Matlab使用循环创建单元格数组

Question

As part of a university project, I have to do the following: Use a database that contains 14 different folders (14 different subjects) and in every folder we have 5 trials of 12 different activities stored in .mat files. The trials files contain 6 columns (of which I want only 3) and different size of rows. Now I want to concatenate the 5 trials in one .mat folder for every subject, and store all the database to the following format : subject x activity x trial .

I have written the following code:

InPath_data = 'C:\Users\olga\Desktop\USC-HAD\';

listname = [InPath_data, 'Subjects.txt'];
folder_subject = textread(listname,'%s','delimiter','\n','whitespace','');
listname = [InPath_data,'ActivityTrial.txt'];
files_act_tr = textread(listname,'%s','delimiter','\n','whitespace','');
fname = [InPath_data,folder_subject{1},'\',files_act_tr{1},'.mat'];
tmp1= [];
k=1;
for i=1:length(folder_subject)
    for j=1:length(files_act_tr)
        fname = [InPath_data,folder_subject{i},'\',files_act_tr{j},'.mat'];
        tmp= load(fname);
        q=tmp.sensor_readings( : ,(1:3));
        tmp1= cat(1,tmp1,q);
        clear q;
        if mod(j,5)==0
         data(j-4*k,:,:)=tmp1;
         k=k+1;
         tmp1=[];
        end
    end
end

Now when clearing tmp1 i cannot use the loop. I suspect that using cell arrays is the key to do what I want, but I have never used them before, so... Any help would be appreciated! :)

Answer 1

clear tmp1; deletes the variable, in the next iteration the variable tmp1 is unknown like it was never used before. Using tmp1=[] instead empties the variable. This should fix your code.

Answer 2

Edit: I managed to do it, by setting a stable array size for trials to 10000x3.Otherwise, the size changes over loops and the 4 D matrix cannot be formed properly. Using @Daniel R solution tmp1=[] and some other alterations, the code goes like this:

InPath_data = 'C:\Users\olga\Desktop\USC-HAD\';

listname = [InPath_data, 'Subjects.txt'];
folder_subject = textread(listname,'%s','delimiter','\n','whitespace','');
listname = [InPath_data,'ActivityTrial.txt'];
files_act_tr = textread(listname,'%s','delimiter','\n','whitespace','');
fname = [InPath_data,folder_subject{1},'\',files_act_tr{1},'.mat'];
tmp1= [];
k=1;

for i=1:length(folder_subject)
    for j=1:length(files_act_tr)
        fname = [InPath_data,folder_subject{i},'\',files_act_tr{j},'.mat'];
        tmp= load(fname);
        q=tmp.sensor_readings( : ,(1:3));
        tmp1= cat(1,tmp1,q);
        clear q;

        if mod(j,5)==0

         data(i,j-4*k,:,:)= tmp1((1:10000), :);
         k=k+1;
         tmp1=[];

        end

    end
   k=1;
end