Plotting data from a mat file that is a struct

I am challenged by structures and cells, not being a programmer by design. I have a mat struct, something like filename.mat with various arrays. Let's say there is one array called counts and another called times, with a single column of numbers each. How does one plot one array against another? Specifically how do you pick up the array you want out of several arrays in that mat file? Apologies if that is too simple a question for some of you.

Answer 1

[Sean de Wolski]

S = load('filename.mat');
plot(S.times,S.counts);

Doc:

http://www.mathworks.com/help/releases/R2013b/matlab/structures.html

[Nick]

Thanks for the prompt reply. First line is fine, second line gives an error

Reference to non-existent field 'times'.

Error in tryplot (line 2)
plot(S.times,S.counts);

In fact any field I try from that struct gives the same error. Also times(:,1) in that expression gives the same error. Typing S.times gives the same error (Reference to non-existent field 'times'). Clicking on the field, however, displays the contents fine. Matlab reports the mat file is a struct.

>> whos
  Name      Size            Bytes  Class     Attributes
  S         1x1              3570  struct

Is this a paradox?

Edit: S.filename.times works, plot(S.filename.times,S.filename.counts); also works. I am mystified but I can at least plow on.

[Image Analyst]

You didn't store counts and times separately. You stored filename, and filename was a structure that you created with members/fields called times and counts. See my answer on how to use fieldnames to check on whether or not fields exists and which ones actually do exist.

[Sean de Wolski]

Nick, I don't know how you created the MAT file but if you created it by saving two variables times and counts, what I have will work. Here is a full example:

times = 1:10;
counts = randperm(10);
save('filename.mat','times','counts');
clear
S = load('filename.mat');
plot(S.times,S.counts);

Answer 2

[Image Analyst]

% Read in structure from mat file.
storedStructure = load('filename.mat');

% For fun, let's see what fields this structure has.
fieldNames = fieldnames(storedStructure) % No semicolon

% You should see counts and times listed in the command window
% if they were actually stored there.

% Tell user whether or not counts is there.
hasField = isfield(storedStructure, 'counts');
if hasField
  uiwait(helpdlg('Your structure has a field called counts'));
else
  uiwait(helpdlg('Your structure DOES NOT HAVE a field called counts'));
end
% Tell user whether or not times is there.
hasField = isfield(storedStructure, 'times');
if hasField
  uiwait(helpdlg('Your structure has a field called times'));
else
  uiwait(helpdlg('Your structure DOES NOT HAVE a field called times'));
end

Tell me what you observe in the command window and in the message boxes.

[Image Analyst]

See the FAQ http://matlab.wikia.com/wiki/FAQ#What_is_a_cell_array.3F for a good description of cell arrays.

[Nick]

Many thanks for your feedback and the helpful code. The answer to your code is that the fields do not exist. However, fieldNames = filename, so the file has a single field with the name of the file (is that not confusing), which field in turn contains the fields we are talking about.

Sean, your code of course works and is the correct answer to my question, although it did not solve my problem. That mat file was produced by commercially available software exporting data into a matlab readable file. Sadly, you do not get to work straight away with simple examples in real life. Thanks to both of you for your feedback.

[Sean de Wolski]

So you'll need:

S.filename.times %?

[Image Analyst]

That's what I would think Sean. I agree with you Nick that using filename as the name of a structure that contains fields that have nothing to do with a filename is very confusing. Personally I would bring it up to the publisher and let them know.