Question

Is it possible to express the cases using the conditional operator? For Example :

 if(rval==1)
     DO THIS 1;
 else if(rval==2)
     DO THIS 2;
 else if (rval ==3)
     DO THIS 3;

Answer 1

You can’t. What you have there looks like a use-case for a switch-statement:

switch(rval) {
    case -1: DO THIS 1; break;
    case  2: DO THIS 2; break;
    case  3: DO THIS 3; break;
    default: ERROR;
}

Edit I was over-eager here. As @Luchian Grigore points out in the comments, you can do this (as he demonstrates in his answer). Note that this is also well defined for cases where one or both expressions are of the type void, as specified in §5.16/2 [expr.cond]:

If either the second or the third operand has type (possibly
cv-qualified) void, then the lvalue-to-rvalue (4.1), array-to-pointer
(4.2), and function-to-pointer (4.3) standard conversions are
performed on the second and third operands, and one of the following
shall hold:

— The second or the third operand (but not both) is a
throw-expression (15.1); the result is of the type of the other and is
an rvalue.

— Both the second and the third operands have type void;
the result is of type void and is an rvalue.

[Note: this includes the
case where both operands are throw-expressions. ]