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- - Recursion in C++ (http://www.daniweb.com/forums/thread181459.html)

kavithabhaskar

Mar 14th, 2009 12:11 am

Recursion in C++

HI I am trying to work on a very simple recursive function but i get o/p as some negative values.. can u please help and tell me what mistake i am doing ?

#include<iostream.h>





void countdown(int x)

{

using namespace std;

cout<<x<<endl;

countdown(x-1);

}



int main(void)

{

countdown(10);

return 0;

}

Ancient Dragon

Mar 14th, 2009 12:16 am

Re: Recursion in C++

1) it goes negative because there is nothing to prevent it. Its an infinit recursive function -- that is until it consumes all available stack space and then it will simply crash. You need to add a line that stops the recursion when x == 0 (or whatever other value you want).

2) replace iostream.h with <iostream>. Current c++ standards do not use the .h extension. If you compiler doesn't support <iostream> then get a newer compiler.

3) move that line

using namespace std;

up above just under the last include directive. It does not good to place it where it is.

kavithabhaskar

Mar 14th, 2009 12:25 am

Re: Recursion in C++

Hi AncientDragon:

Thanks for ur reply.. i made some modifications.. still does not work! i get all 0s now..

#include<iostream.h>



using namespace std;

void countdown(int x)



{

cout<<x<<endl;

while(x > 0)

{

countdown(x-1);

}

}



int main(void)

{

countdown(10);

return 0;

}

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~                                                                                             

~

Quote:

Originally Posted by Ancient Dragon (Post 824666)

using namespace std;

up above just under the last include directive. It does not good to place it where it is.

Ancient Dragon

Mar 14th, 2009 12:30 am

Re: Recursion in C++

you don't want a while statement, but just a simple if statement

if( x > 0)

   countdown(x-1);

kavithabhaskar

Mar 14th, 2009 12:32 am

Re: Recursion in C++

Hey THanks it worked.. but how does it matter.. "while" was supposed to be doing the same thing..as "if" is doing now.. then why did it falter ?

Quote:

Originally Posted by Ancient Dragon (Post 824674)

you don't want a while statement, but just a simple if statement

if( x > 0)

   countdown(x-1);

Ancient Dragon

Mar 14th, 2009 12:50 am

Re: Recursion in C++

The while didn't work because now you have recursion within recursion. Follow the function through with pencil & paper and you will see why it doesn't work like you wanted it to.

kavithabhaskar

Mar 14th, 2009 1:07 am

Re: Recursion in C++

I dont see the difference.. it is going to call the countdown function repetitively anyway..!

Ancient Dragon

Mar 14th, 2009 1:26 am

Re: Recursion in C++

With the while statement, the function will call itself billions of times. Try this version and see the results

#include <iostream>



using namespace std;



long long counter = 0;

void countdown(int x)

{

    ++counter;

    if( counter % 10000000 == 0)

        cout << counter << "\n";

    //cout<<x<<endl;

    while(x > 0)

    {

        countdown(x-1);

    }

}



int main(void)

{

countdown(10);

cout << "counter = " << counter << "\n";

return 0;

}

siddhant3s

Mar 14th, 2009 2:22 am

Re: Recursion in C++

Oh man!! Ancient Dragon would be snatching his hairs!!

Look kavithabaskar, You are basically using recursions to avoid loops.
Every series has Recursive form and a closed form(non-recursive)[Note that it is not possible for every series to have both forms].
Say, the series 1,2,3,4,5,6,7,8 has a recursive form as $A_{n}=A_{n-1}+1 || A_{1}=1$
and open form as $A_{n}=n$
So, if you know the closed form it is best to use it through loops.
But when you dont know closed form you go to recursive functions.

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