(octave.info)Basic Usage and Examples

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6.1.1 Basic Usage and Examples
------------------------------

Here are some examples of using data structures in Octave.

   Elements of structures can be of any value type.  For example, the
three expressions

     x.a = 1;
     x.b = [1, 2; 3, 4];
     x.c = "string";

create a structure with three elements.  The ‘.’ character separates the
structure name from the field name and indicates to Octave that this
variable is a structure.  To print the value of the structure you can
type its name, just as for any other variable:

     x
          ⇒ x =
             {
               a = 1
               b =

                 1  2
                 3  4

               c = string
             }

Note that Octave may print the elements in any order.

   Structures may be copied just like any other variable:

     y = x
          ⇒ y =
             {
               a = 1
               b =

                 1  2
                 3  4

               c = string
             }

   Since structures are themselves values, structure elements may
reference other structures.  The following statements change the value
of the element ‘b’ of the structure ‘x’ to be a data structure
containing the single element ‘d’, which has a value of 3.

     x.b.d = 3;
     x.b
          ⇒ ans =
             {
               d = 3
             }

     x
          ⇒ x =
             {
               a = 1
               b =
               {
                 d = 3
               }

               c = string
             }

   Note that when Octave prints the value of a structure that contains
other structures, only a few levels are displayed.  For example:

     a.b.c.d.e = 1;
     a
          ⇒ a =
             {
               b =
               {
                 c =
                 {
                   1x1 struct array containing the fields:

                   d: 1x1 struct
                 }
               }
             }

This prevents long and confusing output from large deeply nested
structures.  The number of levels to print for nested structures may be
set with the function ‘struct_levels_to_print’, and the function
‘print_struct_array_contents’ may be used to enable printing of the
contents of structure arrays.

 -- : VAL = struct_levels_to_print ()
 -- : OLD_VAL = struct_levels_to_print (NEW_VAL)
 -- : struct_levels_to_print (NEW_VAL, "local")
     Query or set the internal variable that specifies the number of
     structure levels to display.

     When called from inside a function with the "local" option, the
     variable is changed locally for the function and any subroutines it
     calls.  The original variable value is restored when exiting the
     function.

     See also: *note print_struct_array_contents:
     XREFprint_struct_array_contents.

 -- : VAL = print_struct_array_contents ()
 -- : OLD_VAL = print_struct_array_contents (NEW_VAL)
 -- : print_struct_array_contents (NEW_VAL, "local")
     Query or set the internal variable that specifies whether to print
     struct array contents.

     If true, values of struct array elements are printed.  This
     variable does not affect scalar structures whose elements are
     always printed.  In both cases, however, printing will be limited
     to the number of levels specified by STRUCT_LEVELS_TO_PRINT.

     When called from inside a function with the "local" option, the
     variable is changed locally for the function and any subroutines it
     calls.  The original variable value is restored when exiting the
     function.

     See also: Note: struct_levels_to_print.

   Functions can return structures.  For example, the following function
separates the real and complex parts of a matrix and stores them in two
elements of the same structure variable.

     function y = f (x)
       y.re = real (x);
       y.im = imag (x);
     endfunction

   When called with a complex-valued argument, ‘f’ returns the data
structure containing the real and imaginary parts of the original
function argument.

     f (rand (2) + rand (2) * I)
          ⇒ ans =
             {
               im =

                 0.26475  0.14828
                 0.18436  0.83669

               re =

                 0.040239  0.242160
                 0.238081  0.402523

             }

   Function return lists can include structure elements, and they may be
indexed like any other variable.  For example:

     [ x.u, x.s(2:3,2:3), x.v ] = svd ([1, 2; 3, 4]);
     x
          ⇒ x =
             {
               u =

                 -0.40455  -0.91451
                 -0.91451   0.40455

               s =

                  0.00000   0.00000   0.00000
                  0.00000   5.46499   0.00000
                  0.00000   0.00000   0.36597

               v =

                 -0.57605   0.81742
                 -0.81742  -0.57605

             }

   It is also possible to cycle through all the elements of a structure
in a loop, using a special form of the ‘for’ statement (Note: Looping
Over Structure Elements).