Thread: Recursive function that returns the sum of all prime numbers from 2-n.

  1. #1
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    Recursive function that returns the sum of all prime numbers from 2-n.

    When i use 6 as my n, i get 0, which makes sense to me.;however, why do i get 6 for my final result? Why does it not do 0+5+...etc..?
    Code:
    //source.c file
    #include "Header.h"
    unsigned int sum_primes(unsigned int num)
    {
        //base case
        
        
        if (num == 4 || num == 3)
        {
            return 3;
        }
        if (num%2==0||num%3==0)
        {
            return 0;
        }
        else
        {
            return num+sum_primes(num - 1);
        }
        
    }
    //main
    #include "Header.h"
    
    
    
    
    
    
    
    
    int main() 
    {
        printf("%d\n", sum_primes(6));
    }

  2. #2
    Programming Wraith GReaper's Avatar
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    Your code gives zero as a result. What are you talking about?

    Also, that result doesn't make sense at all. The sum of all primes from 2 to 6 should be (2+3+5) = 10.
    Devoted my life to programming...

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    Sorry i meant to ask why i was getting zero as my answer. Yea i understand that it should be 10, but where did i go wrong? I understand why i get 0 for my very first num, because 10 is even; however, why does it not call itself again?

  4. #4
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    because 6 is even*

  5. #5
    Programming Wraith GReaper's Avatar
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    Well, it can't work because you immediately return 0 for all multiples of 2 or 3. In my opinion, this function is irredeemable. Recursion and prime numbers don't mix well...
    Devoted my life to programming...

  6. #6
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    oh i see. This is one of my hw assignments and it stated that recursion was mandatory. Imo i think a for loop would make this process much easier, but i can't do that.

  7. #7
    Programming Wraith GReaper's Avatar
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    Recursion may be mandatory, but is it the only method you're allowed to use? I'm asking because you could easily sneak an 'isPrime(num)' function in there. Like this:
    Code:
    unsigned sum_primes(unsigned num)
    {
        unsigned result = 0;
    
        if (isPrime(num)) {
            result += num;
        }
        if (num >= 2) {
            result += sum_primes(num - 1);
        }
    
        return result;
    }
    There are of course many ways to make this function better, I leave this up to you.
    Devoted my life to programming...

  8. #8
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    Ah i see. Thank you very much for your time.

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