Agricultural and Applied Economics 637 Introduction to matlab the Fundamentals

Note that with a trailing semicolon no output is shown.

As implied the above, you can undertake various tasks using MATLAB one command at a time (i.e., interactive mode) or by running a collection of commands at once contained within a single command file (i.e.; batch mode). To run a file in batch mode you first create an ASCII (or text) file containing these commands. This is often referred to as a script file. This file can be created using your own favorite ASCII file editor (e.g., Notepad++, Tedit, etc) or you can use the native MATLAB program editor. Using the native MATLAB editor is very useful as it checks your syntax as you create the command file. MATLAB expects all script files containing MATLAB commands end with an “m” extension. I highly recommend that you follow this convention.

The first example we will use is contained in the file example_1.m . This file is contained in the location indicated at the right:

• 
  There need not be a semi-colon to end every command. If you have a semi-colon the result of the command will not be shown.
  
• 
  If a statement does not fit on one line, use an ellipsis (three periods), ..., followed by the Enter key to indicate that the statement continues on the next line. A good practice to adopt is that if your command line is more than 80 characters, continue with your code on a 2^nd line, indent, and keep typing.
  
• 
  Any variable to the left of an equals sign is a name I made up, e.g. that I want to define for MATLAB
  
• 
  Anything to the right of an equals sign is either a variable name I already defined by a previous command or a MATLAB command/operation/function/reserved word/etc.
  
• 
  Comments, that don’t impact the operation are located after the “%” . You should adopt a policy of using comments extensively in your code.
  

The MATLAB editor has the normal Windows-based file management system so you can edit and save this file like any other program you have used before if you are a Window$ user (I assume the same is true for Mac users). I would recommend that you save this file to your private network directory or your own removable storage device. In the editor, save the file by clicking on the following icon in the Editor tab: and save it in the desired location.

d=a*b
Save the file and run the program again and notice what happens- it doesn’t work. That is, you receive the error message in the bottom window that looks similar to the following:

Change the last line to read

d=a .* b

Also add the line

e=a*bʹ % Note the transpose
Run the code again. Does it work? What is different about these? What do these commands do?
4. READING IN A DATA SET

There are a number of methods for reading in datasets into your MATLAB software. You could load native MATLAB datasets (and similar to STATA and SAS datasets) that are binary files that cannot be opened and examined using an ASCII (text) editor. For an overview of how to open a variety of file types in MATLAB refer to the User’s Guide and the Data Import and Export section. For these workshops (and for our assignments) we will load Excel files containing the data. The first row of these files contains the variable names and the remaining rows are the data. DBMSCOPY is a useful program that can transform any ASCII or binary file of a particular type (e.g., SAS, STATA, EXCEL, GAUSS) into any other file type. Besides being able to transform data from one form to another, DBMSCOPY also has the nice characteristic of allowing you to view any type of data, undertake simple data management activities and to obtain descriptive statistics of this data. The DBMSCOPY manual is available on-line. Feel free to use your own data conversion software if you prefer.

You can access MATLAB remotely so long as you have internet access and a valid AAE account (If you have a MAC computer you can also access the remote server but I do not know how to do this. Talk to the HECC director for more information.) You can use the Remote Desktop feature of your operating system to connect to the AAE Remote Server. If you are at a remote location, click on START, then PROGRAMS then ACCESSORIES then COMMUNICATIONS then REMOTE DESKTOP CONNECTION. The computer you want to access is our remote servers which have the computer name (address) of: remote.aae.wisc.edu . Once you enter this address, you will then see what looks like another Windows session. What is actually occurring is that your remote machine is in fact starting a session on a virtual Windows machine (i.e., the REMOTE server). You can then log-in to the remote computer using your AAE username and password. The remote machine is set up exactly (or very similar to) the regular AAE lab machines. This means that you have access to all the programs in the AAE lab from your remote location. Since you are logging in using your AAE account, you have the same drive mappings as you have when you log into a lab machine. This means that so long as your data, programs etc are stored on a network resource, you have access to it from your remote location. In addition if you set the remote access options correctly on your home machine you can access not only the network drives but also your local drives. Refer to the HECC help system for assistance (http://www.hecc.wisc.edu/).

Now, I would like you to use the 2^nd MATLAB command file to illustrate the use of MATLAB, example_2.m. In the MATLAB editor close the previously used MATLAB command file (Note: You can have multiple files open at once if desired). Then open up the file example_2.m in the editor The following is a listing of the commands in this file (MATLAB functions in bold).

[full_data,varnames,raw] = xlsread('china_00');

%{

Load all data from an excel dataset

%}

fafh = full_data(:,1);

fah = full_data(:,3);

percinc = full_data(:,4);

[numobs,numvars]=size(full_data); % Determine the number of rows

var_names=char('fafh','region','fah','percinc'); % var_names is a (4 x 1) character array

[num,txt,raw] = xlsread(filename)

[num,txt,raw] = xlsread(filename,sheet)

[num,txt,raw] = xlsread(filename,range)

[num,txt,raw] = xlsread(filename,sheet,range)

[num,txt,raw] = xlsread(filename,-1)

[num,txt,raw] = xlsread(filename,sheet,'','basic')

[num,txt,raw,custom] = xlsread(filename,sheet,range,'',functionHandle)

The command [num,txt,raw] = xlsread(filename) reads data from the first worksheet in the Microsoft Excel spreadsheet file named filename and returns the numeric data in array num (this can be any name I want). With this command structure, it optionally returns the variable names in the cell array txt (this can be any name I want), and the unprocessed data (numbers and text) in cell array raw (this can be any name I want). (http://www.mathworks.com/help/techdoc/ref/xlsread.html)
fafh: Created variable containing the (numobs x 1) vector of food away from home expenditures
region: Created variable containing the (numobs x 1) vector of region of residence
fah: Created variable containing the (numobs x 1) vector of food at home expenditures
percinc: Created variable containing the (numobs x 1) vector of per capita income
size: Returns the dimension of the target matrix in separate variables numobs (rows) and numvars (columns). (http://www.mathworks.com/help/techdoc/ref/size.html)
numobs: A created variable (this can be any name I want) that contains the number of observations in the dataset
numvars: A created variable (this can be any name I want) that contains the number of variables in the data matrix
delete: This command is used to delete the named file. If the file does not exist you will receive an error (nonfatal) message. (http://www.mathworks.com/help/techdoc/ref/delete.html)
diary: Is used to write a copy of all subsequent keyboard input and the resulting output (except it does not include graphics) to the named file, where the filename is the full pathname or filename is in the current MATLAB folder. If the file already exists, output is appended at the end of the file. By deleting it first I avoid the appending of output. (http://www.mathworks.com/help/techdoc/ref/diary.html)
diary off: This command suspends the diary command. (Note that diary on resumes diary mode using the current filename, or the default filename diary if none has yet been specified.)

5. RUN THE CODE, AND CHECK TO MAKE SURE YOU HAVE

SUCCESSFULLY LOADED THE DATA
With the above program in the editor, run this file using the main editor menu. In the Workspace area you should see the four vectors of data plus the underlying matrix each with 2050 observations. You should see the following displayed in the Command window: “I have successfully run the program”.

6. IDENTIFY YOUR VARIABLES

The variables in this data set are the following

Per Capita Income percinc

At home food expenditures fah

Away from home food expenditures fafh

Region (either 1, 2, or 3) region

The use of the “ : ” in the above command file (e.g., lines 9 – 12) tells MATLAB to grab all the rows of the full data matrix for the particular column of data . If you wanted to grab only the 120^th -220^th observations for some reason you would type the following:

and see what is generated.

columns

Let’s do the following simple tasks:

ii) What is the maximum level of income?

3. 
   What percent of households live in region 1?
   
4. 
   What is the minimum, maximum, and average food away from home expenditures in region 1?
   

****************************************************************

A list of commands by category can be obtained from the following URL: http://www.mathworks.com/help/matlab/functionlist.html . An alphabetical list can be obtained via the following: http://www.mathworks.com/help/matlab/functionlist-alpha.html . Here are a few commands that you may find useful:

(http://www.mathworks.com/help/techdoc/ref/cumsum.html)

(http://www.mathworks.com/help/techdoc/ref/mean.html)

(http://www.mathworks.com/help/techdoc/ref/std.html)

(http://www.mathworks.com/help/techdoc/ref/min.html)

(http://www.mathworks.com/help/techdoc/ref/max.html)

(http://www.mathworks.com/help/techdoc/ref/cov.html)

(http://www.mathworks.com/help/techdoc/ref/zeros.html)

(http://www.mathworks.com/help/techdoc/ref/ones.html)

(http://www.mathworks.com/help/techdoc/ref/eye.html)

(http://www.mathworks.com/help/techdoc/ref/randn.html)

sort(x) Sort array elements in ascending or descending order depending on the additional arguments supplied when using this command

(http://www.mathworks.com/help/techdoc/ref/sort.html)

(http://www.mathworks.com/help/techdoc/ref/size.html)

%{ , %} The text enclosed within the %{ and %} symbols is a comment block. Use these symbols to insert comments that take up more than a single line in your script of function code. Any text between these two symbols is ignored by MATLAB. You can use these to have MATLAB not execute a set of commands.

&, |, ~ Element by element logical operations on arrays

Syntax: expr1 & expr2
expr1 | expr2
~expr

expr1 & expr2 represents a logical AND operation between values, arrays, or expressions expr1 and expr2. In an AND operation, if expr1 is true and expr2 is true, then the AND of those inputs returns a true value. If either expression is false, the result is false. Here is a pseudocode example of AND:

IF (expr1: all required inputs are included) AND ...

(expr2: all inputs are valid)

THEN (result: execute the function)

expr1 | expr2 represents a logical OR operation between values, arrays, or expressions expr1 and expr2. In an OR operation, if expr1 is true or expr2 is true, then the OR of those inputs returns a value of true. If both expressions are false, the result is false. Here is a pseudocode example of the use of OR:

IF (expr1: S is a string) OR ...

(expr2: S is a cell array of strings)

THEN (result: parse string S)

~expr represents a logical NOT operation applied to expression expr. In a NOT operation, if expr is false, then the result of the operation is true. If expr is true, the result is false. Here is a pseudocode example of NOT:

IF (expr: function returned a Success status) is NOT true

THEN (result: throw an error)

The function xor(A,B) implements the exclusive OR operation.

eq Test for equality

Syntax: A == B
eq(A, B)

If both A and B are scalar (i.e., 1-by-1 matrices), then the MATLAB software returns a scalar value.

If both A and B are nonscalar arrays, then these arrays must have the same dimensions, and MATLAB returns an array of the same dimensions as A and B.

If one input is scalar and the other a nonscalar array, then the scalar input is treated as if it were an array having the same dimensions as the nonscalar input array. In other words, if input A is the number 100, and B is a 3-by-5 matrix, then A is treated as if it were a 3-by-5 matrix of elements, each set to 100. MATLAB returns an array of the same dimensions as the nonscalar input array.

When comparing handle objects, use eq or the == operator to test whether objects are the same. Use isequal to test if objects have equal property values, even if those objects are different.

eq(A, B) is called for the syntax A == B when either A or B is an object.

gt Test for greater than

Syntax: A > B

gt(A, B)

If both A and B are scalar (i.e., 1-by-1 matrices), then the MATLAB software returns a scalar value.

If both A and B are nonscalar arrays, then these arrays must have the same dimensions, and MATLAB returns an array of the same dimensions as A and B.

If one input is scalar and the other a nonscalar array, then the scalar input is treated as if it were an array having the same dimensions as the nonscalar input array. In other words, if input A is the number 100, and B is a 3-by-5 matrix, then A is treated as if it were a 3-by-5 matrix of elements, each set to 100. MATLAB returns an array of the same dimensions as the nonscalar input array.

gt(A, B) is called for the syntax A>B when either A or B is an object.

ge Test for greater than or equal to

Syntax: A >= B

ge(A, B)

If both A and B are scalar (i.e., 1-by-1 matrices), then the MATLAB software returns a scalar value.

If both A and B are nonscalar arrays, then these arrays must have the same dimensions, and MATLAB returns an array of the same dimensions as A and B.

If one input is scalar and the other a nonscalar array, then the scalar input is treated as if it were an array having the same dimensions as the nonscalar input array. In other words, if input A is the number 100, and B is a 3-by-5 matrix, then A is treated as if it were a 3-by-5 matrix of elements, each set to 100. MATLAB returns an array of the same dimensions as the nonscalar input array.

ge(A, B) is called for the syntax A >= B when either A or B is an object.

le Test for less than or equal to

Syntax: A <= B
le(A, B)

If both A and B are scalar (i.e., 1-by-1 matrices), then the MATLAB software returns a scalar value.

If both A and B are nonscalar arrays, then these arrays must have the same dimensions, and MATLAB returns an array of the same dimensions as A and B.

If one input is scalar and the other a nonscalar array, then the scalar input is treated as if it were an array having the same dimensions as the nonscalar input array. In other words, if input A is the number 100, and B is a 3-by-5 matrix, then A is treated as if it were a 3-by-5 matrix of elements, each set to 100. MATLAB returns an array of the same dimensions as the nonscalar input array.

le(A, B) is called for the syntax A < =B when either A or B is an object.

lt Tests for less than

Syntax: A < B
lt(A, B)

If both A and B are scalar (i.e., 1-by-1 matrices), then the MATLABsoftware returns a scalar value.

If both A and B are nonscalar arrays, then these arrays must have the same dimensions, and MATLAB returns an array of the same dimensions as A and B.

If one input is scalar and the other a nonscalar array, then the scalar input is treated as if it were an array having the same dimensions as the nonscalar input array. In other words, if input A is the number 100, and B is a 3-by-5 matrix, then A is treated as if it were a 3-by-5 matrix of elements, each set to 100. MATLAB returns an array of the same dimensions as the nonscalar input array.

lt(A, B) is called for the syntax A < B when either A or B is an object.

ne Test for inequality

Syntax: A ~= B
ne(A, B)

If both A and B are scalar (i.e., 1-by-1 matrices), then the MATLAB software returns a scalar value.

If both A and B are nonscalar arrays, then these arrays must have the same dimensions, and MATLAB returns an array of the same dimensions as A and B.

If one input is scalar and the other a nonscalar array, then the scalar input is treated as if it were an array having the same dimensions as the nonscalar input array. In other words, if input A is the number 100, and B is a 3-by-5 matrix, then A is treated as if it were a 3-by-5 matrix of elements, each set to 100. MATLAB returns an array of the same dimensions as the nonscalar input array.

ne(A, B) is called for the syntax A ~= B when either A or B is an object.

C. Use of FOR LOOPS

The following illustrates the use of a “For Loop”. You should refer to your reference material for instructions concerning its use. It is very important to remember that the use of the For loop can make your life much easier if you are doing repetitive tasks. The For loop is designed to execute statements a specified number of times.

the loop goes from 1 to 20 in increments of 1, e.g., 20 steps %}

another element equal to the current value of “i” */

Now you are ready to run a longer MATLAB program. I sent you a 3^rd command file with the name demo_matlab.m . I assume you have either saved it in your network drive or in your USB storage device. This file undertakes some basic matrix manipulations. After examining this file, run it in MATLAB and make sure you understand the various operations.

1. 
   From the main program toolbar choose Preferences > Editor/Debugger > Language
   
2. 
   From the Language drop-down menu, select a language
   
3. 
   Select or clear Apply smart indenting while typing, depending on whether you want indenting applied automatically, as you type. If you clear this option, you can manually apply indenting by selecting the lines in the Editor to indent, right-clicking, and then selecting Smart Indent from the context menu.
   
4. 
   Do one of the following:
   
   1. 
      If you chose any language other than MATLAB in step 2, click OK.
      
   2. 
      If you chose MATLAB in step 2, select a Function indenting format, and then click OK. Function indent formats are:
      
      1. 
         Classic: The Editor aligns the function code with the function declaration.
         
      2. 
         Indent nested functions: Editor indents function code within a nested function.
         
      3. 
         Indent all functions — The Editor indents the function code for both main and nested functions.