Question

I have a problem that I thought I knew how I can fix, but apparently I failed ..

I got a .mat file that I created. It has two columns and 25 rows of numbers. I would like to do a loop to get each value in the first column and put it in the X value, and the second column in the Y value. I then need to plot the points on the graph.

I know how to do the loop, and the plotting .. but I failed to extract the data and put them in X and Y values.

This is my trial code:

        load figureinfo.mat
        for K=1:25
            x=X(:,K) ~~ I remember that the code looks something like that to extract ..
            y=Y(:,K)
            plot(x,y,'o')
            hold on
        end

How do I get the data and put it in X and Y?

In addition, where is “ROWS” in (:,b)? b=Columns, but where do I put the rows?

Answer 1

Try the following:

load figureinfo.mat; %# assume this contains a matrix called figureinfo
X = figureinfo(:,1); %# numbers from all rows, column 1, into X
Y = figureinfo(:,2); %# numbers from all rows, column 2, into Y
plot(x,y,'o');

Or more simply,

load figureinfo.mat;
plot(figureinfo(:,1), figureinfo(:,2), 'o');

If you don’t know the name of the matrix in your .mat file, I recommend:

clear %# clear all variables from workspace
load figureinfo.mat;
whos

which will show the the name, size, and datatype of whatever you just loaded.

If you really want to extract the data in a loop, you have two options:

load figureinfo.mat; %# assume this contains a matrix called figureinfo
X = [];
Y = [];
for ctr = 1:length(figureinfo)
    X = [X figureinfo(ctr,1)];
    Y = [Y figureinfo(ctr,2)];
end

or (faster because it doesn’t keep reallocating X and Y)

load figureinfo.mat; %# assume this contains a matrix called figureinfo
X = zeros(length(figureinfo),1);
Y = zeros(length(figureinfo),1);
for ctr = 1:length(figureinfo)
    X(ctr) = figureinfo(ctr,1);
    Y(ctr) = figureinfo(ctr,2);
end