how to initialize an M×N array whose size is unknown in for-loop

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AP
AP 2011-11-10
Hi All,
I am trying to read 150 text files. Each contains the value of x and y. x is the input of an experiment and y is the measured value. Each experiment has different number of data. The number of data cannot be determined until I have read each file.
How should I use the arrays which store x and y in a for-loop efficiently to speed up the process of reading? How should I initialize them while each experiment has different number of elements? I tried to use a code similar to the following code.
x=zeros(??,150);
y=zeros(??,150);
for i=1:150
fid=fopen(fname);
data=textscan(fid,'%f %f');
x(??,i)=data{1}
y(??,i)=data{2}
end
By ?? I mean I don't exactly know what to use. Could someone please help me?
Thanks.

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采纳的回答

Walter Roberson
Walter Roberson 2011-11-10
x = cell(150,1);
y = cell(150,1);
for K = 1 : 150
fid=fopen(fname);
data = textscan(fid, '%f %f');
x{K} = data{1};
y{K} = data{2};
end
You can always put them in to a matrix after you have collected them all. If you do that, you will need to decide how you want to pad the shorter columns.

更多回答（3 个）

Naz
Naz 2011-11-10
One but not very intelligent way to do it is allocate more memory that you need. You probably have an idea of what what would be a maximum number of values in any of your txt. file. If you know that it should be less than a thousand, allocate a thousand rows (or 2000...). Also, you can fill the matrix with distinct value that you know does not exist in your data:
x=zeros(150,1000); % 1000 columns of x data
y=zeros(150,1000);
x(x==0)=pi; % change your zeros to pi or (-1) to have it as a mark of the end of data (similar how they use 0 at the end of the string in C)
y(y==0)=pi;
then perform the same loop as you wrote above. When you read data, you know where to stop since there is a mark.
  1 个评论
Jonathan
Jonathan 2011-11-11
I use a similar technique often. I would initialize x as "x = nan(150, 1000);". This way, whenever the data is expected to contain no nans, there is no need to pick another value (such as pi or -1) as a default value. If using pi as the default value is desirable, I use "x = pi*ones(150,1000);".

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Bård Skaflestad
Bård Skaflestad 2011-11-10
As your data is all numeric, you can do something akin to the following
x = []; y = [];
for i = 1:150,
% ... like before ...
x(1:numel(data{1}), i) = data{1};
y(1:numel(data{2}), i) = data{2};
end
This will reallocate x and y to appropriate sizes whilst reading the input. I'm not, however, advocating this solution due to the reallocation cost. Personally, I'd just use something like
x = cell([1, 150]); y = cell([1, 150]);
for i = 1:150,
% ... like before ...
x{i} = data{1};
y{i} = data{2};
end
although the effectiveness depends greatly on what you intend to do to 'x' and 'y' once the data is in MATLAB.
Sincerely, Bård Skaflestad
Jonathan
Jonathan 2011-11-10
A. Hill,
You can probably get a good estimate of the number of elements by examining the file sizes. Suppose the files are in the directory 'C:\files' and the extensions are '.dat'. You can use something like the following.
dirName = 'C:\files';
listing = dir(fullfile(dirName, '*.dat'));
sizeAllFiles = sum([listing.bytes]);
sizeOneEntry = 8; % or some more appropriate number.
numRowsAll = sizeAllFiles / sizeOneEntry;
x=zeros(numRowsAll,150);
y=zeros(numRowsAll,150);
iRow = 1;
for i=1:150
fid = fopen(fname);
data = textscan(fid,'%f %f');
numRowsOne = size(data{1},1);
x(iRow:iRow+numRowsOne,i) = data{1};
y(iRow:iRow+numRowsOne,i) = data{2};
iRow = iRow + numRowsOne;
end
x = x(1:iRow-1);
y = y(1:iRow-1);
Let me know if this helps.
~Jonathan

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