The first function is performing the calculations but the second, when ran, won't graph the results

I'm calling these two functions one after another from a menu created using switch (choice), the first one runs fine as I can see the reults come out but the second does not plot the results of the first.

function [x, v, t] = performcalcs(m, s1, ~, ~, t0, dt, t_total, ~, v, x, g)
    i=1;
    for t = t0: dt: t_total
        F = - s1 .* x(i) - m .* g;
        a = F ./ m;
        v(i+1) = a .* dt + v(i);
        x(i+1) = x(i) + v(i+1) .* dt;
        i = i + 1;
    end
end

function plotpos(t,x)
    plot(t,x,'-');
    title('Vertical position of mass');
    xlabel('t[s]');
    ylabel('x[m]');
end

[Cris LaPierre]

The plotting code appears to be fine, so the issue is likely with how you are calling it. Please share the calling script as well.

[Cris LaPierre]

Nothing wrong here, either. Perhaps the issue is in your performcalcs function? Check that t and x contain the values you expect.

I can't actually run your code because all the variables needed are missing.

[Cris LaPierre]

User is deleting their comment and code. Restoring from the cache so this still makes sense.

Gabriel Earle

while (exit_flag == 0)
    fprintf('Numerical simulation of dampened oscillating weight\n');
    fprintf('1. Perform calculations\n');
    fprintf('2. Graph (t,x)\n')
    fprintf('3. Graph (t,v)\n');
    fprintf('4. Display initial parameters\n');
    fprintf('5. Change initial parameters\n');
    fprintf('6. Exit program\n');
    choice = input('Please select: ');
    switch (choice)
        case{1}
            [x, v, t] = performcalcs(m, s1, s2, c, t0, dt, t_total, d, v, x, g); %perform calculations
        case{2}
            plotpos(t,x); %plot position
        case{3}
            plotvel(t,v); %plot velocity
        case{4}
            printtable(m_c, s1, s2, c, d, x0); %display initial parameters
        case{5}
            [m_c, s1, s2, c, d, x0] = changeparams; %change initial parameters
        case{6}
            exit_flag = 1;
        otherwise
            disp('Invalid entry');
    end

I run it by pressing 1 and then 2

Gabriel Earle

Could it also be the way t is assigned?

[Cris LaPierre]

@Gabriel Earle It is recomended you do not delete your posts, especially once it has been answered. It leaves the remaining content confusing to anyone who may come across it in the future with a similar question, and it adversely impacts the likelihood you will get answers to future questions you may ask.

[Rik]

Responding to the flag ("I would like for my posts to be removed from Matlab answers for risk of them being plagiarised by others."):

@Gabriel Earle You published your code under a creative commons license, see the Terms of Use. If people attribute the code to you and publish it under a similar license, it can't be plagirism. You decided to publish it. You were free to not do that. You don't seem to have responded to what Cris commented, adding to your rudeness.

Here is the cache I will restore your question from: link.

[Adam Danz]

@Gabriel Earle, just out of curiosity I wonder if you're going to cite Cris LaPierre in your work since you're using at least parts of Cris' answer.

At the time of this comment, the 3 people who have commented here (Cris, Rik, and myself) have a total of exactly 6900 answers in this forum. Those answers undoubtedly have contributed to doctoral theses, patents, published academic papers, and private proprietary projects and I think I've been cited less than 5 times that I know of.

When a user asks to delete their perfectly fine question I imagine that they may have committed plagiarism and are about to claim my (unpaid) work as their own. But at least for me, the benefits of paying forward my help to others outweigh any lack of recognition I may get. One of two scenarios could happen with your perfectly reasonable question. 1) Its view count will bottom out and no one will really ever see it among the nearly half a million questions or 2) the question and its answer will help others in the future. But if it's deleted, well so much for the time people gave you I guess.

[Walter Roberson]

AFAIK, I've been cited twice on over 45000 Answers.

[Adam Danz]

That egregious. Mine are from the file exchange except for 1.

[Adam Danz]

...and now the sum total number of answers from contributors to this thread has more than sextupled 🤣

[Walter Roberson]

I was wrong, Google Scholar says I have been cited 3 times.

[Stephen23]

http://web.archive.org/web/20210215061507/https://www.mathworks.com/matlabcentral/answers/724932-the-first-function-is-performing-the-calculations-but-the-second-when-ran-won-t-graph-the-results

Answer 1

[Cris LaPierre]

"Could it also be the way t is assigned?"

Yes, that is the issue. Your variable t is a loop counter, meaning it only ever has a single value assigned to it at a time. When you try to plot, t=t_total, not t0:dt:t_total. When you plot, then, it will plot each data point in x as a new series. Series are plotted as separate lines. Here, each of your lines is a single point, so you can't see them. To see this, in your figure window menu, select Edit > Plot Browser.

You don't even use t inside your for loop. Try this instead.

function [x, v, t] = performcalcs(m, s1, ~, ~, t0, dt, t_total, ~, v, x, g)    
    t = t0: dt: t_total
    for i=1:length(t)
        F = - s1 .* x(i) - m .* g;
        a = F ./ m;
        v(i+1) = a .* dt + v(i);
        x(i+1) = x(i) + v(i+1) .* dt;
    end
end

[Cris LaPierre]

x has one more data point than t

x(i+1) = x(i) + v(i+1) .* dt;
   ^^ % i+1

Try starting at i=2, and use i-1 and i instead of i and i+1

for i=2:length(t)
    F = - s1 .* x(i-1) - m .* g;
    a = F ./ m;
    v(i) = a .* dt + v(i-1);
    x(i) = x(i-1) + v(i) .* dt;
end

[Cris LaPierre]

Addition comments the OP made but has since deleted

Gabriel Earle

This seems to be progress but now I am getting an error saying:

Error using plot 
  Vectors must be the same length.
  
Error in Weight>plotpos (line 62 ) 
  plot(t,x,'-');
  
 
Error in Weight (line 36 ) 
plotpos(t,x); %plot position

Gabriel Earle

Thank you so much for your help !

That works now