plot graph according to matrix index

r = [cos(theta), sin(theta); -sin(theta), cos(theta)];
b = [x_mat; y_mat]
a = r*b
x_mat_new = a(1,:)
y_mat_new = a(2,:)

for the above code, I can plot a graph of y_mat_new against x_mat_new, but I cant generate a graph if I change the colon to 1.

But based on my understanding, the matrix product should contain (1,1) and (1,2) element. I wonder why a graph cannot be obtained.

Answer 1

[Voss]

Changing a colon to a 1 means providing plot with a vector for x and a scalar for y (or vice versa). When you do that, one line is created for each element of the vector, and each line has one point. You cannot see a line with one point unless it has a data marker. So either use a data marker for plots like that, or modify your plot call to provide two vectors.

theta = pi/6;
x_mat = rand(1,10);
y_mat = rand(1,10);

r = [cos(theta), sin(theta); -sin(theta), cos(theta)];
b = [x_mat; y_mat];
a = r*b;

plot(a(1,:),a(2,:)) % original

plot(a(1,1),a(2,:),'o') % replacing 1st colon with 1, and using a data marker

plot(a(1,:),a(2,1),'o') % replacing 2nd colon with 1, and using a data marker

plot(a(1,1)*ones(1,size(a,2)),a(2,:)) % plotting 2 vectors

plot(a(1,:),a(2,1)*ones(1,size(a,2))) % plotting 2 vectors