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From: Mike on 18 Feb 2010 10:20
I am preallocating a cell array because MATLAB suggested it for greater efficiency. In the suggestion box there was this sentence:

"If you do not know the size of an array before the loop begins, preallocate it, and then if necessary, shrink it after the loop completes."

I have been searching for hours and I have not been able to find anything about shrinking the cell array. I tried checking the first element of each row starting at the end, but I can't make comparisons on cells to determine which row is the last with data. Can anyone help this MATLAB rookie?

Thank you.
From: Jan Simon on 18 Feb 2010 10:31
Dear Mike!

> I am preallocating a cell array because MATLAB suggested it for greater efficiency. In the suggestion box there was this sentence:
>
> "If you do not know the size of an array before the loop begins, preallocate it, and then if necessary, shrink it after the loop completes."
>
> I have been searching for hours and I have not been able to find anything about shrinking the cell array. I tried checking the first element of each row starting at the end, but I can't make comparisons on cells to determine which row is the last with data. Can anyone help this MATLAB rookie?

It would be helpful if you post the number of dimensions of the used cell.

You can use the logical array:
CELLFUN('isempty', C)

Or, assumed to be the easiest method, store the largest indices during the creation of the cell elements.

For more detailed answers, post your code.

Kind regards, Jan
From: Sadik on 18 Feb 2010 10:32
Did you try converting your cell array into a matrix first, using cell2mat? Maybe that would help.

Best.

"Mike " <mimoran(a)wfubmc.edu> wrote in message <hljlr9$a96$1(a)fred.mathworks.com>...
> I am preallocating a cell array because MATLAB suggested it for greater efficiency. In the suggestion box there was this sentence:
>
> "If you do not know the size of an array before the loop begins, preallocate it, and then if necessary, shrink it after the loop completes."
>
> I have been searching for hours and I have not been able to find anything about shrinking the cell array. I tried checking the first element of each row starting at the end, but I can't make comparisons on cells to determine which row is the last with data. Can anyone help this MATLAB rookie?
>
> Thank you.
From: Sadik on 18 Feb 2010 10:39
But you're saying there are gaps in the cell array, so cell2mat wouldn't work. I like Jan's solution.

Best.

"Sadik " <sadik.hava(a)gmail.com> wrote in message <hljmhi$pv8$1(a)fred.mathworks.com>...
> Did you try converting your cell array into a matrix first, using cell2mat? Maybe that would help.
>
> Best.
>
> "Mike " <mimoran(a)wfubmc.edu> wrote in message <hljlr9$a96$1(a)fred.mathworks.com>...
> > I am preallocating a cell array because MATLAB suggested it for greater efficiency. In the suggestion box there was this sentence:
> >
> > "If you do not know the size of an array before the loop begins, preallocate it, and then if necessary, shrink it after the loop completes."
> >
> > I have been searching for hours and I have not been able to find anything about shrinking the cell array. I tried checking the first element of each row starting at the end, but I can't make comparisons on cells to determine which row is the last with data. Can anyone help this MATLAB rookie?
> >
> > Thank you.
From: Mike on 18 Feb 2010 11:33
HERE IS THE CODE:

%=================================================
values = cell(50, 4);

% Loop through the events based on the number of colons in our input data
for i = 1:numberOfColons

individualEvent = substr(compressedEvents, startOfEvent, (endi(i) - startOfEvent - 1));

% Assign the current : index to be the index of the beginning of the
% next event in compressedEvents
startOfEvent = endi(i);

boxAndNeuron = substr(individualEvent, 0, 1);

% Determine which box and neuron we are dealing with based on the
% received character value
switch boxAndNeuron
case '*' % v-- Box 1 --v
box = 1;
neuron = 1;
case '+'
box = 1;
neuron = 2;
case '\'
box = 1;
neuron = 3;
case '-'
box = 1;
neuron = 4;
case '.' % v-- Box 2 --v
box = 2;
neuron = 1;
case '/'
box = 2;
neuron = 2;
case '0'
box = 2;
neuron = 3;
case '1'
box = 2;
neuron = 4;
case '2' % v-- Box 3 --v
box = 3;
neuron = 1;
case '3'
box = 3;
neuron = 2;
case '4'
box = 3;
neuron = 3;
case '5'
box = 3;
neuron = 4;
case '6' % v-- Box 4 --v
box = 4;
neuron = 1;
case '7'
box = 4;
neuron = 2;
case '8'
box = 4;
neuron = 3;
case '9'
box = 4;
neuron = 4;
end

% Convert the character from substr into the ASCII equivalent decimal
% and subtract 64 to get the proper channel number
channel = ((substr(individualEvent, 1, 1))-0) - 64;

% Break apart the retrieved timestamp into the integer and fraction
% portion, convert them independently back to decimal from hexadecimal,
% and then compute them back into a decimal timestamp
eventLength = length(individualEvent);

timeStampSubstring = substr(individualEvent, 2);
[endInteger] = regexp(timeStampSubstring,'[.]'); % endInteger will contain the index of the decimal point skipping the first two characters

integer = hex2dec(substr(timeStampSubstring, 0, (endInteger(1) - 1)));
fraction = hex2dec(substr(timeStampSubstring, endInteger(1), (eventLength - endInteger(1))));

timestamp = integer + (fraction / 1000000);


% Add the values from this decompressed data to the cell array that
% will be returned from this function.
values(i,:) = { box channel neuron timestamp };
end
%=================================================

Which goes back to

%==================================================
decompressedArray = decompress(dataReceived); % Unpack and decompress the data we received

% Concatenate the new data on to the previous data
dataArray = cat(1, dataArray, decompressedArray);
%==================================================

I need any empty rows at the end of the returned cell array to be removed before concatenating. I will look at teh current suggestions and see if they are what I need.

Thanks.
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