Click here to Skip to main content
12,951,971 members (62,898 online)
Click here to Skip to main content
Add your own
alternative version

Stats

187.5K views
342 downloads
27 bookmarked
Posted 5 Nov 2007

Iterative vs. Recursive Approaches

, 5 Nov 2007 CPOL
Rate this:
Please Sign up or sign in to vote.
Implication of not thinking of iterative solutions over recursive from performance (response time) point of view

Introduction

This article was originally posted at blogs.microsoft.co.il/blogs/Eyal.

Recursive function – is a function that is partially defined by itself and consists of some simple case with a known answer. Example: Fibonacci number sequence, factorial function, quick sort and more.
Some of the algorithms/functions can be represented in an iterative way and some may not.

Iterative functions – are loop based imperative repetitions of a process (in contrast to recursion which has a more declarative approach).

Comparison between Iterative and Recursive Approaches from Performance Considerations

Factorial

//recursive function calculates n!
static int FactorialRecursive(int n)
{
    if (n <= 1) return 1;
    return n * FactorialRecursive(n - 1);
}

//iterative function calculates n!
static int FactorialIterative(int n)
{
    int sum = 1;
    if (n <= 1) return sum;
    while (n > 1)
    {
        sum *= n;
        n--;
    }
    return sum;
}
NRecursiveIterative
10334 ticks11 ticks
100846 ticks23 ticks
10003368 ticks110 ticks
100009990 ticks975 ticks
100000stack overflow9767 ticks

As we can clearly see, the recursive is a lot slower than the iterative (considerably) and limiting (stackoverflow).

The reason for the poor performance is heavy push-pop of the registers in the ill level of each recursive call.

Fibonacci

//--------------- iterative version ---------------------    
static int FibonacciIterative(int n)
{
    if (n == 0) return 0;
    if (n == 1) return 1;
        
    int prevPrev = 0;
    int prev = 1;
    int result = 0;
        
    for (int i = 2; i <= n; i++)
    {
        result = prev + prevPrev;
        prevPrev = prev;
        prev = result;
    }
    return result;
}
    
//--------------- naive recursive version --------------------- 
static int FibonacciRecursive(int n)
{
    if (n == 0) return 0;
    if (n == 1) return 1;
        
    return FibonacciRecursive(n - 1) + FibonacciRecursive(n - 2);
}
    
//--------------- optimized recursive version ---------------------
static Dictionary<int> resultHistory = new Dictionary<int>();

static int FibonacciRecursiveOpt(int n)
{
    if (n == 0) return 0;
    if (n == 1) return 1;
    if (resultHistory.ContainsKey(n)) 
        return resultHistory[n];

    int result = FibonacciRecursiveOpt(n - 1) + FibonacciRecursiveOpt(n - 2);
    resultHistory[n] = result;
        
    return result;
}
NRecursiveRecursive opt.Iterative
55 ticks22 ticks9 ticks
1036 ticks49 ticks10 ticks
202315 ticks61 ticks10 ticks
30180254 ticks65 ticks10 ticks
100too long/stack overflow158 ticks11 ticks
1000too long/stack overflow1470 ticks27 ticks
10000too long/stack overflow13873 ticks 190 ticks
100000too long/stack overflowtoo long/stack overflow3952 ticks

As before, the recursive approach is worse than iterative however, we could apply memorization pattern (saving previous results in dictionary for quick key based access), although this pattern isn't a match for the iterative approach (but definitely an improvement over the simple recursion).

Notes

  1. The calculations may be wrong in big numbers, however the algorithms should be correct.
  2. For timer calculations, I used System.Diagnostics.Stopwatch.

Points of Interest

  1. Try not to use recursion in system critical locations.
  2. Elegant solutions not always the best performing when used in "recursive situations".
  3. If you required to use recursion, at least try to optimize it with dynamic programming approaches (such as memorization).

History

  • Post: November 06, 2007

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Share

About the Author

Eyal Lantzman
Software Developer (Senior) Taldor
Israel Israel
In the last couple of years I'm working as Microsoft sub contractor in various project types - LOB, applications, CnC applications and Distributed applicaitons all of them considered to be extra large in tems of man power (or brain power), duration and geographic destribution between connected sites.

You may also be interested in...

Pro

Comments and Discussions

 
GeneralGood Job Pin
merlin9816-Nov-07 4:24
membermerlin9816-Nov-07 4:24 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.

Permalink | Advertise | Privacy | Terms of Use | Mobile
Web02 | 2.8.170525.1 | Last Updated 6 Nov 2007
Article Copyright 2007 by Eyal Lantzman
Everything else Copyright © CodeProject, 1999-2017
Layout: fixed | fluid