Error "The length of X must match either the number of rows or the number of columns of Y" while trying to plot data

I am trying to plot data following this code:

% Data calculation
load CovidData.mat covid_data
           [m,n]=size(covid_data);
           country_daily_cases=zeros(1,n);
            z=0;
            index = find(strcmp([covid_data],app.CountryListBox.Value)); % This allows me to look for data for a particular country
            cond = length(index)==1; % Some of these countries have regions associated to them (cond = 0), while some others don't (cond = 1)
                      
            if cond == 1 
                for w=3:n
                    if w==3
                        z = z + covid_data{index,w}(1); 
                    else
                    z = z + covid_data{index,w}(1)-covid_data{index,w-1}(1); % I want to compute the daily cases, so I used this for loop to substract the cases from the previous day                    end
                    country_daily_cases(w-2)=z;
                    z=0;
                    end 
                end
            end
            app.CovidDailyCases = country_daily_cases(1:end-2); % Due to my code I get two extra 0 at the end of my vector, so I remove them, getting a 1x707 vector
                  
            % The code for cond == 0 is not part of the problem, so I removed it for better understanding
                    
           % Time points
           dates=datetime(covid_data(1,3:end),'Format','dd-MMM-uuuu');
           app.TimePoints=datenum(dates);
           
           % Plotting code
           cla (app.UIAxes,"reset");
            if app.CasesButton.Value
                
                if app.DailyButton.Value
                bar(app.UIAxes,app.TimePoints,app.CovidDailyCases, 'blue');
                app.UIAxes.YColor = 'b';
                datetick(app.UIAxes,'x','mmm yy');
                end
            end
                    
                          
            

The app.TimePoints vector has 1x707 dimensions, and the app.CovidDailyCases has also 1x707 dimensions, but when I try to run the script, the following error message appears:

Error using bar (line 171)

The length of X must match either the number of rows or the number of columns of Y.

Error in CovidData/OptionButtonGroupSelectionChanged (line 422)

app.plotData();

Why does it say that lengths must match if they do actually match? Sorry if the information provided is scarce, it comes from long parts of code. If you need any extra data or code, I would be happy to provide it. Thank you

Answer 1

[Walter Roberson]

When cond == 0 then country_daily_cases=zeros(1,n); which is 1 x number of columns in the covid data.

                bar(app.UIAxes,app.TimePoints,app.CovidDailyCases, 'blue');

we have no idea where app.TimePoints comes from. You are not load()'ing it, and it seems to be independent of the number of columns in the data. It seems unlikely that it happens to exactly match the number of columns in the file.

[Pablo Rodríguez Suárez]

Ups, my bad, I thought I had put the code for that:

dates=datetime(covid_data(1,3:end),'Format','dd-MMM-uuuu');
app.TimePoints=datenum(dates);

This is it. It's a 1 x (number of columns in the covid data - 2), and that's why I remove the last two zeros from the country_daily_cases vector, so both of them have the same length.

[Walter Roberson]

                    country_daily_cases(w-2)=z;

You never write into country_daily_cases(w-1) or country_daily_cases(w) in the context of for w=3:n -- those slots just exist because of your initialization with zeros(1,n) . It would seem to make more sense to intialize zeros(1,n-2) and then not remove the final two entries.

[Pablo Rodríguez Suárez]

Same problem...the weird thing is that when I copy the exact same code into Matlab (not in the app designer), I get a nice plot...so I don't understand why it doesn't work on the gui...

[Pablo Rodríguez Suárez]

Ok, it's solved...I forgot to add call the function that computes the daily cases before calling the function to plot the data...

  % Selection changed function: OptionButtonGroup
        function OptionButtonGroupSelectionChanged(app, event)
            app.dailyCountryCases(); % This line solved everything
            app.plotData();    

Thank you!