CODEWITHSUNDEEP
c++c-preprocessor
xorxorxor
xorxorxor

Reputation: 65

DEFINE() macro usage in expressinions

So I defined..

#define RADIAN(x) x*3.14159265f/180.0f

and then used it like this:

RADIAN(theta-90)

My program constantly gave me incorrect results, it took me a couple hours to realize that there was a huge difference between the above statement and the statement below.

RADIAN((theta-90))

Now my program is running perfectly fine. Why is the first statement incorrect?

Upvotes: 2

Views: 955

Answers (4)

RobH
RobH

Reputation: 3338

The answers above are all correct. However, one point has not yet been made...

This is C++, not C. Stop using preprocessor macros.

Upvotes: 2

Shamim Hafiz - MSFT
Shamim Hafiz - MSFT

Reputation: 22104

This is because, in the first case X will be replaced by (theta-90) so your function would evaluate to: 

theta - 90* 3.14159265f/180.0f

Upvotes: 0

x13n
x13n

Reputation: 4153

#define makes just text substitution, so RADIAN(theta-90) was really theta-90*3.14159265f/180.0f, what obviously wasn't what you meant. Try

#define RADIAN(x) ((x)*3.14159265f/180.0f)

instead.

Upvotes: 7

forsvarir
forsvarir

Reputation: 10839

Macro's largley do text based replacement so

RADIAN(theta-90)

expands to:

theta - 90* 3.14159265f/180.0f

which because of operator precedence, evaluates as:

theta - (90* 3.14159265f/180.0f)

Upvotes: 4

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