>  I was reviewing the leapyear3 program and it does not make sense.
>  In the if statement, why don't the conditions inside the parantheses 
>  not complete.  For example, "year%4" has no condition.  Why?  

This was discussed in lecture in some detail.  The expression
"year%4" is zero if year is divisible by 4 and nonzero otherwise.
Used as a logical condition, a nonzero value is converted to "true"
and a zero value is converted to "false."   Hence,
"if ( year%4 )"   is equivalent to "if ( year%4 != 0)", and   
"if ( !(year%4) )"   is equivalent to "if ( year%4 == 0)".
The former checks whether year is *not* divisible by 4; the latter
checks whether year *is* divisible by 4.