# Using #define action with externs and registers

**URL:** <https://forum.p4.org/t/using-define-action-with-externs-and-registers/424>\
**Category:** P4 Dev\
**Created:** [July 13, 2022, 2:26pm UTC](https://forum.p4.org/t/using-define-action-with-externs-and-registers/424 "2022-07-13T14:26:22Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![feliperampazzo](https://avatars.discourse-cdn.com/v4/letter/f/a4c791/32.png) [@feliperampazzo](https://forum.p4.org/u/feliperampazzo)\
**Post date:** [July 13, 2022, 2:26pm UTC](https://forum.p4.org/t/using-define-action-with-externs-and-registers/424/1 "2022-07-13T14:26:22Z")

</div>

Hi, guys! I’m new with P4 and I have lots of doubts but for now my question is how to implement (if it is possible) this code/idea:

I want to reduce my code to generate random values and set them into a register like that:

```auto
register<bit<32>>(16) register_pub_matrix;
.
.
.
#define gen_matrix(INDEX) action gen_matrix_r##INDEX##() {
        bit<32> coef;
        random(coef, 0, PARAM_Q);
        register_pub_matrix.write(##INDEX##, coef);
      }
      gen_matrix(0)
      gen_matrix(1)
      gen_matrix(2)
      gen_matrix(3)
      gen_matrix(4)
      gen_matrix(5)
      gen_matrix(6)
      gen_matrix(7)
      gen_matrix(8)
      gen_matrix(9)
      gen_matrix(10)
      gen_matrix(11)
      gen_matrix(12)
      gen_matrix(13)
      gen_matrix(14)
      gen_matrix(15)

```

However, I got this error message:

> syntax error, unexpected IDENTIFIER, expecting ACTION or CONST or TABLE  
> random  
> ^^^^^^^

If I understood correctly, I can only use a constant here, not call methods, externs, etc…  
Is there any way to implement this without copy and paste multiple times the random call?

Thanks in advance!

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<div class="post-metadata">

**Author:** ![andyfingerhut](https://sea1.discourse-cdn.com/flex019/user_avatar/forum.p4.org/andyfingerhut/32/17_2.png) [@andyfingerhut](https://forum.p4.org/u/andyfingerhut)\
**Post date:** [July 13, 2022, 10:01pm UTC](https://forum.p4.org/t/using-define-action-with-externs-and-registers/424/2 "2022-07-13T22:01:22Z")

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P4 compilers typically use the C preprocessor for handling #define, and the entire definition of the macro must either (a) fit on a single line, or (b) you must put a backslash () character as the very last character of a line in order to make the definition continue with the following line. Thus if you put a backslash character at the end of your #define line, and each of the next 3 lines, but NOT the one containing ‘}’ (since you do not want to continue the macro definition after that), I suspect it might work.

In general, explicitly invoking the C preprocessor and looking at the output to see if it looks like legal P4 code is a good way to help debug such things.

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<div class="post-metadata">

**Author:** ![feliperampazzo](https://avatars.discourse-cdn.com/v4/letter/f/a4c791/32.png) [@feliperampazzo](https://forum.p4.org/u/feliperampazzo)\
**Post date:** [July 14, 2022, 4:29pm UTC](https://forum.p4.org/t/using-define-action-with-externs-and-registers/424/3 "2022-07-14T16:29:30Z")

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Thank you very much, Mr. Andy! It works. I still have difficulties compiling without using the scripts contained in [GitHub - p4lang/tutorials: P4 language tutorials](https://github.com/p4lang/tutorials). I need to understand better how to debug the code, but thanks for the help!!