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6.1 Data Structures

Octave includes support for organizing data in structures. The current implementation uses an associative array with indices limited to strings, but the syntax is more like C-style structures. 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. To print the value of the structure, you can type its name, just as for any other variable:

octave:2> 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.

octave:1> 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.

octave:1> x.b.d = 3
x.b.d = 3
octave:2> x.b
ans =
{
  d = 3
}
octave: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,

octave:1> a.b.c.d.e = 1;
octave:2> a
a =
{
  b =
  {
    c =
    {
      d: 1x1 struct
    }
  }
}

This prevents long and confusing output from large deeply nested structures.

Built-in Function: val = struct_levels_to_print ()

Built-in Function: old_val = struct_levels_to_print (new_val)

Query or set the internal variable that specifies the number of structure levels to display.

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.

octave:1> 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.

octave:2> 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,

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

  -0.40455  -0.91451
  -0.91451   0.40455

x.s =

  0.00000  0.00000  0.00000
  0.00000  5.46499  0.00000
  0.00000  0.00000  0.36597

x.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 (see section 10.5 The for Statement)