## 6 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 Variable: struct_levels_to_print

> You can tell Octave how many structure levels to display by setting the built-in variable `struct_levels_to_print`. The default value is 2.

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 The for Statement)

The following functions are available to give you information about structures.

— Built-in Function: isstruct (expr)

> Return 1 if the value of the expression expr is a structure.

— Built-in Function: fieldnames (struct)

> Return a cell array of strings naming the elements of the structure struct. It is an error to call `fieldnames` with an argument that is not a structure.

— Built-in Function: isfield (expr, name)

> Return true if the expression expr is a structure and it includes an element named name. The first argument must be a structure and the second must be a string.