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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;
==> // ** Could not get 'IdxFile'.
==> // ** Either set environment variable 'SINGULAR_IDX_FILE' to 'IdxFile',
==> // ** or make sure that 'IdxFile' is at "%D/singular/singular.idx"
==> // ** Displaying help in browser 'dummy'.
==> // ** Use 'system("--browser", <browser>);' to change browser,
==> // ** where <browser> can be: "dummy", "emacs".
==>    ? No functioning help browser available.
==>    ? error occurred in or before ./examples/Example_of_a_procedure_defini\
   tion_in_a_library:.sing line 7: `  help tab;`