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Problems with Parametric Macros

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  Problems with Parametric Macros emanuele cannizzo <emacannizzo@gmail.com> - 2021-02-07 09:30 -0800
    Re: Problems with Parametric Macros James Kuyper <jameskuyper@alumni.caltech.edu> - 2021-02-07 13:27 -0500
    Re: Problems with Parametric Macros Hans-Bernhard Bröker <HBBroeker@t-online.de> - 2021-02-07 21:11 +0100

#6213 — Problems with Parametric Macros

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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#6214

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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#6215

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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