How do I preallocate memory for a structure in MATLAB 7.6 (R2008a)?

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I would like to know how to preallocate memory for a structure.

Accepted Answer

MathWorks Support Team
MathWorks Support Team on 10 Dec 2021
Edited: MathWorks Support Team on 13 Dec 2021
There is a way to preallocate memory for a structure in MATLAB 7.6 (R2008a) using the STRUCT and REPMAT commands.
EXAMPLE 1: A structure with two fields
s.field1
s.field2
field_names = {'field1','field2'}; % Cell with field names
empty_cells = repmat(cell(1),1,numel(field_names));
entries = {field_names{:} ; empty_cells{:}};
s = struct(entries{:});
EXAMPLE 2: A structure with a field with a subfield
s.field1.subfield
s = struct('field1',struct('subfield',cell(1)));
EXAMPLE 3: An array of structures
v(1).field1
...
v(100).field1
s = struct('field1',cell(1));
v = repmat(s,100,1);
Even though the data elements pointed to by the different fields in a structure are not stored contiguously, a benefit of preallocating structures is that you avoid the (possibly time-consuming) operation of dynamically growing the structure field header. Please refer to the following documentation page for more information:
  2 Comments
Walter Roberson
Walter Roberson on 15 Feb 2017
The line
empty_cells = repmat(cell(1),1,numel(field_names));
creates a cell array of copies of [] . The line
entries = {field_names{:} ; empty_cells{:}};
arranges those copies below the field names, like
{ 'first', 'second', 'third';
[], [], [] }
and the line
s = struct(entries{:});
converts that to
a = struct('first', [], 'second', [], 'third', [])
so already [] is being used.
The code given in the first part of the solution is code to automatically generate this just given the field names.

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