Why is my plot plotting blank?

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Xingkai Yang
Xingkai Yang on 2 Nov 2015
Commented: John on 4 Nov 2015
>> 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)
  1 Comment
John
John on 4 Nov 2015
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)

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Answers (2)

Walter Roberson
Walter Roberson on 2 Nov 2015
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
  1 Comment
Xingkai Yang
Xingkai Yang on 2 Nov 2015
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?

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Nitin Khola
Nitin Khola on 4 Nov 2015
Edited: Nitin Khola on 4 Nov 2015
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.

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