University of Alaska Fairbanks
Geophysical Institute

Beyond the Mouse 2010 - The geoscientist's computational chest.

Lab 4: Matlab Flow Control

Lab slides

flow control, functions (pdf)

Introduction

In Monday's lecture we introduced constructs that allow you to increase the complexity of problems you can solve with your programs. Flow control language features like if-else, for, or while allow you to create tree like structures of code rather than one long list of function calls. Great! Today you'll expand the problem we worked on last Friday - the GPS time series. The goal is to have you create a Matlab script which will process a bunch of data files: load data, put it in an array of structs, create figures.

Exercise (Commented Solution)

You will use the data from last week. Download this archive (zip) to get the data files and necessary functions. Let me write out in detail what I expect you to do:

• Create a cell array that contains the names of the three GPS stations: BZ09, FAIR and WHIT.
• Iterate over this cell array using it's length as a terminating condition; use a for-loop. In general, we want to call such parts of the script 'Initialization' which is the part where you get your variables into defined states such that you can later work on them.
• We are in the for loop.
• For each station, add a new struct to an array of structs, the struct contains the 5 fields: 'time', 'lon', 'lat', 'height', and 'name' just like the example from class on Monday.
• Add the name of the GPS station you are currently working on to the name field
• In as a temporary variable, create the filename that contains the data for this station using the entry in the cell-array you are currently iterating through, the format is: /path/to/STATIONAME.dat

  There are at least three ways to construct a composite string:
  filename = ['/path/to/' STATIONAME '.dat'];
filename = sprintf('/path/to/%s.dat', STATIONAME);
filename = strcat('/path/to/', STATIONAME, '.dat');


• Call the function read_gps_data, presented on Monday. Use the temporary variable that holds the filename as input parameter and populate the other fields of your structure with the return values you get from this call.
• You can imagine that there are quite a few things you could do with this array of structs. There are other things you could do after the initialization that don't have much to do with the struct. We want to leave this up to your imagination; but please: imagine it!
• The next thing you will do is plot the timeseries for each GPS station. We want you to use a while loop. Not necessarily because it's the best thing to do (in fact, it isn't), we just want you to have used it.
• Define and initialize a counter variable
• Start your while loop with the length of your struct as termination condition.
• Call a slightly modified version of the plot_gps_timeseries function presented in class. You will get the figure handle as a return value; assign it to a variable!
• You will have Matlab automatically save the resulting figure (hence the need for a figure handle). First create a new output filename. This time make sure it ends in one of the graphics formats supported by the saveas function.
• Save your figure by calling saveas and handing it the figure handle, filename, and optionally a format defintion it's supposed to use (i.e. if you want colored postscript, add 'psc2' as third parameter).
• increment your counter
• CONGRATULATIONS, YOU JUST CREATED PUBLISHABLE FIGURES THAT ARE REPRODUCIBLE! AND WE'RE ONLY A MONTH IN THE SEMESTER.

If you are done early, consider this: textread will complain (throw an error) if a file does not exist. Given that you might be dealing with hundreds of stations at a time it would be frustrating to stop possibly lenghty processing whenever there is a typo in the cell array that holds the sites or certain files are just not there. Guess what: try-catch will help!

Try putting the read_gps_data function call in a try-block and catch possible errors. Give a useful error message (consult last lectures introduction of the try-catch-block and the respective documentation pages for information the error message gives you) OR display your very own error message, i.e. file X does not exist, where X is the parameter you handed to read_gps_data. Then add a fantasy string to the cell array that holds the station names and run your script. You should get at least one empty plot and a meaningful message, but the script should not abort - yeah!

Helpful Matlab Functions:

Here is a list of functions that might be of help.

• figure: create a canvas to plot on; get a handle so you can refer to it
• length, size: determine the length/size of arrays/matrices
• saveas: save a figure in a specified format
• struct: create a struct
• sprintf: create a formatted string
• strcat: concatenate many strings to get one single string