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recursion - how to calculate sum?

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krizz

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recursion - how to calculate sum?

Oct 10th, 2007

I need help with the following:

How do we calculate the sum of consecutive numbers from 1 to n and not from n to 1 by using recursion.

Eg. if n = 10

it should be 1+2+...+9+10 and not 10+9+...+2+1.

Last edited by krizz; Oct 10th, 2007 at 1:06 am.

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vmanes

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Re: recursion - how to calculate sum?

Oct 10th, 2007

Assuming you understand how to write a recursive function, you need to establish what the base case is for this problem - when a parameter for the current term passed in is equal to the upper limit (10 in this case, but best also passed as a parameter for flexibility of use). Rather than the usual decreasing value passed in (the 10-1 version), your current term will be 1 in the first call, 2 in the next, and so on.

Val

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Re: recursion - how to calculate sum?

Oct 10th, 2007

according to me the code can be writen in this way.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
 
#include <conio>
 
int sum(int);
 
int x;
 
void main()
{
       int n ; 
 
       cin >> n ;
 
       sum(n) ;
 
       cout<<" the sum is"<<x ;
 
       getch();
 
}
 
int sum(int a)
{
    if(a)
    {
        return  sum(a-1)+a;
 
     }
}
#include <iostream>

#include <conio>

int sum(int);
  
int x;

void main()
{
       int n ; 
 
       cin >> n ;
  
       sum(n) ;

       cout<<" the sum is"<<x ;

       getch();

}

int sum(int a)
{
    if(a)
    {
        return  sum(a-1)+a;
        
     }
}

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vmanes

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Re: recursion - how to calculate sum?

Oct 10th, 2007

It can be written that way, but it's not correct. It doesn't even compile.

Once you fix the two problems in your preprocessor directives, you'll find it is not a correct solution.

Once you've corrected the problem in main that causes you to not even see the answer your sum( ) function returns, note that you have not solved the original problem. Your function works in the 10 9 8... direction.

Your if( a ) is clever approach, however it leaves your function with a warning of "not all control paths are followed."

You should test code before posting it to the world.

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iamthwee

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Re: recursion - how to calculate sum?

Oct 10th, 2007

>it should be 1+2+...+9+10 and not 10+9+...+2+1

But it gives you exactly the same answer, and all you are asking for is the end total right?

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Re: recursion - how to calculate sum?

Oct 10th, 2007

Sometimes the journey is more important than the destination?

Val

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ChaseVoid

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Re: recursion - how to calculate sum?

Oct 10th, 2007

The quest here is not to get the result, but to get that scenario. If you just wanted to get the sum then this is suffice :

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include<iostream>
using namespace std;
 
int Sum(int s)
{
	int result;
	if( s == 1 )
		return 1;
	else
	{
		result = Sum(s-1) + s;
		return result;
	}
 
}
 
int main()
{
	int n = 0; 
	cout<<"Enter a limit : ";
	cin>>n;
 
	cout<<"The sum of numbers up to "<<n<<" is "<<Sum(n);
 
	cin.ignore();
	cin.get();
}
#include<iostream>
using namespace std;

int Sum(int s)
{
	int result;
	if( s == 1 )
		return 1;
	else
	{
		result = Sum(s-1) + s;
		return result;
	}
	
}

int main()
{
	int n = 0; 
	cout<<"Enter a limit : ";
	cin>>n;
	
	cout<<"The sum of numbers up to "<<n<<" is "<<Sum(n);

	cin.ignore();
	cin.get();
}

But we want to get the result such that it calculates from 1 till the limit.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
for ( int i = 1; i < limit; i++ )
      sum += i;
for ( int i = 1; i < limit; i++ )
      sum += i;

We want that scenario with recursion. I've trying to figure it out, but I can't

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Re: recursion - how to calculate sum?

Oct 10th, 2007

You don't need to figure it out as it isn't your homework.

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Re: recursion - how to calculate sum?

Oct 10th, 2007

The end total can be found easily by calculating from 10+9+...+2+1 but i am more interested in the scenario as one of my friends has already said..ie. I need it to process from 1 to n and not n to 1.

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Originally Posted by iamthwee

>it should be 1+2+...+9+10 and not 10+9+...+2+1

But it gives you exactly the same answer, and all you are asking for is the end total right?

Last edited by krizz; Oct 10th, 2007 at 5:49 pm.

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Re: recursion - how to calculate sum?

#10

Oct 10th, 2007

ChaseVoid's code gives a good implementation of the normal (1+2+3...) method of recursively solving this problem. Reread my first comments, and you should be able to modify that function to make it solve in the desired way.

Val