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.