The difference in defining PI in C

Question

I'm a beginner in programming and I have a question. Mainly, what is the difference in defining PI exactly as I state below;

#define PI  (4.*atan(1))
#define PIa  4.*atan(1)
#define PIb  4*atan(1)

I have printed this code:

    printf( "%lf\n", 3 / PI );
    printf( "%lf\n", 3 / PIa );
    printf( "%lf\n", 3 / PIb );

The results are:

 0.954930
 0.589049
 0.000000

According to my calculator the first one is correct, and the rest should also be. Why aren't they?

The difference is in the literal expansion of the macros. `3 / PIb` will become `3 / 4*atan(1)` - and here the first operation is *integer division* `3/4`, which is giving `0`. In `3 / PIa` the expansion leads to division to be carried out first too, which is not the correct order of operations. — Eugene Sh., Oct 14 '21 at 15:58

score 2 · Accepted Answer · answered Oct 14 '21 at 15:59

2

The macro is text substitution thus:

3 / PIa

is parsed as:

3 / 4.*atan(1) = (3/4.) * atan(1)

This is different from the first case where 3 / PI is expanded as:

3 / (4.*atan(1))

The similar case is for the third example:

3 / 4*atan(1) = (3/4) * atan(1)

The expression 3/4 is integer division, which result is 0.

Generally, it is strongly advised to put parentheses around the whole macro and all parameters it takes like in the following example:

#define ADD(a,b) ((a) + (b))

answered Oct 14 '21 at 15:59

tstanisl

1

Macro replacement substitutes preprocessor tokens, not text. Otherwise `#define x -a` could make `b-x` into `b` `--` `a` instead of `b` `-` `-` `a`. – Eric Postpischil Oct 14 '21 at 19:20
I get it now, thank you! :) – Dylikk Oct 14 '21 at 19:37

1 Answers1