Output : 

The reversed numbers are : 6 4 2 0 

Time complexity: O(N)
Auxiliary space: O(1)

Method #2 : Using range(N, -1, -2) This particular task can also be performed using the conventional range function which, if provided with the third argument performs the alternate skip and second argument is used to start from backwards. 

Python3

# Python3 code to demonstrate
# Alternate Rear iteration
# using range(N, -1, -2)

# Initializing number from which
# iteration begins
N = 6

# using reversed() to perform the Alternate Rear iteration
print ("The reversed numbers are : ", end = "")
for num in range(N, -1, -2) :
    print (num, end = " ")

Output : 

The reversed numbers are : 6 4 2 0 

Time complexity: O(N/2), where N is the value of the variable N.
Auxiliary space: O(1), as we are not using any additional data structure to store the values.

Method 3: use list comprehension

Python3

# Python3 code to demonstrate
# Alternate Rear iteration
# using list comprehension and join()

# Initializing number from which
# iteration begins
N = 6

# using list comprehension to generate the Alternate Rear iteration sequence
nums = [str(num) for num in range(N, -1, -2)]

# using join() to concatenate the list elements into a string
result = " ".join(nums)

# printing the result
print("The reversed numbers are:", result)

Output

The reversed numbers are: 6 4 2 0

Time complexity: O(N/2) for generating the list and O(N) for joining the elements, 
Auxiliary space: O(N) for storing the list. However, it may be slightly less efficient than the other methods due to the overhead of creating and joining a list.

Method 4: Using a while loop

Step-by-step approach:

1. Initialize a variable num to N.
2. While num is greater than or equal to 0, print its value and decrement it by 2.
3. Repeat step 2 until num becomes less than 0
4. Print a newline character to end the output.

Python3

# Python3 code to demonstrate
# Alternate Rear iteration
# using while loop

# Initializing number from which
# iteration begins
N = 6

# Using while loop to perform the Alternate Rear iteration
print("The numbers are: ", end="")
num = N
while num >= 0:
    print(num, end=" ")
    num -= 2

# Print newline character
print()

Output

The numbers are: 6 4 2 0 

Time Complexity: O(N/2), as we iterate through N/2 numbers, where N is the starting number.
Auxiliary Space: O(1), as we only use a constant amount of extra memory to store the num variable.