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

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azam ghamari
azam ghamari 2019-4-12
评论： Walter Roberson 2019-4-14
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

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采纳的回答

azam ghamari
azam ghamari 2019-4-14
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
  1 个评论
Walter Roberson
Walter Roberson 2019-4-14
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.

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更多回答（2 个）

Image Analyst
Image Analyst 2019-4-12
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.
  3 个评论
显示 1更早的评论
Image Analyst
Image Analyst 2019-4-14
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.

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azam ghamari
azam ghamari 2019-4-14
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
  1 个评论
Walter Roberson
Walter Roberson 2019-4-14
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), '*-')

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