How can I reference a matrix in a loop - matlab

Question

I have a series of 11 large matrices corresponding to earthquake data. I want to draw data from individual matrices in a loop. I will use dummy matrices data1 and data2 to illustrate my problem.
load('data1');
load('data2'); %data1 and data2 are large matrices
matrixname={data1 data2};
for j=1:2
matrix=matrixname(j);
latitude=matrix(:,1);
longitude=matrix(:,2);
geoshow(latitude,longitude);
end
So in my loop I want to extract columns from different matrices depending on the index. But I cannot figure out how to do this - I get error messages saying that the index exceeds the matrix dimensions.
Appreciate the help!

Answer 1

As mentioned, use {} brackets for cell arrays. Also, if you want to store the latitude and longitude into new matrices then add the 'j' term to your matrices so that you don't overwrite the data in your for loop.
Also when writing for loops, it's better to use size/length/numel instead of a fixed variable in case the number of files your analysing changes.
So taking your code;
for j=1:size(matrixname,2)
matrix = []; % reformat matrix in case of size differences
matrix = matrixname{j}; % gets the jth matrix
latitude(j) = matrix(:,1); % outputs column 1
longitude(j) = matrix(:,2); % outputs column 2
geoshow(latitude(j),longitude(j)); % runs function on jth set of data
end
Remember, [] brackets are for matrices. Using the wrong brackets is what gave you the error, as pointed out above.

Answer 2

To get the matrix you need from a cell array (which is what {data1 data2} is), you need to use cell2mat():
matrix=cell2mat(matrixname(j));

Answer 3

Since data1 and data2 are matrices, when you do:
matrixname={data1 data2};
You don't get an array of doubles as you're expecting, but a cell array. That's ok and good since data1 and data2 doesn't have the same size (number of rows or columns).
Then, inside the for loop, when accessing one of the original matrices (data1 or data2) from matrixname, you should convert it back to an array of doubles. The simplest and fastest way of doing it is by:
matrix = matrixname{j};
Look at the difference from your code: I'm using curly brackets {} instead of parenthesis (). Doing that way, matrix is a array of doubles. Doing matrix=matrixname(j), matrix is a cell array.
Finally, it's always good to clear temporary variables inside a loop. Then, your code should like:
load('data1');
load('data2'); %data1 and data2 are large matrices
matrixname = {data1 data2};
for j=1:2
matrix=matrixname{j};
latitude=matrix(:,1);
longitude=matrix(:,2);
geoshow(latitude,longitude);
clear matrix latitude longitude
end
For example, if:
data1 = [1 1; 2 2; 3 3];
data2 = [10 10; 20 20; 30 30; 40 40];
matrixname = {data1 data2};
matrixname{1} gives you exactly data1 and matrixname{2} gives you data2.