(octave.info)Creating Structures

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6.1.3 Creating Structures
-------------------------

Besides the index operator ".", Octave can use dynamic naming "(var)" or
the ‘struct’ function to create structures.  Dynamic naming uses the
string value of a variable as the field name.  For example:

     a = "field2";
     x.a = 1;
     x.(a) = 2;
     x
          ⇒ x =
             {
               a =  1
               field2 =  2
             }

Dynamic indexing also allows you to use arbitrary strings, not merely
valid Octave identifiers (note that this does not work on MATLAB):

     a = "long field with spaces (and funny char$)";
     x.a = 1;
     x.(a) = 2;
     x
          ⇒ x =
             {
               a =  1
               long field with spaces (and funny char$) =  2
             }

The warning id ‘Octave:language-extension’ can be enabled to warn about
this usage.  Note: warning_ids.

   More realistically, all of the functions that operate on strings can
be used to build the correct field name before it is entered into the
data structure.

     names = ["Bill"; "Mary"; "John"];
     ages  = [37; 26; 31];
     for i = 1:rows (names)
       database.(names(i,:)) = ages(i);
     endfor
     database
          ⇒ database =
             {
               Bill =  37
               Mary =  26
               John =  31
             }

   The third way to create structures is the ‘struct’ command.  ‘struct’
takes pairs of arguments, where the first argument in the pair is the
fieldname to include in the structure and the second is a scalar or cell
array, representing the values to include in the structure or structure
array.  For example:

     struct ("field1", 1, "field2", 2)
     ⇒ ans =
           {
             field1 =  1
             field2 =  2
           }

   If the values passed to ‘struct’ are a mix of scalar and cell arrays,
then the scalar arguments are expanded to create a structure array with
a consistent dimension.  For example:

     s = struct ("field1", {1, "one"}, "field2", {2, "two"},
             "field3", 3);
     s.field1
          ⇒
             ans =  1
             ans = one

     s.field2
          ⇒
             ans =  2
             ans = two

     s.field3
          ⇒
             ans =  3
             ans =  3

   If you want to create a struct which contains a cell array as an
individual field, you must wrap it in another cell array as shown in the
following example:

     struct ("field1", {{1, "one"}}, "field2", 2)
          ⇒ ans =
             {
               field1 =

             {
               [1,1] =  1
               [1,2] = one
             }

               field2 =  2
             }

 -- : S = struct ()
 -- : S = struct (FIELD1, VALUE1, FIELD2, VALUE2, ...)
 -- : S = struct (OBJ)

     Create a scalar or array structure and initialize its values.

     The FIELD1, FIELD2, ... variables are strings specifying the names
     of the fields and the VALUE1, VALUE2, ... variables can be of any
     type.

     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.

     If the argument is an object, return the underlying struct.

     Observe that the syntax is optimized for struct *arrays*.  Consider
     the following examples:

          struct ("foo", 1)
            ⇒ scalar structure containing the fields:
              foo =  1

          struct ("foo", {})
            ⇒ 0x0 struct array containing the fields:
              foo

          struct ("foo", { {} })
            ⇒ scalar structure containing the fields:
              foo = {}(0x0)

          struct ("foo", {1, 2, 3})
            ⇒ 1x3 struct array containing the fields:
              foo


     The first case is an ordinary scalar struct—one field, one value.
     The second produces an empty struct array with one field and no
     values, since being passed an empty cell array of struct array
     values.  When the value is a cell array containing a single entry,
     this becomes a scalar struct with that single entry as the value of
     the field.  That single entry happens to be an empty cell array.

     Finally, if the value is a non-scalar cell array, then ‘struct’
     produces a struct *array*.

     See also: Note: cell2struct, *note fieldnames:
     XREFfieldnames, Note: getfield, *note setfield:
     XREFsetfield, Note: rmfield, *note isfield:
     XREFisfield, Note: orderfields, *note isstruct:
     XREFisstruct, Note: structfun.

   The function ‘isstruct’ can be used to test if an object is a
structure or a structure array.

 -- : isstruct (X)
     Return true if X is a structure or a structure array.

     See also: Note: ismatrix, Note: iscell,
     Note: isa.