Why is my plot plotting blank?

>> t = 0:20;
>> Vx = 1000;
>> for ii = 1:2:length(t)
Vx = Vx - (0.0004*Vx^2)*t(ii)
end

Vx =

          1000

Vx =

     200

Vx =

     136

Vx =

     91.6096

Vx =

     64.7542

Vx =

     47.9818

Vx =

     36.9310

Vx =

     29.2931

Vx =

     23.8014

Vx =

     19.7225

Vx =

     16.6107

>> plot (t,Vx)

[John]

You are plotting a single point. You are overwriting Vx every loop iteration. You need to index Vx. Try this:

t = 0:20;
Vx = zeros(size(t));
Vx(1) = 1000;
for ii = 2:length(t)
    Vx(ii) = Vx(ii - 1) - (0.0004*Vx(ii - 1)^2)*t(ii);
end

figure, plot(t, Vx)

Answer 1

[Walter Roberson]

Your plot is not empty. You can see that with a minor change:

plot (t,Vx, '*')

You should ask yourself how many items are in t and how many items are in Vx

[Xingkai Yang]

Thank you very much. I see I have t 0 to 20 and only have one value in Vx. I wanted to plot all the values of Vx during for loop, so what should do for this?

Answer 2

[Nitin Khola]

As Walter has mentioned, Vx in this case is a scalar. Refer to the last bullet in the description for plot(X,Y) by following the link below: http://www.mathworks.com/help/matlab/ref/plot.html#description

Your code indicates that you have a variable "Vx" that is dependent on the variable "t". Also, most for loops can be avoided as discussed in following documentation. http://www.mathworks.com/help/matlab/matlab_prog/vectorization.html

If there is no possibility for eliminating the "for" loop, you can do data linking so that the plot updates its data. Refer to the following documentation for details on data linking: http://www.mathworks.com/help/matlab/data_analysis/making-graphs-responsive-with-data-linking.html

I do not recommend it as it will seriously affect the performance of the code, however, just for illustration purposes, you can modify your existing code to include a "hold on" and a "scatter" in the loop. Note, that I do not recommend it. It will just help you see how these commands work.

for ii = 1:2:length(t)
Vx = Vx - (0.0004*Vx^2)*t(ii)
scatter(t(ii),Vx);
hold on
end

I would suggest reading in detail about vectorization and data linking is the best approach.