Question

I have loaded a set of 50 files into matlab workspace. (from data1 to data50).
The size of each one is 721*176. I want to extract only the first 144 lines of each data and store it in another one.

For example:

newData1 = data1(1:144,:);

My question is: How can I do this using a loop to process all data at once?

Answer 1

That depends on how you store the data.

The best way in your case would be 3D array:

First, allocate the array (As @HighPerformanceMark points out correctly):

data = zeroes(721,176,50);

Then, populate it with values:

data(:,:,1)  =  read('firstFile.bmp');
data(:,:,2)  =  read('secondFile.bmp');
...

data(:,:,50) = ...

In that case, just use the following selection:

data(1:144,:,:)

---

Another plausible way to store the data is cell array. It is useful since 3D array cannot handle different size matrixes.

 data{1} = read('firstFile.bmp');
 data{2} = read('secondFile.bmp');
 ...

In this case, use cellfun :

 selection = cellfun(@(x) {x(1:144,:)}, data);

---

If you still insist on storing the data in separate variables rather than in an array, you can use eval. That is the least recommended way.

 for i=1:50
    eval(' selection{i} = data%d',i)
 end