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Which Of These Is Faster?


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#1 flyingfisch

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Posted 05 December 2012 - 11:58 AM

Which of these loops is faster, and which would be preferred?

while 1 do
   --[[ ... ]]--
end

or

function something()
   --[[ ... ]]--
   something()
end


#2 Forty-Two

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Posted 05 December 2012 - 09:02 PM

while 1. The recursive one will eventually give you a stack overflow.

#3 flyingfisch

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Posted 05 December 2012 - 11:08 PM

while 1. The recursive one will eventually give you a stack overflow.


Ah, ok. Is there any reason you would want to use the recursive one?

#4 MicroPro

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Posted 06 December 2012 - 08:36 PM

Even if program flow is interrupted before the stack fills up, the recursive one is slower because of the time spent during setting up the stack and stuff for calling/returning from the function.

On the other hand writing non-recursive code for solving some problems can be a pain, e.g Towers of Hanoi; in which recursive approach is preferred.

Source: textbook :P

#5 flyingfisch

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Posted 06 December 2012 - 09:06 PM

Even if program flow is interrupted before the stack fills up, the recursive one is slower because of the time spent during setting up the stack and stuff for calling/returning from the function.

On the other hand writing non-recursive code for solving some problems can be a pain, e.g Towers of Hanoi; in which recursive approach is preferred.

Source: textbook :P


Ah, ok, thank you for the clarification. :)

I have wondered about that for a while.

#6 Forty-Two

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Posted 06 December 2012 - 11:38 PM

Also: many sorting algorithms use recursivity to partition data into smaller parts to sort (I believe quicksort and mergesort work like this).

#7 MicroPro

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Posted 21 December 2012 - 07:40 PM

Another case is when there is multiple recursive calls with the same inputs. For example write a non-recursive and a recursive function that generate Fibonacci numbers and compare the timings. How long do you think it will take for the recursive function to calculate the 50th number in the sequence?

#8 Forty-Two

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Posted 25 December 2012 - 05:45 PM

It depends on the language, really. In some languages (lisp), the recursive solution is not only faster, but more elegant. In others (C, for example), it is easier to do it with a loop.

#9 MicroPro

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Posted 30 December 2012 - 02:19 PM

Lisp has complex compilation and optimization steps that turns the recursive code into hardware-friendly fast programs. In contrast, in most procedural languages, recursive function calls are compiled into "real" function calls which have much overhead for the program.

How long does it generating 50th Fibonacci number take in lisp?




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