Define in C does not work when input is composite

Question

Problems using define in C. Works well when I call OP(9), but when I call OP(7+2) I get 23. Why?

 #include<stdio.h>
 #include<stdlib.h>

 #define OP(x) x*x;

 int main() {
 int x,y;

 x = 2;
 y = OP(7+2);
 printf("%d", y);

 return 0;

 }

Why prints 23 and not 81?

Parentheses — or lack thereof. Look at `7 + 2*7 + 2` – that is `23`. If you want `(7+2)*(7+2)`, add the parentheses: `#define OP(x) ((x)*(x))` — all of them! — Jonathan Leffler, Feb 09 '17 at 17:39
"...but when I call `OP(7+2)`..." You cannot "call" `OP(7 + 2)`. `OP` is a macro, not a function. Macros cannot be "called". They can only be substutuded/replaced. The rest follows. — AnT stands with Russia, Feb 09 '17 at 17:45

score 4 · Accepted Answer · edited Feb 09 '17 at 17:42

4

You should wrap x in parentheses in order to force precedence. However, it is also essential to wrap the entire expression in parentheses.

#define OP(x) ((x)*(x))

edited Feb 09 '17 at 17:42

Jonathan Leffler

answered Feb 09 '17 at 17:39

Ryan

Better yet: `#define OP(x) ((x)*(x))` – David Ranieri Feb 09 '17 at 17:40
1

going from -1 to +1 for the correction, reason can be seen by running these lines `#define ABS(a) (a) < 0 ? -(a) : (a)` `printf("ABS(-3) - 1): %d\n", ABS(-3) - 1);` – Kami Kaze Feb 09 '17 at 17:42

1 Answers1