Question

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:

1, 2, 3, 5, 8, 13, 21, 34, 55, 89, …
I made the program but my answer doesnt match.

#include<stdio.h>
int main()
{
 long unsigned int i,sum=0,x=1,y=2,num;
 for(i=0;i<4000000;i++)
 {
  num=x+y;
  if(i%2==0)
   sum+=num;
  x=y;
  y=num;
 }
 printf("%lu\n",sum);
 getchar();
 return 0;
}

Answer 1

Three problems I can see:

• You should start with x = 1, y = 1, since otherwise you skip the first even-valued Fibonacci;
• Your loop condition should be (x + y) <= 4000000
• You should test num for even-ness, not i.

(After these changes, it should be obvious that you can omit i entirely, and therefore replace the for loop with a while loop)