it does not plot V1,V2 untill I put * in plot order. why?

Hi guys, I want to plot s/th in matlab. But my matlab does not give me the right answer until I put * in plot in V1,V2,x1 plot. why?

My code is:

clc;
clear all;
% syms x1(t)
% syms x2(t)
% syms x3(t)
% syms x4(t)
x1(1)=57.5;
x2(1)=8.5;
x3(1)=0.051;
x4(1)=4.8;
deltat=10^-3;
B=[0.0015 -0.0015 -6.9*10^-12;-5.9*10^-5 -9*10^2 5.9*10^-11;3.46*10^-5 5.2*10^-5 3.24*10^-11;-0.0167 0.0239 -1.0733*10^-9];
u=[32;32;80];
for i=1:100
% (x1(i+1)-x1(i))/deltat=-1.5*x1(i)+x2(i);
% (x2(i+1)-x2(i))/deltat=(x1(i)+x2(i))*sin(x1(i))-3*x2(i);
x1(i+1)=(3.7*10^-15*x1(i)+7.6*10^-6*x2(i))*deltat+x1(i);
x2(i+1)=(-4*10^-16*x1(i)-6.5*10^-5*x2(i)-2.8)*deltat+x2(i);
x3(i+1)=(2.37*10^-16*x1(i)-0.142*x3(i))*deltat+x3(i);
x4(i+1)=(-8.15*10^-14*x1(i)-0.5535*x2(i)+18.21*x3(i)+0.0833*x4(i))*deltat+x4(i);
V1(i)=0.5*(x1(i).^2+x2(i).^2+x3(i).^2+x4(i).^2);
V2(i)=0.5*x1(i).^2+x2(i).^2+x3(i).^2+x4(i).^2;
end 
for i=1:100
    figure(1)
plot(x1(1,:),x2(1,:))
hold on;
    plot(x1(1,1),x2(1,1),'bo') % starting point
    hold on;
     plot(x1(1,end),x2(1,end),'ks') % ending point 
     hold on;
     figure(2);
     plot(i,x1(1,i),'*-')
     hold on;
 figure(3)
 plot(i,V1(1,i),'*-')
 hold on
figure(4) 
 plot(i,V2(1,i),'*-')
 hold on;
end

Answer 1

[azam ghamari]

yes it may works, thanyou, but I said about difference between using *- and not using. I said why it plot just when we use *-?

Thanks

[Walter Roberson]

The default is not to use any marker. When you use '*-' you tell it to use * as a Marker. plot will plot markers when you tell it to plot individual points. plot() needs at least two adjacent finite points in order to plot a line, but it only needs isolated finite points in order to plot markers.

Answer 2

[Image Analyst]

In MATLAB, type this control-a (to select all), control-i (to fix indenting), control-c (to copy). Then paste back here, highlight and click the code icon.

You say "I want to plot s/th". What are s and th? I don't see them in the code. The rest of the code runs, but it's a bit unnecessary to have it plot every single point one at a time unless you were going to put a break point in the code to watch it slowly.

When you plot a single element of V1 or V2, at a particular i value.....

If you don't use * to place a marker, and only use - (which connects two data points with a line), then you will see nothing because you're plotting only a single point, not two points that can be connected by a line.

[azam ghamari]

Dear Sir/madam

i mean of s/th is V1,V2, it is clear as I wrote in the question.

however, - connects the points, why do you say that we will see nothing with it.

however, if i don't put any thing also it does show any thing.!!! why?

thanks

[Walter Roberson]

Please read https://www.mathworks.com/matlabcentral/answers/455433-plot-not-appearing-in-the-axes#comment_692085 which explains why you do not see anything.

[Image Analyst]

Yes, using '-' will connect the points, but you're not plotting multiple points in any of your calls to plot V1 or V2. Like I said, you're plotting only ONE point, NOT two or more:

plot(i,V1(1,i),'*-')  % Plot one single value, V1(1,i)

The above will plot a marker * but no line since only one point is being plotted, not two or more.

Answer 3

[azam ghamari]

Dear Walter

I read that, it says taht save the items in a vector and then plot, it isn't matter hear, my problem is that why with putting '*-' in plot order it shows nut without that it does not show any thing. By the way, I have the values of V2 in workspace. they are for 1000 points, not just one point. Why do you say that it plots only a single point?!

Thanks

[Walter Roberson]

Your i is a scalar because you are in a for i loop. plot(i,V2(1,i),'*-') is plot() of a scalar i value and a scalar value extracted from V2. This is a request to plot() only one point in that one call. plot() never creates a line unless you plot() at least two adjacent finite values in the same call . For example, plot(i:i+1, V2(1,i:i+1), '*-') would be a request to plot two adjacent points and it would be willing to create a line between the two.

You should remove the

figure(4)
plot(i,V2(1,i),'*-');
hold on

from the loop, and then after the loop you should put in

figure(4)
plot(1:100, V2(1,1:100), '*-')