Save to array in for loop, with steps - Matlab

You can reshape the data and then either split it up into different variables or just know that each column is a different variable (I'm now assuming each measurement occurs the same number of times i.e. length(data) is a multiple of 4)

data = reshape(data, 4, []).';

So now if you want

X = data(:,1);
Y = data(:,2);
%// etc...

But also you could just leave it as data all in one variable since calling data(:,1) is hardly more hassle than X.

Now, you should NOT use for-loops for this, but I'm gong to address what's wrong with your loops and how to solve this using loops purely as an explanation of the logic. You have a nested loop:

for i=1:3;
     for j=1:4:12;
        X(i)=data(j);
     end
end

Now what you were hoping was that i and j would each move one iteration forward together. So when i==1 then j==1, when i==2 then j==5 etc but this is not what happens at all. To best understand what's going on I suggest you print out the variables at each iteration:

disp(sprintf('i:  \tj:'));
for i=1:3;
     for j=1:4:12;
        disp(sprintf('   %d\t   %d',i,j));
     end
end

This prints out

i:      j:
   1       1
   1       5
   1       9
   2       1
   2       5
   2       9
   3       1
   3       5
   3       9

What you wanted was

disp(sprintf('i:  \tj:'));
for i=1:3;
     disp(sprintf('   %d\t   %d',i,4*i-3));
end

which outputs:

i:      j:
   1       1
   2       5
   3       9

applied to your problem:

%// preallocation!
X = zeros(size(data,1)/4, 1)
for i=1:3
    X(i)=data(i*4 - 3);
end

Or alternatively you can keep a separate count of either i or j:

%// preallocation!
X = zeros(size(data,1)/4, 1)
i = 1;
for j=1:4:end;
    X(i)=data(j);
    i = i+1;
end

Just for completeness your own solution should have read

 i = 0;
 for j=1:4:12;
    i = i+1;
    X(i)=data(j);
 end

Of course am304's answer is a better way of doing it.