lvalue required??

Discussion in 'C' started by IndiraP, Nov 17, 2012.

1. IndiraPNew Member

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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 (ā%d%d\nā, sum, product);
}
```
y am i getting an error stating lvalue required at --TWICE(no); above??
wat does it mean???

Last edited by a moderator: Nov 18, 2012
2. xpi0t0sMentor

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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.

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