the code below I'm trying to plot a loglog graph, but everytime I run the code I keep getting an empty graph, then when i use the brush i can see the points. can anyone help

n=10;
x=0.2;

for i=1:n

     h=1/i;
     cf=dot([1 -2 1],[(cos(pi*(x+h))) cos(pi*x) cos(pi*(x-h))])/h^2;
     rf=dot([2 -5 4 -1],[cos(pi*x) ;cos((pi*(x+h))) ;cos((pi*(x+2*h))); cos(pi*(x+3*h))])/h^2;
     ec=abs((-pi^2*cos(pi*x))-cf);
     er=abs((-pi^2*cos(pi*x))-rf);

     loglog(h,ec,'g');
     hold on
     loglog(h,er,'r');
     hold on

end

xlabel('h')
ylabel('ec + er')

Answer 1

[Walter Roberson]

You are only plotting one point at a time so you will not get connecting lines. But you have not specified any marker shape or marker size so you cannot see the markers.

Are you trying for lines or for a scatter plot?

[Rayhaan Bhikha]

I'm trying for lines, I thought i was plotting against the value of h?

[Walter Roberson]

At the time you do

loglog(h,ec,'g');

your h is a scalar (1/i) and your ec is a scalar calculated just above. You are therefore using loglog() of one scalar against another. That plots one point. However, when you use plot() without specifying a data marker, the points are not plotted... so you get no visible output. If you had used

loglog(h,ec,'*g');

you would have gotten visible individual points.

But you want lines. To get lines you should store the computations and then plot() the results.

n=10;
x=0.2;
for i=1:n

       h(i) = 1/i;
       cf = dot([1 -2 1],[(cos(pi*(x+h(i)))) cos(pi*x) cos(pi*(x-h(i)))])/h(i)^2;
       rf = dot([2 -5 4 -1],[cos(pi*x) ;cos((pi*(x+h(i)))) ;cos((pi*(x+2*h(i)))); cos(pi*(x+3*h(i)))])/h(i)^2;
       ec(i) = abs((-pi^2*cos(pi*x))-cf);
       er(i) = abs((-pi^2*cos(pi*x))-rf);
  end

loglog(h, ec, 'g');
hold on
loglog(h, er, 'r');
hold off

xlabel('h')
ylabel('ec + er')

[Walter Roberson]

Is there a reason you did not copy the code I gave that shows how to save the points?