Plot Using Variable Name From For Loop

I have a number of variables (for example timeoffset0 - timeoffset20 and receivedpoweroffset0 - receivedpoweroffset20). I would like to plot all of these variables within a loop or similar method so that I can write a script which will plot all of the variables for any number of variables that are given as an input.

I have tried to do this using a for loop in the two following methods:

for fractionoffset=0:1:20
plot(['timeoffset',fractionoffset],['receivedpoweroffset',fractionoffset])
end

or

for fractionoffset=0:1:20
plot(sprintf('tdownsampoffset%d', fractionoffset),[sprintf('r1dbdownsampoffset%d', fractionoffset)
end

Neither of these work since MATLAB is trying to plot the text rather than the variable referenced by the text.

The error is: Error using plot Invalid first data argument.

How would I go about doing this? The different numbered variables have different numbers of elements so they cannot be easily combined into one variable.

EDIT: moved "Answer" here:

Here attached are two pairs of files which I am struggling to read into one variable (i.e. one variable for td and one variable for db) (one column for 2 and one column for 3 within each variable). If this can be done without having to add a zero at the beginning or end of the variables ending in three (which contain one less element) then I can easily plot them from a loop.

[jonas]

May I avsked where those variables come from? It'd be easier to avoid this situation.

[Benjamin Bloomfield]

The variables are text files generated from a third party program which I have read into MATLAB using dlmread. My first attempt was to write them all into one file with different columns using

example(:,fractionoffset)

however as the files have different numbers of elements MATLAB would not let them be assigned in this way. I could assign the array with zeros first but then these show up in the plot which is undesirable.

Consequently they are read into different variables within a loop using assignin (I know this is bad practice) but plotting them from a loop is proving tricky.

If there is a better way to do this that avoids using asignin then I am happy to do this if it makes plotting easier.

[jonas]

Surely! Can you provide 2-3 tricky txt-files to work with?

[Stephen23]

"The different numbered variables have different numbers of elements so they cannot be easily combined into one variable."

Then use a cell array. With a cell array you can trivially use efficient indexing, which is much simpler than buggy, inefficient dynamic variable names:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Using dynamic fieldnames is a significant improvement on dynamic varaible names, but requires more complexity than you need: indexing is really simple, so why not use it?

Answer 1

[Stephen23]

Here is an outline of one simple way to do this:

N = 20;
db = cell(1,N);
td = cell(1,N);
for k = 0:N
    db{k} = dlmread(sprintf('db%d.txt',k));
    td{k} = dlmread(sprintf('td%d.txt',k));
end

and then plotting in a loop is trivial too, using simple, very efficient indexing:

for k = 1:N
    plot(db{k},td{k})
    hold on
end

Much better than magically accessing the variable names!

Answer 2

[jonas]

Here is one way to store the data in a single struct using dynamic field names. The data is stored in a single folder, without any other txt-files.

data=struct;
files=dir('filepath\*.txt');

%%Preallocate array with fieldnames
fieldnames = strings(numel(files),1)

%%Create vector with fieldnames and save data
for i=1:numel(files)
  str=strsplit(files(i).name,'.');
  fieldnames{i}=str{1}
  data.(fieldnames{i})=dlmread(files(i).name);
end

Now you can easily call a subset of data, e.g.:

plot(data.td2,data.db2)

or something like this

plot(data.(fieldnames{3}),data.(fieldnames{1}))

or if you want to use a loop to plot pairs

figure; hold on
for i=2:3
  plot(data.(['td',num2str(i)]),data.(['db',num2str(i)]))
end

which gives the attached figure.

In conclusion, dynamic field names are much more convenient to work with than dynamic variable names :)