MATLAB: Plot inside a For Loop

Question

Pardon my ignorance here, but I cannot seem to understand how to plot data inside a simple MATLAB for loop. I currently have the following:

sigma=[.9 .9250 .95 .95];
gamma=[1 .0784 .54 .4862];
F=[0 0 .25 0;0 0 0 0;0 0 0 0;0 0 0 0];
for e=0:.01:.2
   R_0=max(eig(F*inv((eye(4)-[sigma(1)*(1-gamma(1)) 0 0 0;...
       sigma(1)*gamma(1) sigma(2)*(1-gamma(2)) 0 0;...
       0 sigma(2)*gamma(2) sigma(3)*(1-gamma(3))*(1-e) sigma(4)*gamma(4);...
       0 0 sigma(3)*gamma(3)*(1-e) sigma(4)*(1-gamma(4))]))))
end

I am trying to plot R_0 with respect to e. The for loop works in that for each value of e (0, then .01, then .02, up until 2), the code gives me values for R_0 (1.1049, then 1.0138, then .9365, up until .3949). So basically I have a group of points that I am trying to plot and then connect with a line, but I cannot seem to figure out how to plot this.

Once again, this question seems extremely simple, but I would greatly appreciate any help.

il_raffa · Accepted Answer · 2016-12-27 07:41:41Z

2

First you have to collect the R_0 values in each iteratiion (in the current version of your code, you are overwritting the value at each iteration).

To save all the value, you have to use R_0 an array and introduce a counter to be incremented in the loop; then you can use the plot function to plot the data.

In the following code you can see:

the initialization of the conter cnt

its increment at each itaration

the use of the plot function after the loop

sigma=[.9 .9250 .95 .95];
gamma=[1 .0784 .54 .4862];
F=[0 0 .25 0;0 0 0 0;0 0 0 0;0 0 0 0];

% Initialize the counter
cnt=0;

for e=0:.01:.2
   % Increment the counter
   cnt=cnt+1;
   % Use R_0 as an array and store the value of each iteration

   R_0(cnt)=max(eig(F*inv((eye(4)-[sigma(1)*(1-gamma(1)) 0 0 0;...
      sigma(1)*gamma(1) sigma(2)*(1-gamma(2)) 0 0;...
      0 sigma(2)*gamma(2) sigma(3)*(1-gamma(3))*(1-e) sigma(4)*gamma(4);...
      0 0 sigma(3)*gamma(3)*(1-e) sigma(4)*(1-gamma(4))]))))
end
% Plot the results
plot([0:.01:.2],R_0,'o-')

Hope this helps,

Qapla'

answered Dec 27, 2016 at 7:41

il_raffa

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3 Comments

Dibbs Over a year ago

Thanks. Just as a follow up questions, if I were to add a line to the graph at y=1 (as is shown here: i.sstatic.net/fO7sc.jpg) what would be the best way to find this intersection

Dibbs Over a year ago

Sorry, I keep pressing enter; graph1=plot([0:.01:.2],R_0) hold on graph2=plot(xlim,[1 1])

il_raffa Over a year ago

I'm not sure this will be the best way, but you can use polyfit to find the coefficients of a polynomial fitting the R_0 data, then use roots to get the roots of the polynomial. To find the intersection of a specific y value, you have to subtract the target y value (in your case 1) when evaluating the coefficients of the polynomial. One you've got the roots, you use imag to downselect only the real roots.

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