Question

bool stop = false;
           int f1 = 1;
           int f2 = 2;
           int f3 = 0;
           int sum = 2;
           while (!stop)
           {
               f3 = f1 + f2;
               sum += f3 % 2 == 0 ? f3 : 0; //THIS ONE
               stop = f3 > 4000000 ? true : false;//AND THIS ONE.
               f1 = f2;
               f2 = f3;
           }

What is that conditional operator? This is the first time I’ve seen anything like this.

Answer 1

The line:

sum += f3 % 2 == 0 ? f3 : 0; //THIS ONE

is the same as:

if (f3 % 2 == 0)
  sum += f3;
else
  sum += 0;

which could of course be rewritten as

if (f3 % 2 == 0) sum += f3;

and the line

stop = f3 > 4000000 ? true : false;//AND THIS ONE.

is the same as

if (f3 > 4000000)
   stop = true;
else 
   stop = false;

Or better yet:

stop = f3 > 4000000;