lvalue required??

IndiraP's Avatar, Join Date: Nov 2012
Go4Expert Member
Code:
# define TWICE(i) 2*i 
# define TWO(i) i+i 
main () 
{ 
     int no, sum, product; 
     no = 1; 
     sum = -- TWICE(no); 
     --sum; 
     product = --TWo(no); 
     printf (“%d%d\n”, sum, product); 
}
y am i getting an error stating lvalue required at --TWICE(no); above??
wat does it mean???

Last edited by shabbir; 18Nov2012 at 10:31.. Reason: Code blocks
xpi0t0s's Avatar, Join Date: Aug 2004
Mentor
It means you aren't using #defines and the predecrement operator correctly.

--TWICE(no) expands to --2*no and you can't predecrement a constant.
However, in the case of TWICE, even brackets don't fix the problem because --(2*no) still won't work because 2*no is not an lvalue. An lvalue ("left value") is something you can put on the left of an assign, so in i=5, i is the lvalue and this is valid because you can assign a value to it. 5=i is invalid because you can't assign a value to 5. (2*no)=7 is invalid because you can't assign a value to (2*no).

--TWO(no) will "work" - in the sense that it won't throw an error, but you're still abusing the -- operator. This will expand to --i+i which gives undefined results.
IndiraP's Avatar, Join Date: Nov 2012
Go4Expert Member
Thank u for giving me a clear view..