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

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

Answer 1

[MathWorks Support Team]

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:

https://www.mathworks.com/help/matlab/matlab_prog/memory-requirements-for-a-structure-array.html

[Walter Roberson]

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.