File: chap8.txt

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  8 Records
  
  
  8.1 Functions for records
  
  8.1-1 AssignGlobals
  
  AssignGlobals( rec )  function
  
  This function has been transferred from package RCWA.
  
  It  assigns  the  record components of rec to global variables with the same
  names.
  
    Example  
    
    gap> r := rec( a := 1, b := 2, c := 3 );;                                      
    gap> AssignGlobals( r );
    The following global variables have been assigned:
    [ "a", "b", "c" ]
    gap> [a,b,c];
    [ 1, 2, 3 ]
    
  
  
  
  8.2 Option records for functions
  
  8.2-1 OptionRecordWithDefaults
  
  OptionRecordWithDefaults( defaults, useroptions )  function
  
  This  functions has been transferred by Chris Jefferson from other packages.
  It  simplifies  the  handling  of  records which are intended to be used for
  expressing  configuration options. defaults represents the "default record",
  and useroptions lets the user give new values for values in defaults.
  
  The  function returns a record with the same component names as defaults and
  which  has  the same values as defaults, except for those component names in
  useroptions,  where  the values in useroptions are used instead. An error is
  given  if  useroptions  contains  any  component  names  not in defaults. If
  useroptions  is  an  empty  list  it  is  treated as an empty record, and if
  useroptions  is  a list of length 1 containing a record, this record is used
  as useroptions.
  
    Example  
    
    gap> defaults := rec( a := 1, b := 2, c := 3 );;
    gap> OptionRecordWithDefaults( defaults, rec( a := 6) );
    rec( a := 6, b := 2, c := 3 )
    gap> OptionRecordWithDefaults( defaults, rec( b := 7, c := 8 ) );
    rec( a := 1, b := 7, c := 8 )
    gap> OptionRecordWithDefaults( defaults, [ ] );
    rec( a := 1, b := 2, c := 3 )
    gap> OptionRecordWithDefaults( defaults, [ rec( c := 8 ) ] );
    rec( a := 1, b := 2, c := 8 )
    gap> OptionRecordWithDefaults( defaults, rec( d := 9 ) );
    Error, Unknown option: d
    gap> OptionRecordWithDefaults( defaults, [ rec( b := 7 ), rec( c := 8 ) ] );
    Error, Too many arguments for function
    gap> OptionRecordWithDefaults( defaults, [6,7,8] );
    Error, Too many arguments for function
    
  
  
  This function is designed to support functions with optional arguments given
  as   a  variable  record,  of  the  form  function(x,y,options...).  In  the
  following,  very  contrived, example function, PrintDimensions, the defaults
  are given by the variable order which takes values h, w and d having default
  values   1,   2   and   3.   If   there   is   a   second   argument,   then
  OptionRecordWithDefaults(  order,  arg[2]  ); is used to cvhange the values.
  These  three  values  then determine the order in which the three dimensions
  are printed using a SortParallel command.
  
  
    
    PrintDimensions := function( arg ) 
        local nargs, dim, order, V, L, len, K, i; 
        nargs := Length( arg ); 
        dim := [ arg[1]!.height, arg[1]!.width, arg[1]!.depth ]; 
        order := rec( h := 1, w := 2, d := 3 ); 
        V := [ "height", "width", "depth" ]; 
        if ( nargs > 1 ) and IsRecord( arg[2] ) then 
            order := OptionRecordWithDefaults( order, arg[2] ); 
        fi; 
        L := [ order!.h, order!.w, order!.d ]; 
        len := Length( L );
        K := [ 1..len ]; 
        SortParallel( L, K ); 
        Print( "dimensions: " ); 
        Print( V[K[1]], " = ", dim[K[1]], ", " );
        Print( V[K[2]], " = ", dim[K[2]], ", " );
        Print( V[K[3]], " = ", dim[K[3]], "\n" );
    end;;
  
  
  In  the  example  below  the  first  call  to  PrintDimensions  has just one
  parameter,  mydim,  so  the  default  order  is  used.  In  the second call,
  alternate  values  for h, w and d are given, causing the width to be printed
  first, and then the depth and height.
  
    Example  
    
    gap> mydim := rec( height := 45, width := 31, depth := 17 ); 
    rec( depth := 17, height := 45, width := 31 )
    gap> PrintDimensions( mydim );
    dimensions: height = 45, width = 31, depth = 17
    gap> PrintDimensions( mydim, rec( h:=3, w:=1, d:=2 ) );
    dimensions: width = 31, depth = 17, height = 45