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Task 13.3 Detail: Using structures

Summary of new tools and commands.

• no new tools

Task: Write a script to read the data in E7.dat which has 4 columns (day, V1, V2,V3). Create a structure to hold these data. Include in the structure the name of the data file and a character array identifying the variable in each column.

Structures are composite variables. They can be used to contain data values along with supporting information.

The following script fragment creates a structure called S containing some data with info on units and a file name:

  S.filename='data13.dat';
  S.units.time='hours';
  S.units.length='meters';
  S.time = [ 0 2 3 4 6  7  9 12 15 18 22];
  S.length=[10 3 2 6 4 19 24 18 13  9  4];
  save('Data.mat','S');

As you see with the units variable, sub-variables can themselves have sub-variables.

Structures can be arrays as well. Imagine that you are doing an experiment where you measure properties at a set of locations. Then, the data can be held in the following structure:

  Data(1).station=1  %  station number
  Data(1).latitude=37.3;
  Data(1).longitude=-85.2;
  Data(1).temp=19;
  Data(1).wind=25;
  Data(1).humidity=75;
  Data(1).elevation=120;
%     and so forth
  Data(2).station=2  %  station number
  Data(2).latitude=37.2;
  Data(2).longitude=-85.3;
%  and so forth
  Data(3).station=3  %  station number
  Data(3).latitude=37.1;
  Data(3).longitude=-85.4;
%  and so forth

 disp(['Longitude: ' num2str(Data(10).longitude)])
 disp(['Latitude: '  num2str(Data(10).latitude)])

There is no way to create a bunch of entries in a structure. You simply create a structure starting with entry (1) and keep adding. If you create Data(1) and then Data(10), MATLAB will also create entries (2) through (9).

Flow chart for task:

%%%  read data file
%%%  separate data columns
%%%  create structure with date file name
%%%  add data columns to structure
%%%  add cell array for data descriptors
%%%  add cell array for data units

Script to accomplish task:

%%%  read data file
%%%  separate data columns
  data=load('E7.dat');
  day=data(:,1);V1=data(:,2);V2=data(:,3);V3=data(:,4);
%%%  create structure with data file name
  DATA.file='E7.dat';
  DATA.dir='../Data';
%%%  add data columns to structure
  DATA.day=day;
  DATA.V1=V1;
  DATA.V2=V2;
  DATA.V3=V3;
%%%  add cell array for data descriptors
  DATA.variables={'day: time in days';...
             'V1: values for variable 1';...
             'V2: values for variable 2';...
             'V3: values for variable 3'};
%%%  add cell array for data units
  DATA.units={'days';'meters';'deg C';'mg/L'};

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