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  • Problems with Parametric Macros

    From emanuele cannizzo@emacannizzo@gmail.com to comp.std.c on Sun Feb 7 09:30:33 2021
    From Newsgroup: comp.std.c

    In my code I have a lot of vectors with a similar name:
    vector1, vector2, vector3, vector4, ...
    Because I don't want to write the same code with the only difference of the names of vectors I tried creating this macro:

    #define VECTOR(x) vector##x

    I tried this code as an example:
    int n = 1;
    printf("%d", VECTOR(n)[0]);

    I thought that was the same as printf("%d", vector1[0]); but the compiler substitutes the macro VECTOR(n)[0] with vectorn[0]

    Can you give me a solution to this problem or suggest me other tecniques to use? Thanks

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  • From James Kuyper@jameskuyper@alumni.caltech.edu to comp.std.c on Sun Feb 7 13:27:11 2021
    From Newsgroup: comp.std.c

    On 2/7/21 12:30 PM, emanuele cannizzo wrote:
    In my code I have a lot of vectors with a similar name:
    vector1, vector2, vector3, vector4, ...
    Because I don't want to write the same code with the only difference of the names of vectors I tried creating this macro:

    #define VECTOR(x) vector##x

    I tried this code as an example:
    int n = 1;
    printf("%d", VECTOR(n)[0]);

    I thought that was the same as printf("%d", vector1[0]); but the compiler substitutes the macro VECTOR(n)[0] with vectorn[0]

    Can you give me a solution to this problem or suggest me other tecniques to use? Thanks

    The problem is that macro substitution occurs at compile time
    (specifically, during translation phase 4), and it is purely textual.
    As far as the preprocessor is concerned, "n" is just a character string,
    which is why the result of the macro expansion is "vectorn". "n" doesn't
    become the name of a variable with a value until translation phase 7,
    and it doesn't become the variable itself (as opposed to the name of a variable) until run time.

    There's several alternatives you could use, one of which is

    int *vector[] = {vector0, vector1, vector2, vector3};

    and then

    vector[n][0].
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  • From =?UTF-8?Q?Hans-Bernhard_Br=c3=b6ker?=@HBBroeker@t-online.de to comp.std.c on Sun Feb 7 21:11:57 2021
    From Newsgroup: comp.std.c

    Am 07.02.2021 um 18:30 schrieb emanuele cannizzo:
    In my code I have a lot of vectors with a similar name:
    vector1, vector2, vector3, vector4, ...

    Don't do that. As a rule of thumb, whenever you feel the urge to put
    running numbers of things into the name of things, you're almost
    certainly missing out on the obvious solution to make an array out of
    those things, instead, turning that number into an array index.

    You want

    vectortype vector[10]; // or however many of these you need

    Either that, you or you need to get up to speed with pointers to
    "vectortype" things.


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