If we declare such if statements and for loops is it valid..?

I have seen such kind of statements in some online programs i searched for other purpose but i don't know how it works,is such kind of statements valid.

condition 1:
if(!y)
a++;
condition 2:
if(y)
b++;
and inside this loop
for(int i=0;n;n\2,i++)

and also what happens if we put a ";" after a for loop and we declare nothing inside that loop braces{}.