### 3.7.1 Procedure definition

Syntax:
  [`static`] `proc` proc_name [(<parameter_list>)]
  [<help_string>]
  `{`
  <procedure_body>
  `}`
  [`example`
  `{`
  <sequence_of_commands>
  `}`]
Purpose:
  - Defines a new function, the `proc` proc_name.
  - The help string, the parameter list, and the example section are optional. They are, however, mandatory for the procedures listed in the header of a library. The help string is ignored and no example section is allowed if the procedure is defined interactively, i.e., if it is not loaded from a file by the `LIB` or `load` command (see LIB and see load ).
  - Once loaded from a file into a SINGULAR session, the information provided in the help string will be displayed upon entering `help proc_name;`, while the `example` section will be executed upon entering `example proc_name;`. See Parameter list, Help string, and the example in Procedures in a library.
  - In the body of a library, each procedure not meant to be accessible by users should be declared static. See Procedures in a library.

#### Example of an interactive procedure definition and its execution:

      proc milnor_number (poly p)
      {
        ideal i= std(jacob(p));
        int m_nr=vdim(i);
        if (m_nr<0)
        {
          "// not an isolated singularity";
        }
        return(m_nr);         // the value of m_nr is returned
      }
      ring r1=0,(x,y,z),ds;
      poly p=x^2+y^2+z^5;
      milnor_number(p);
    ==> 4

#### Example of a procedure definition in a library:

First, we define the library (and store it as `sample.lib`):

    // Example of a user accessible procedure
    proc tab (int n)
    "USAGE:    tab(n);  n integer
    RETURNS:  string of n space tabs
    EXAMPLE:  example tab; shows an example"
    { return(internal_tab(n)); }
    example
    {
      "EXAMPLE:"; echo=2;
      for(int n=0; n<=4; n=n+1)
      { tab(4-n)+"*"+tab(n)+"+"+tab(n)+"*"; }
    }

    // Example of a static procedure
    static proc internal_tab (int n)
    { return(" "[1,n]); }

Now, we load the library and execute its procedures:

      LIB "sample.lib";        // load the library sample.lib
      example tab;             // show an example
    ==> // proc tab from lib sample.lib
    ==> EXAMPLE:
    ==>   for(int n=0; n<=4; n=n+1)
    ==>   { tab(4-n)+"*"+tab(n)+"+"+tab(n)+"*"; }
    ==>     *+*
    ==>    * + *
    ==>   *  +  *
    ==>  *   +   *
    ==> *    +    *
    ==>
      "*"+tab(3)+"*";          // use the procedure tab
    ==> *   *
      // the static procedure internal_tab is not accessible
      "*"+internal_tab(3)+"*";
    ==>    ? `internal_tab(3)` is not defined
    ==>    ? error occurred in or before ./examples/Example_of_a_procedure_defini\
       tion_in_a_library:.sing line 5: `  "*"+internal_tab(3)+"*";`
      // show the help section for tab
      help tab;
    ==> // proc tab from lib sample.lib
    ==> proc tab (int n)
    ==> USAGE:    tab(n);  n integer
    ==> RETURNS:  string of n space tabs
    ==> EXAMPLE:  example tab; shows an example

User manual for Singular version 4-0-3, 2016, generated by texi2html.