## 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 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 The for Statement)

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

— Built-in Function: struct ("field", value, "field", value, ...)

> Create a structure and initialize its value.

> If the values are cell arrays, create a structure array and initialize its values. The dimensions of each cell array of values must match. Singleton cells and non-cell values are repeated so that they fill the entire array. If the cells are empty, create an empty structure array with the specified field names.

— 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.

— Function File: [v1, ...] = getfield (s, key, ...)

> Extract fields from a structure. For example

>               ss(1,2).fd(3).b=5;
>               getfield (ss, {1,2}, "fd", {3}, "b")
>               => ans = 5

> Note that the function call in the previous example is equivalent to the expression

>                        i1= {1,2}; i2= "fd"; i3= {3}; i4= "b";
>                        ss(i1{:}).(i2)(i3{:}).(i4)

> See also: setfield, rmfield, isfield, isstruct, fieldnames, struct.

— Function File: [t, p] = orderfields (s1, s2)

> Return a struct with fields arranged alphabetically or as specified by s2 and a corresponding permutation vector.

> Given one struct, arrange field names in s1 alphabetically.

> Given two structs, arrange field names in s1 as they appear in s2. The second argument may also specify the order in a permutation vector or a cell array of strings.

> See also: getfield, rmfield, isfield, isstruct, fieldnames, struct.

— Built-in Function: rmfield (s, f)

> Remove field f from the structure s. If f is a cell array of character strings or a character array, remove the named fields.

> See also: cellstr, iscellstr, setfield.

— Function File: [k1, ..., v1] = setfield (s, k1, v1, ...)

> Set field members in a structure.

>               oo(1,1).f0= 1;
>               oo = setfield(oo,{1,2},'fd',{3},'b', 6);
>               oo(1,2).fd(3).b == 6
>               => ans = 1

> Note that this function could be written

>                        i1= {1,2}; i2= 'fd'; i3= {3}; i4= 'b';
>                        oo( i1{:} ).( i2 )( i3{:} ).( i4 ) == 6;

> See also: getfield, rmfield, isfield, isstruct, fieldnames, struct.

— Built-in Function: struct2cell (S)

> Create a new cell array from the objects stored in the struct object. If f is the number of fields in the structure, the resulting cell array will have a dimension vector corresponding to `[`F `size(`S`)]`.

> See also: cell2struct, fieldnames.

— Function File: substruct (type, subs, ...)

> Create a subscript structure for use with `subsref` or `subsasgn`.

> See also: subsref, subsasgn.