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

938 views (last 30 days)

Show older comments

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.

##### 0 Comments

### Accepted Answer

Walter Roberson
on 10 Nov 2011

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.

##### 0 Comments

### More Answers (3)

Naz
on 10 Nov 2011

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 Comment

Jonathan
on 11 Nov 2011

Bård Skaflestad
on 10 Nov 2011

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

##### 0 Comments

Jonathan
on 10 Nov 2011

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

##### 0 Comments

### See Also

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!