Bhread: Blog Threads

I’m starting a new category of posts called “Code Snippets” where I share code I find interesting.

In this first installment is a simple code snippet I originally wrote in Powershell to scan a local port continuously and do something when the port is found:

function deploy(): 
	if ( port_exists(port) ):
		do_something()
	else: 
		deploy()

deploy()

Recursion has been a hard concept for me to put into practice and this served as a good introduction to it. However, this doesn’t feel like a ‘proper’ recursion to me.

I was so satisfied with this code so I showed it to my programmer co-worker friend. He asked me what will happen if the program doesn’t ever find the port? I didn’t think much of it because I know the user can just close the program and the port scanning is slow enough (around 1 sec for each function call).

I later realized what he meant. Being a better programmer than me, he knew something was missing. And so, let’s try to improve the code.

function deploy(retries):
	
	if ( retries < 1 ): 
		raise Exception("Port not found, Maximum retries reached.")

	if ( port_exists(port) ):
		do_something()
	else: 
		deploy(retries-1)

# Limit retries to n
n = 10
deploy(n) 

This feels more like recursion. The program will now only do a maximum of n retries. After that, if the port still doesn’t exist, it will throw an error.

The reason why I said the previous code didn’t feel like a proper recursion was because it didn’t limit the number of times it can call itself.

This limit is called the base case. In the code:

function deploy(retries):
	
	if (retries < 1 ): # <--- This is the Base case
		raise Exception("Port not found, Maximum retries reached.")

	if ( port_exists(port) ):
		do_something()
	else: 
		deploy(retries-1) 

# Limit retries to n
n = 10
deploy(n) 

The base case is a condition where if it is true, it stops the recursion.

Without the base case, some recursive functions could result to infinite recursions — something that should not happen, hence why the computer will throw an error (usually a stack overflow) when it finds an infinite recursive function.

For example, you can try to run this python code:

def a(b):
	return a(b+1)

a(1)

In this code, nothing will stop a from calling itself again which could be problematic. This is why adding the base case and a way to achieve the base case is important.

Back to the code snippet, what else can we do to improve the code?

Here’s an idea: add a time.sleep() before each recurse to prevent the code from executing too fast and eating unnecessary memory:

import time
function deploy(retries):
	
	if (retries < 1 ): 
		raise Exception("Port not found, Maximum retries reached.")

	if ( port_exists(port) ):
		do_something()
	else: 
		time.sleep(1) # <--- Do a 1-second sleep before every recurse
		deploy(retries-1)

# Limit retries to n
n = 10
deploy(n) 

Without the time.sleep(1), the program will loop as fast as it can, faster than what the code needs to be which will just waste our computer’s resources especially if the port_exists function consumes a lot of memory or cpu when called.

Note: I mentioned earlier that the port scanning takes 1 second so adding a time.sleep(1) will make the program scan every 2 seconds. I didn’t add that to my original code but in this case, I added it since it’s a different program.

That’s it for this code snippet. Try to improve the code, and maybe publish it. I will be happy to read them :).

To end this episode, I’ll give you something to think about. Here’s the same program but instead of recursion, it uses a while loop:

function deploy(retries):
	
	while not port_exists(port):
		if ( retries < 1 ): 
			raise Exception("Port not found, Maximum retries reached.")
		retries -= 1

	do_something()

# Limit retries to n
n = 10
deploy(n) 

Practice your programming skills and think of these questions:

What are the advantages and disadvantages of using recursion or iteration (i.e. a while loop)?
What types of situations does it make sense to use recursion versus iteration?

I will leave you, the reader to answer these questions. See you in the next Code Snippet!

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