Matlab for loop plots curves in random order

Hi everyone,

What I am doing with the code below is to plot a curve of res_p vs P_e for every value of def_err. However, this produces over 50 plots and I would like to plot specific indices within def_err, namely 10, 20, 30, 40 and 50.

I suspect that I may be indexing into the variable def_err incorrectly such that the output plots are in random order(see plots below). And the values of the phase error seem oddly small as well.

clear all;
% P_e=pi/8;
lambda= 1.94e-2; %Ang
res_p= linspace(0,1.5,50); %Ang^-1
def_err = (0.4:0.4:20);%Ang

idx=[10, 20, 30, 40, 50];
A=def_err(idx);

for j=1:length(A);
    P_e(:,j)= pi*lambda.*def_err(:,j).*res_p.^2;
for i=1:length(res_p) 
 hold on
    plot(res_p,P_e.*(180/pi));
       
end 
 legend ('4 A','8 A','12 A','16 A','20 A','Location','best','Fontsize',14);
end 
ylabel('Phase error (degrees)','Fontsize',14)
xlabel('Spatial frequency (q) (Ang^{-1})', 'Fontsize',14)

Answer 1

[Star Strider]

The code required a bit of tweaking.

Try this —

lambda= 1.94e-2; %Ang
res_p= linspace(0,1.5,50); %Ang^-1
def_err = (0.4:0.4:20);%Ang
idx=[10, 20, 30, 40, 50];
A=def_err(idx);
for j=1:length(A);
    P_e(:,j)= pi*lambda.*def_err(:,j).*res_p.^2;
end

figure
hold on
for i=1:length(A) 

    plot(res_p,P_e(:,i).*(180/pi), 'DisplayName',sprintf('%2d A',A(i)));
       
end 
legend ('Location','best','Fontsize',14);
ylabel('Phase error (degrees)','Fontsize',14)
xlabel('Spatial frequency (q) (Ang^{-1})', 'Fontsize',14)
hold off

.

[Arthur Moya]

@Star Strider,

legendary stuff.

Indexing in for loops really confuses me at times.

There is something fundamental that I miss about multiple for loops that makes it difficult for me to structure them successfully. Any recommendations or thoughts on how to get better at these would be great.

Thank you.

[Star Strider]

As always, my pleasure!

The loop indexing is correct in the first loop, however not in the second.

Also, the second loop is not strictly necessary, since plot is intelligent enough to figure out on its own how to plot the matrix (unless the matrix is square, then plot needs help, specifically transposition if necessary, to plot it the way you intend).

This illustrates that approach —

lambda= 1.94e-2; %Ang
res_p= linspace(0,1.5,50); %Ang^-1
def_err = (0.4:0.4:20);%Ang
idx=[10, 20, 30, 40, 50];
A=def_err(idx);
for j=1:length(A);
    P_e(:,j)= pi*lambda.*def_err(:,j).*res_p.^2;
end

figure
plot(res_p,P_e.*(180/pi));
ylabel('Phase error (degrees)','Fontsize',14)
xlabel('Spatial frequency (q) (Ang^{-1})', 'Fontsize',14)
legend(compose('%2d A',A), 'Location','best')

Since you got the indexing correct in the first loop, there is not much I can recommend. Like everything, this takes practice (and learning from fixing the code that did not work exactly as desired when it is first run).

[Arthur Moya]

@Star Strider,Thank you again.

I guess I have to keep doing this until I build enough experience-backed confidence.

Cheers,

Arthur Moya

[Star Strider]

No worries!

I first encountered MATLAB in 1993, and I have been involved in Answers on and off since 2012. It just takes practice!