Objective c : Using #define constant values in math expressions

Question

I'm new to iPhone development and I'm just trying out some simple drawing routines and I'm having trouble using defined values in simple math.

I have a line like this:

int offset = (((myValue - min_value) * 6) - middle);

and this works fine - but I don't like using the hard coded 6 in there (because I'll use it lots of places.

So I thought I'd define a constant using #define:

#define WIDTH_OFFSET 6;

then I could use:

int offset = (((myValue - min_value) * WIDTH_OFFSET) - middle);

however - this gets a compiler error : "Expected Expression."

I can get round this by breaking up the calculation onto several lines:

int offset = myValue - min_value;
offset = offset * WIDTH_OFFSET;
offset = offset - middle;

The compiler thinks this is fine.

I'm guessing there's some implicit cast or some other language feature at work here - can anyone explain to me what is happening?

Answer 1

Remove the semicolon ; after #define:

#define WIDTH_OFFSET 6

#define substitutes its arguments literally, so your expression after preprocessing becomes

(((myValue - min_value) * 6;) - middle);

As you can see, there is a semicolon in the middle of the expression, which is a syntax error.

On the other hand, your other expression

int offset = myValue - min_value;
offset = offset * WIDTH_OFFSET;

does not exhibit such problem, because having two semicolons in a row as in

 offset = offset * 6;;

is syntactically valid.

Answer 2

When you #define something where you use it is exactly the same as if you typed it in yourself there. So where you are using WIDTH_OFFSET you are getting 6; in its place - which of course is not your intention. So just remove the semicolon.

Answer 3

As dasblinkenlight said, remove the semi colon. The explanation for this is that #defines are a literal substitution into your code. Thus with the semi colon your broken code read:

int offset = (((myValue - min_value) * 6;) - middle);

The working code read:

offset = offset * 6;;

Which is syntactically fine as the ;; is effectively a blank 'line' of code.

Answer 4

Basically, macros are convenient functions that are placed inline by the preprocessor. So you can think that they are doing copy/paste for matching entries, in your case it will substitute any occurency of WIDTH_OFFSET with 6; so, just like others said, remover the semicolon ; and you are all set.

Also, when defining macros for simple math functions, remeber to put them in brackets ( and ) otherwise, you could end up with some math operation order bugs( like unintended part multiplication before addition)