How to plot in a for loop?

my graph is showing up, but the plot is blank. Does anyone know why? Thank you.

TimeOut=input('Press Enter to view velocity and acceleration of the wiper process in increments of 5 degrees')
hold on

 for i=[0:5:90]

    L1=3; %in feet
    L2=3; %in feet

    r1=[-L1*sind(i),L1*cosd(i),0];
    i2 = asind(-L1*sind(i)/(L2));
    r2=[L2*sind(i2),L2*cosd(i2),0]; %array
    r0 = r1(2)+r2(2); %r0=length of travel of mechanism
    disp(sprintf('Length of travel of mechanism is %0.2f feet\n',r0(1)));

    vb=cross(W, r1);
    w2=[0, 0, L1*cosd(i)/(L2*cosd(i2))];
    vc=[0,-W(3)*-L1*sind(i)-w2(3)*L2*sind(i2),0];

    %Make sure the velocity of the mechanism is not to exceed 480mm/s 

    if abs(vc)>=1.5748; %480mm/s = 1.5748ft/s
        disp('Velocity exceeds 480mm/s. Refine inputs.')
      break

    end

    %Find Acceleration 
    anb=cross(W, cross(W, r1)); %Normal Acceleration of b 
    atb=cross(alpha1, r1); %Tangential Acceleration of b
    ancb=cross(w2, cross(w2, r2)); %Normal Acceleration of c/b
    alpha2=[0, 0, -(anb(2)+atb(2)+ancb(2))/r2(1)];%Angular Acceleration of link 2
    atcb=cross(alpha2, r2);%Tangential Acceleration of b
    ac=anb + atb + ancb + atcb;%Acceleration of wiper (point C)

   disp(sprintf('Velocity of wiper is %0.2f ft/s\n',vc(2)));
   disp(sprintf('Acceleration of wiper is %0.2f ft/s^2\n',ac(1)));

    subplot(2,1,1)
    plot(i,vc(2))
    grid
    xlabel('Angle in Degrees')
    ylabel('Velocity')

    subplot(2,1,2)
    plot(i,ac(1))
    grid
    xlabel('Angle in Degrees')
    ylabel('Acceleration')
    end

[Cam Salzberger]

Nicole - you've removed the majority of your question, including all of your code. Can you add that back so that future visitors can have context to your question?

Are you still running into issues, or is this question resolved?

[Cam Salzberger]

I would suggest inspecting the data that you are plotting. If that all looks correct (a long vector with varying values), then check the plot itself. You can see how many lines are present in a plot with:

hLines = get(gca, 'Children')

You can look at the 'XData' and 'YData' of the lines to ensure that there is proper data there. Then check that the axes limits are big enough that they would show more than one point. Then check coloring - white on white doesn't show up very well.

[Cam Salzberger]

Looks like your line has only a single point then. See the XData and YData fields. If you want the line to be all your data, the call to plot needs to be with vectors.

Make sure that you are building the vectors a point at a time. In other words, that you are doing:

velVals(k) = vc(2);
accelVals(k) = ac(1);

and not:

velVals = vc(2);
accelVals = ac(1);

[Cam Salzberger]

You are plotting just "i" against "vc(2)" outside the loop. It will only display the last value those variables were assigned. You should be plotting "x" against "V" and "x" against "A".

[Cam Salzberger]

I had assumed you were cutting out the rest of your loop code in your post not literally cutting it out. Yes, "V" and "A" will contain the same value for every element if you loop through each element and assign the same value to it. When I suggested you follow a structure similar to this:

for i = iVals
    ...
    velVals(i) = vc(2);
    ...
    accelVals(i) = ac(1);
    ...
end

I meant:

for i = iVals
    ...PUT CALCULATIONS FOR vc HERE...
    velVals(i) = vc(2);
    ...PUT CALCULATIONS FOR ac HERE...
    accelVals(i) = ac(1);
    ...DO ANYTHING ELSE YOU NEED TO DO IN THE LOOP HERE...
end

I'm not here to write your code for you fully and completely. I'm just trying to give you suggestions on how you can resolve the issues you are facing and allow you to apply them to your own code.

Hopefully this helps clear up the misunderstanding.

-Cam

[Rena Berman]

(Answers Dev) Restored edit

Answer 1

[Cam Salzberger]

Hello Nicole,

Assuming you have just opened MATLAB, your axes are first created when you do "hold on" (which isn't really the best way to create axes, since it's not obvious to people who read your code). Later, when you use subplot, it actually deletes these axes and replaces them. This is important, because the property that "hold on" affects ('NextPlot' property) is reset to its default value. So you are replacing the existing line every time you plot a new line.

The second issue is that you are plotting (as far as I can tell) one point at a time. However, you are using the default line specification, which is a simple line between points. If you have only one point, it won't draw the line. If you want to plot one point at a time, specify a line specification that includes a marker. For example:

plot(i, vc(2), 'o')

However, I would recommend computing all the points you would like to plot, saving them to an array, and then plot at once outside the loop. It's just cleaner and better for the graphics operations.

-Cam

[Cam Salzberger]

Preallocate outside the loop so you're not growing the variables inside of it:

iVals = 0:5:90;
velVals = zeros(size(iVals));
accelVals = zeros(size(iVals));

Then assign the values to the arrays as you compute them:

for i = iVals
    ...
    velVals(i) = vc(2);
    ...
    accelVals(i) = ac(1);
    ...
end

Then do your plotting at the end:

subplot(2,1,1)
plot(iVals, velVals)
grid
xlabel('Angle in Degrees')
ylabel('Velocity')

subplot(2,1,2)
plot(iVals, accelVals)
grid
xlabel('Angle in Degrees')
ylabel('Acceleration')

[Cam Salzberger]

Right, sorry, I didn't pay attention to the actual loop values. The error is because it's trying to use 0 as an index. A better way would be:

x=0:5:90;
V=zeros(size(x)); 
A=zeros(size(x))
for i=1:numel(x)
    ...(using x(i) in calculations)...
    V(i)=vc(2);
    A(i)=ac(1);
end

Of course, I technically wouldn't recommend using "i" or "j" as a variable name, since it has a built-in meaning as the imaginary unit. I prefer "k" for single loops, "r"/"c" for looping over a matrix, or the ever-popular "ii", "jj", etc.