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Re: What's wrong with "char op[8]=(istest?"ef":"df");"

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On 2022-01-25, John Forkosh <forkosh@panix.com> wrote:
> Minimal working (or should I say non-working example)...
>
> #include <stdio.h>
> int main () {
>   int istest=1;
>   char op[8] = (istest?"ef":"df");
>   /*char op[8] = "ef";*/
>   printf("op=%s\n",op); }
>
> for which cc emits the error...
>
> test.c: In function 'main':
> test.c:4:16: error: invalid initializer
>     4 |   char op[8] = (istest?"ef":"df");
>       |                ^
>
> But if I comment out that line and uncomment the one below it,
> then it compiles and runs fine. I've used this construction
> a million times without problems. But a-million-and-one seems
> to somehow be a problem. What's wrong???

Initialization is not assignment, as others have answered.
An array of char may be initialized by a literal, and that
has special meaning.

Note that arrays cannot be assigned at all in C:

  char a[5];
  a = "abc"; // error

A solution which is semantically close to the apparent
intended meaning of what you're trying to do is this:

  char op[8] = { istest ? 'e' : 'd', 'f' };

(This uses a non-constant expression for initializing an aggregate
member, which was not required to work in ISO C C90, but became standard
as of C99, at least for aggregates defined in automatic storage.)

By semantically close I mean that op is the the type that you seem to
want: a modifiable array of 8 characters, and that the goal of
initializing that array one way or the other based on the flag is
satisfied.

Whereas:

  char *op = istest ? "ef" : "df";

is certainly succinct, op is not an array but a pointer.
Therefore sizeof (op) calculates sizeof (char *) which is
unrelated to how many characters there are in the underlyinga
array. It points to an array which is not of 8 chars but 3, and
that is not required to be modifiable; this is not well-defined
behavior, under this definition of op:

 op[0] = 'g';

Another solution:

  char op[8];
  strcpy(op, istest ? "ef" : "df");

If istest can be a compile-time condition:

  #define istest 1

  #if istest
  #define iftest_init(THEN, ELSE) THEN
  #define iftest(THEN, ELSE) (THEN)
  #else
  #define iftest_init(THEN, ELSE) ELSE
  #define iftest(THEN, ELSE) (ELSE)
  #endif

  char op[8] = iftest_init("ef", "df");

iftest() generates parentheses, which we want almost all the time,
except here; so we introduce iftest_init() which doesn't do that.

Defining "if" macros like this requires a bit of upfront verbiage,
but the resulting code can be preferrable to code like:

  #if test
  char op[8] = "ef";
  #else
  char op[8] = "df";
  #endif

or even

  char op[8] =
  #if test
  "ef"
  #else
  "df"
  #endif
  ;

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal

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What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-25 13:00 +0000
  Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-25 13:09 +0000
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" Bart <bc@freeuk.com> - 2022-01-25 14:03 +0000
      Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-25 14:33 +0000
  Re: What's wrong with "char op[8]=(istest?"ef":"df");" Mateusz Viste <mateusz@xyz.invalid> - 2022-01-25 14:36 +0100
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-25 14:41 +0000
  Re: What's wrong with "char op[8]=(istest?"ef":"df");" Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-01-25 06:39 -0800
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-25 14:56 +0000
  Re: What's wrong with "char op[8]=(istest?"ef":"df");" James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-25 11:08 -0500
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-26 08:18 +0000
  Re: What's wrong with "char op[8]=(istest?"ef":"df");" Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-25 14:37 -0800
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-26 08:27 +0000
  Re: What's wrong with "char op[8]=(istest?"ef":"df");" Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-25 19:49 -0800
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-01-25 19:56 -0800
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" James Kuyper <jameskuyper@alumni.caltech.edu> - 2022-01-25 23:37 -0500
      Re: What's wrong with "char op[8]=(istest?"ef":"df");" Andrey Tarasevich <andreytarasevich@hotmail.com> - 2022-01-25 20:54 -0800
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-26 08:37 +0000
  Re: What's wrong with "char op[8]=(istest?"ef":"df");" Kaz Kylheku <480-992-1380@kylheku.com> - 2022-01-26 05:08 +0000
    Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-26 09:22 +0000
  Re: What's wrong with "char op[8]=(istest?"ef":"df");" John Forkosh <forkosh@panix.com> - 2022-01-26 09:24 +0000

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