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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.