Why is using a while loop better?

Question

I am making a program that uses a loop to run forever and I have a snippet of code below to show you how I acheive the loop. This is just an example and not the actual program. It is the same idea though.

import time
def start():
    print "hello"
    time.sleep(0.2)
    start()
start()

All of my programmer friends tell me not to do this, and use a while loop instead. Like this:

import time
def start():
    while True:
        print "Hello"
        time.sleep(0.2)
start()

Why should I use the while loop instead when both methods work perfectly fine?

Does the recursive method actually "work fine"? Don't you get an error about max recursion depth rather quickly? — Wooble
– Wooble, Commented Apr 5, 2014 at 2:58
I have a sneaky suspicion that neither approach is better, but there is a third approach involving lazy evaluation that can only be speculated upon until we have more of the code. #justsaying — kojiro
– kojiro, Commented Apr 5, 2014 at 3:04
@kojiro isn't that just complicating things? he merely 'seems' to want to run an infinite loop with some sleep (as a throttling parameter). I don't get why lazy eval is required in this scenario? — Srikar Appalaraju
– Srikar Appalaraju, Commented Apr 5, 2014 at 3:06
Your first approach might be a viable option in a truly functional language where tail call optimizations are performed. Since python neither is a functional language nor does tail call optimizations, it will fail. Had the optimization been done, both approaches would be more or less equivalent. — Jeff Mercado
– Jeff Mercado, Commented Apr 5, 2014 at 3:12

Srikar Appalaraju · Accepted Answer · 2014-04-05 03:27:07Z

6

Each time you are recursing, you are pushing a frame context onto your program stack. Soon you would have used up your entire allotted stack space causing stackoverflow, no pun intended ;)

The second approach has no such flaws. Hence by the looks of it 2nd is better than 1st approach (unless more of the program is presented).

edited Apr 5, 2014 at 3:27

answered Apr 5, 2014 at 3:02

Srikar Appalaraju

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1 Comment

kojiro Over a year ago

The pun is certainly intended, just not by you. :P

falsetru · Accepted Answer · 2014-04-05 03:03:07Z

2

If you run the program continuously with the recursion, you will get RuntimeError:

Traceback (most recent call last):
  File "t.py", line 8, in <module>
    start()
  File "t.py", line 7, in start
    start()
  File "t.py", line 7, in start
    start()
  ...
  File "t.py", line 7, in start
    start()
RuntimeError: maximum recursion depth exceeded

>>> import sys
>>> sys.getrecursionlimit()
1000

answered Apr 5, 2014 at 3:03

falsetru

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2 Comments

Ethan Bierlein Over a year ago

You could also set the recursion limit higher to with sys.setrecursionlimit()

Srikar Appalaraju Over a year ago

not a good idea to inc. recursion limit. since you are running this indefinitely, you will sooner or rather hit the limit, causing program crash...

Pruthvi Raj · Accepted Answer · 2014-04-05 03:03:31Z

1

the while loop amd recursion both have their advantages and disadvantages but while loop is just a test and conditional jump. wheras ecursion involves pushing a stack frame, jumping, returning, and popping back from the stack.So they might have preferred while loop :)

answered Apr 5, 2014 at 3:03

Pruthvi Raj

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1 Comment

Pruthvi Raj Over a year ago

ofcourse, testing and jumping would be more fast when compared to the steps involved in recursion.So it will be less CPU intensive

user2864740 · Accepted Answer · 2014-04-05 03:12:38Z

1

Python does not have Tail Call Optimization (TCO) and cannot elide the stack. As such, recursion should not be used for an unbound depth (in languages without TCO) lest you get a stack-overflow error!

While the current code might appear to be fine, consider this simple change which will quickly reveal a problem.

def start():
    print "hello"
    # let's go faster! -- time.sleep(0.2)
    start()
start()

answered Apr 5, 2014 at 3:12

user2864740

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6 Comments

Ethan Bierlein Over a year ago

I see no difference with the exception of the comment. What is the change?

Lily Mara Over a year ago

@TheNotGoodAtCodeGuy the comment.

user2864740 Over a year ago

@TheNotGoodAtCodeGuy It runs much faster - that is, instead of only recursing 5 times a second, it might recurse 50000 times per second (and will very quickly fail).

Ethan Bierlein Over a year ago

See, that is why I have the time.sleep(), first off, I have a slow computer, so it uses less processing power, and it doesn't cause immediate stack overflow and or crash my computer.

user2864740 Over a year ago

@TheNotGoodAtCodeGuy Recursion should not be used for an unbound depth as shown in this case. The sleep doesn't matter and having it only hides the problem with this approach.

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