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Mar 14th, 2009

Recursion in C++

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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 ?

C++ Syntax (Toggle Plain Text)
#include<iostream.h>
 
 
void countdown(int x)
{
using namespace std;
cout<<x<<endl;
countdown(x-1);
}
 
int main(void)
{
countdown(10);
return 0;
}
#include<iostream.h>


void countdown(int x)
{
using namespace std;
cout<<x<<endl;
countdown(x-1);
}

int main(void)
{
countdown(10);
return 0;
}

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kavithabhaskar

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Mar 14th, 2009

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.

Last edited by Ancient Dragon; Mar 14th, 2009 at 12:19 am.

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Mar 14th, 2009

Re: Recursion in C++

Hi AncientDragon:

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

C++ Syntax (Toggle Plain Text)
#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;
}
~                                                                                             
~                                                                                             
~
#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;
}
~                                                                                             
~                                                                                             
~

Click to Expand / Collapse Quote originally posted by Ancient Dragon ...

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

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Mar 14th, 2009

Re: Recursion in C++

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

C++ Syntax (Toggle Plain Text)
if( x > 0)
   countdown(x-1);
if( x > 0)
   countdown(x-1);

Ancient Dragon

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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 ?

Click to Expand / Collapse Quote originally posted by Ancient Dragon ...

you don't want a while statement, but just a simple if statement
C++ Syntax (Toggle Plain Text)
if( x > 0)
   countdown(x-1);
if( x > 0) countdown(x-1);

kavithabhaskar

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Mar 14th, 2009

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.

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Re: Recursion in C++

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

kavithabhaskar

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Mar 14th, 2009

Re: Recursion in C++

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

C++ Syntax (Toggle Plain Text)
#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;
}
#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;
}

Ancient Dragon

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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.

Last edited by siddhant3s; Mar 14th, 2009 at 2:47 am.

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