Thread Subject: Preallocating Structure Arrays

Hi,

I have a structure array with two subarrays. One subarray
is an array of doubles, and the other is an array of
chars. I want to preallocate my structure array before I
add my data to it, so that I don't take a huge hit in speed.

My question is what would be the best way to preallocate my
strucutre array, especially if it has a subarray of chars?
In my subarray of chars, I'd like to store words of
different size (i.e. 'blue', 'green', 'red', etc). Also,
I'd like to preallocate a large amount of space for both of
my subarrays.

Thank you for any help you may provide.

"C M" <cmikki17@yahoo.com> wrote in message
<g431s2$6fp$1@fred.mathworks.com>...
> Hi,
>
> I have a structure array with two subarrays. One subarray
> is an array of doubles, and the other is an array of
> chars. I want to preallocate my structure array before I
> add my data to it, so that I don't take a huge hit in speed.
>
> My question is what would be the best way to preallocate my
> strucutre array, especially if it has a subarray of chars?
> In my subarray of chars, I'd like to store words of
> different size (i.e. 'blue', 'green', 'red', etc). Also,
> I'd like to preallocate a large amount of space for both of
> my subarrays.
>
> Thank you for any help you may provide.

You could try something like this:

% max sizes for double subarray, char subarray, and overall
array
maxDoubleSize = 100;
maxCharSize = 100;
arrSize = 100;

% a representative element
element =
struct('arrDoubles',zeros(1,maxDoubleSize),'arrChars',char(zeros(1,maxCharSize)));

% your final array, which replicates the elements
data = repmat(element,1,100);

Thank you

"Ian Clarkson" <ovoidkumquat@hotmail.com> wrote in message
<g433tm$uh$1@fred.mathworks.com>...
> "C M" <cmikki17@yahoo.com> wrote in message
> <g431s2$6fp$1@fred.mathworks.com>...
> > Hi,
> >
> > I have a structure array with two subarrays. One
subarray
> > is an array of doubles, and the other is an array of
> > chars. I want to preallocate my structure array before
I
> > add my data to it, so that I don't take a huge hit in
speed.
> >
> > My question is what would be the best way to
preallocate my
> > strucutre array, especially if it has a subarray of
chars?
> > In my subarray of chars, I'd like to store words of
> > different size (i.e. 'blue', 'green', 'red', etc).
Also,
> > I'd like to preallocate a large amount of space for
both of
> > my subarrays.
> >
> > Thank you for any help you may provide.
>
> You could try something like this:
>
> % max sizes for double subarray, char subarray, and
overall
> array
> maxDoubleSize = 100;
> maxCharSize = 100;
> arrSize = 100;
>
> % a representative element
> element =
> struct('arrDoubles',zeros(1,maxDoubleSize),'arrChars',char
(zeros(1,maxCharSize)));
>
> % your final array, which replicates the elements
> data = repmat(element,1,100);

"C M" <cmikki17@yahoo.com> writes:

> Hi,
>
> I have a structure array with two subarrays. One subarray
> is an array of doubles, and the other is an array of
> chars. I want to preallocate my structure array before I
> add my data to it, so that I don't take a huge hit in speed.
>
> My question is what would be the best way to preallocate my
> strucutre array, especially if it has a subarray of chars?
> In my subarray of chars, I'd like to store words of
> different size (i.e. 'blue', 'green', 'red', etc). Also,
> I'd like to preallocate a large amount of space for both of
> my subarrays.

You actually don't need to preallocate the contents of a struct array.
As long as you preallocate the struct array itself (with the correct
size), with empty fields, you'll be fine. Also preallocate each
subarray before writing to it in a loop, but you can do that when you
need it.

Entries in a cell/struct array are simply more MATLAB variables, so
adding an element to a cell/struct (as long as the cell/struct is not
expanding itself) does not require reallocating or copying data.

% This is important:
a = repmat(struct('field1', [], 'field2', []), 100, 1);

for i = 1:100
  % This is fine:
  a(i).field1 = zeros(100);
  a(i).field2 = 'blue';

  % This is not fine:
  for j = 1:200
    a(i).field1(j) = j;
  end

  % But this is fine:
  a(i).field1 = zeros(200,1);
  for j = 1:200
    a(i).field1(j) = j;
  end
 
end


Does that make sense?


-Peter

Yes, thank you Peter

Peter Boettcher <boettcher@ll.mit.edu> wrote in message
<muyiqvu25jd.fsf@G99-Boettcher.llan.ll.mit.edu>...
> "C M" <cmikki17@yahoo.com> writes:
>
> > Hi,
> >
> > I have a structure array with two subarrays. One
subarray
> > is an array of doubles, and the other is an array of
> > chars. I want to preallocate my structure array before
I
> > add my data to it, so that I don't take a huge hit in
speed.
> >
> > My question is what would be the best way to
preallocate my
> > strucutre array, especially if it has a subarray of
chars?
> > In my subarray of chars, I'd like to store words of
> > different size (i.e. 'blue', 'green', 'red', etc).
Also,
> > I'd like to preallocate a large amount of space for
both of
> > my subarrays.
>
> You actually don't need to preallocate the contents of a
struct array.
> As long as you preallocate the struct array itself (with
the correct
> size), with empty fields, you'll be fine. Also
preallocate each
> subarray before writing to it in a loop, but you can do
that when you
> need it.
>
> Entries in a cell/struct array are simply more MATLAB
variables, so
> adding an element to a cell/struct (as long as the
cell/struct is not
> expanding itself) does not require reallocating or
copying data.
>
> % This is important:
> a = repmat(struct('field1', [], 'field2', []), 100, 1);
>
> for i = 1:100
> % This is fine:
> a(i).field1 = zeros(100);
> a(i).field2 = 'blue';
>
> % This is not fine:
> for j = 1:200
> a(i).field1(j) = j;
> end
>
> % But this is fine:
> a(i).field1 = zeros(200,1);
> for j = 1:200
> a(i).field1(j) = j;
> end
>
> end
>
>
> Does that make sense?
>
>
> -Peter