pluraltx.how

来自「国外网站上的一些精典的C程序」· HOW 代码 · 共 40 行

Q: How do the plural_text(), plural_text2(), and plural_text3() macros work?

A: OK, it's pretty straightforward. The thing relies on two defined behaviors
   of standard C:

1.  Logical comparison operators return 0 for false and 1 for true. Although
    any non-zero value will test true, logical comparisons are defined to
    return (int)1.

2.  A quoted string is simply a shorthand method of defining an anonymous
    constant array of char whose length is that of the string plus one for the
    terminating NUL character.

  Given these facts, let's go through it... The "s" and "es" in the macros
are strings - constant character arrays. Like any character array, you can
select specific elements of the array by subscripting it. In order to form
proper plurals, you need to determine if the number is one or something else
since, in English, only the quantity one is singular (makes sense). The
comparison to one returns either 1 or 0 which is used to index into the
character array. In the case of single-character plurals, it will point to
either the 's' or the terminating NUL. In 2-character plurarls, it is
multiplied by 2 (i.e. shifted left by 1) so it will point to either the 'e'
or the terminating NUL. Since an indexed array points to a single element of
that array, the ampersand is there to take to the address of the character
pointed to. The character plurals also mulitply by two, but the embedded NUL
character causes either "y" or "ies" for the singular and plural cases.

  Now, walk through it... If the number is not 1, the comparison will be
false and the zero'th element of the array will be selected. Taking the
adrress of the zero'th element of either array will simply point to the whole
string. If the number is exactly 1, the comparison will test true and the
returned address will be that of the terminating NUL in either case.
Returning a character pointer to a NUL character is the same as using the
string "", i.e. nothing gets printed.

  There are alternative ways to express this same algorithm in a macro.
Here's one that uses pointer arithmetic rather than array subscripts.

#define plural_text(n)  "s"+(1==(n))