fabs(x) vs. (x) < 0 ? -(x) : (x)

[Israel Pinkas]

In article <4477 at ut-ngp.UUCP> jjr at ngp.UUCP (Jeff Rodriguez) writes:
>What's the difference between 
>     double x, y;
>     y = fabs(x)
>and
>     #define abs(X) ((X) < 0 ? -(X) : (X))
>     double x, y;
>     y = abs(x);
>I.e., why isn't fabs() implemented as a macro?

K & R point out (in their discussion of min and max macros) that creating a
macro like this would cause X to be evaluted twice.  While this might be OK
in most cases, consider if you had a function called read_float, which read
a float in from stdin.  Then the call

y = fabs(read_float())

would call read_float() twice if it was implemented as a macro, but only
once if it was implemented as a function.  Remembering that read_float()
reads a new float every time it is called, the macro implementation would
use the sign of the first number to determine whether to change the sign of
the second number.

-Israel
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