Thread: Godbach conjecture!!

Hi everyone,
Its my first post, and by the way its my first semester studying C language. And it's the first programming experience.
I was asked to write a program that , for each even number between 700 and 1100 , it lists its equivalent value as a sum of primes (assuming godbach was right so far!! lol ) ..the thing is that i couldn't do it using ordinary loops. so i used an array in which i have stored the primes i want to process.
For that said, Can anyone tell me why the output of my program outputs only relatively bigger primes as operands for the sum. I mean it's correct but why it skip situations in which small prime numbers can be used like 3.
700 = 419 + 281 ya right but why not 700 = 23 + 677 .
here is my code

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define start 700
#define finish 1100
int is_prime(int);
int check(int);
int main(){
           for(int i  = start ; i <= finish ; i += 2){
                   check(i);
                   }
           system("PAUSE");
           return(0);
    }
int is_prime(int n){
                 for(int i = 2 ; i <= sqrt(n) ; i++){
                           if(!(n%i))
                                   return(0);
                 }
                 return(1);
    }
int check(int x){
                 int k[x];
                 int j = 0;
                 for (int i = 1 ; i < x ; i += 2){
                          if (is_prime(i)){
                             k[j] = i;
                             j++;
                          }
                 }         
                 
                 for (int n = 0 ; n < j ; n++){
                          for (int a = 0 ; a <= j/2 ;a++){
                          if (x == k[n]+k[a]){
                          printf("%d = %d + %d\n",x,k[n],k[a]);
                          return (0);
                          }
                          }
                 }
                 }

it's goldbach and not Godbach, sorry everyone!

Ya but if remove that return, than the program will list all possible cases. which was not asked..
anyway can anybody tell me how to use just loops to solve the problem??

Originally Posted by Leojeen

Ya but if remove that return, than the program will list all possible cases. which was not asked..
anyway can anybody tell me how to use just loops to solve the problem??

If you mean, why is it picking middle numbers, then the answer is

Code:

for (int a = 0; a <= j/2; a++)

which says that when the first prime is 23, the second prime can only go up to 11.

Ya I see..but is there any other possible solution to the problem, without using an array?

Do the for loop correctly? (I was not suggesting that your for-loop on a was right.)

But my program works!! I'm talking about a way without using an array in the function check, it could be some nested loops..right??

I guess there is not!!

Ya that's right , it shows me all the possible cases this time, but i can handle it..
thanks tabstop ..